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SCIENCE EVERYDAY

THINGS

OF

volume 1: REAL-LIFE CHEMISTRY

edited by NEIL SCHLAGERwritten by JUDSON KNIGHT

A SCHLAGER INFORMATION GROUP BOOK

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289SC IENCE OF EVERYDAY TH INGS VOLUME 1: REAL-LIFE CHEMISTRY

OX IDAT ION -REDUCT IONREACT IONSOxidation-Reduction Reactions

CONCEPTMost people have heard the term “oxidation” atsome point or another, and, from the sound ofthe word, may have developed the impressionthat it has something to do with oxygen. Indeedit does, because oxygen has a tendency to drawelectrons to itself. This tendency, rather than thepresence of oxygen itself, is actually what identi-fies oxidation, defined as a process in which asubstance loses electrons. The oxidation of onesubstance is always accompanied by reduction, orthe gaining of electrons, on the part of anothersubstance—hence the term “oxidation-reductionreaction,” sometimes called a redox reaction. Theworld is full of examples of this highly significantform of chemical reaction. One such example iscombustion, or an even more rapid form of com-bustion, explosion. Likewise the metabolism offood, as well as other biological processes,involves oxidation and reduction reactions. So,too, do a number of processes that take place onthe surfaces of metals: when iron rusts; whencopper turns green; or when aluminum forms acoating of aluminum oxide that prevents it fromrusting. Oxidation-reduction reactions also playa major role in electrochemistry, which has ahighly useful application to daily life in the formof batteries.

HOW IT WORKS

Chemical Reactions

A chemical reaction is a process whereby thechemical properties of a substance are changedby a rearrangement its atoms. The change pro-duced by a chemical reaction is quite differentfrom a purely physical change, which does not

affect the fundamental properties of the sub-stance itself. A piece of copper can be heated,melted, beaten into different shapes, and soforth, yet throughout all those changes, itremains pure copper, an element of the transi-tion metals family.

But suppose a copper roof is exposed to theelements for many years. Copper is famous for itshighly noncorrosive quality, and this, combinedwith its beauty, has made it a favored material foruse in the roofs of imposing buildings. (Becauseit is relatively expensive, few middle-class peopletoday can afford a roof entirely made of copper,but sometimes it is used as a decorative touch—for instance, over the entryway of a house.) Even-tually, however, copper does begin to corrodewhen exposed to air for long periods of time.

Over the years, exposed copper develops athin layer of black copper oxide, and as timepasses, traces of carbon dioxide in the air con-tribute to the formation of greenish copper car-bonate. This explains why the Statue of Liberty,covered in sheets of copper, is green, rather thanhaving the reddish-golden hue of new, uncorrod-ed copper.

EXTERNAL VS. INTERNAL

CHANGE. The preceding paragraphsdescribe two very different phenomena. The firstwas a physical change in which the chemicalproperties of a substance—copper—remainedunaltered. The second, on the other hand,involved a chemical change on the surface of thecopper, as copper atoms bonded with carbon andoxygen atoms in the air to form something dif-ferent from copper. The difference between thesetwo types of changes can be likened to varietiesof changes in a person’s life—an external change

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Oxidation-ReductionReactions

on the one hand, and a deeply rooted change onthe other.

A person may move to another house, job,school, or town, yet the person remains the same.Many sayings in the English language express thisfact: for instance, “Wherever you go, there youare,” or “You can take the boy out of the country,but you can’t take the country out of the boy.”Moving is simply a physical change. On the otherhand, if a person changes belief systems, over-comes old feelings (or succumbs to new ones),changes lifestyles in a profound manner, or inany other way changes his or her mind aboutsomething important—this is analogous to achemical change. In these instances, the person,like the surface of the copper described above,has changed not merely in external properties,but in inner composition.

“LEO the Lion Says ‘GER’”

Chemical reactions are addressed in depth with-in the essay devoted to that subject, which dis-cusses—among other subjects—many ways ofclassifying chemical reactions. These varieties ofchemical reaction are not all mutually exclusive,as they relate to different aspects of the reaction.As noted in the review of various reaction types,one of the most significant is an oxidation-reduction reaction (sometimes called a redoxreaction) involving the transfer of electrons.

As its name implies, an oxidation-reductionreaction is really two processes: oxidation, inwhich electrons are lost, and reduction, in whichelectrons are gained. Though these are definedseparately here, they do not occur independent-ly; hence the larger reaction of which each is apart is called an oxidation-reduction reaction.In order to keep the two straight, chemistryteachers long ago developed a useful, if nonsensi-cal, mnemonic device: “LEO the lion says ‘GER’.” LEO stands for “Loss of Electrons, Oxida-tion,” and “GER” means “Gain of Electrons,Reduction.”

Many, though not all, oxidation-reductionreactions involve oxygen. Oxygen combinesreadily with other elements, and in so doing, ittends to grab electrons from those other ele-ments’ atoms. As a result, the oxygen atombecomes an ion (an atom with an electriccharge)—specifically, an anion, or negativelycharged ion.

In interacting with another element, oxygenbecomes reduced, while the other element is oxi-dized to become a cation, or a positively chargedion. This, too, is easy to remember: oxygen itself,obviously, cannot be oxidized, so it must be theone being reduced. But since not all oxidation-reduction reactions involve oxygen, perhaps thefollowing is a better way to remember it. Elec-trons are negatively charged, and the elementthat takes them on in an oxidation-reductionreaction is reduced—just as a person who thinksnegative thoughts are “reduced” if those negativethoughts overcome positive ones.

Oxidation Numbers

An oxidation number (sometimes called an oxi-dation state) is a whole-number integer assignedto each atom in an oxidation-reduction reaction.This makes it easier to keep track of the electronsinvolved, and to observe the ways in which theychange positions. Here are some rules for deter-mining oxidation number.

• 1. The oxidation number for an atom of anelement not combined with other elementsin a compound is always zero.

• 2. For an ion of any element, the oxidationnumber is the same as its charge. Thus asodium ion, which has a charge of +1 and isdesignated symbolically as Na+, has an oxi-dation number of +1.

• 3. Certain elements or families form ions inpredictable ways:

• a. Alkali metals, such as sodium, alwaysform a +1 ion; oxidation number = +1.

• b. Alkaline earth metals, such as magne-sium, always form a +2 ion; oxidation num-ber = +2.

• c. Halogens, such as fluorine, form –1 ions;oxidation number = –1.

• d. Other elements have predictable ways toform ions; but some, such as nitrogen, canhave numerous oxidation numbers.

• 4. The oxidation number for oxygen is –2for most compounds involving covalentbonds.

• 5. When hydrogen is involved in covalentbonds with nonmetals, its oxidation num-ber is +1.

• 6. In binary compounds (compounds withtwo elements), the element having greaterelectronegativity is assigned a negative oxi-dation number that is the same as its charge

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when it appears as an anion in ionic com-pounds.

• 7. When a compound is electrically neutral,the sum of its elements’ oxidation states iszero.

• 8. In an ionic chemical species, the sum ofthe oxidation states for its constituent ele-ments must equal the overall charge.

These rules will not be discussed here;rather, they are presented to show some of thecomplexities involved in analyzing an oxidation-reduction reaction from a structural stand-point—that is, in terms of the atomic or molecu-lar reactions. For the most part, we will beobserving oxidation-reductions phenomenologi-cally, or in terms of their outward effects. A goodchemistry textbook should provide a moredetailed review of these rules, along with a tableshowing oxidation numbers of elements andbinary compounds.

Oxidation-reduction reactions are easier tounderstand if they are studied as though theywere two half-reactions. Half the reactioninvolves what happens to the substances andelectrons in the oxidizing portion, while theother half-reaction indicates the activities of sub-stances and electrons in the reduction portion.

REAL-L IFEAPPLICATIONS

Combustion and Explosions

As with any type of chemical reaction, combus-tion takes place when chemical bonds are brokenand new bonds are formed. It so happens thatcombustion is a particularly dramatic type ofoxidation-reduction reaction: whereas we can-not watch iron rust, combustion is a noticeableevent. Even more dramatic is combustion thattakes place at a rate so rapid that it results in anexplosion.

Coal is almost pure carbon, and its combus-tion in air is a textbook example of oxidation-reduction. Although there is far more nitrogenthan oxygen in air (which is a mixture ratherthan a compound), nitrogen is very unreactive atlow temperatures. For this reason, it can be usedto clean empty fuel tanks, a situation in whichthe presence of pure oxygen is extremely danger-ous. In any case, when a substance burns, it isreacting with the oxygen in air.

As one might expect from what has alreadybeen said about oxidation-reduction, the oxygenis reduced while the carbon is oxidized. In termsof oxidation numbers, the oxidation number of

Batteries use oxidation-reduction reactions to produce electrical current. (Lester V. Bergman/Corbis. Reproduced by permission.)


tary application, incidentally, was “Greek fire,”created by the Byzantines in the seventh centuryA.D. A mixture of petroleum, potassium nitrate,and possibly quicklime, Greek fire could burn onwater, and was used in naval battles to destroyenemy ships.

For the most part, however, the range ofactivities to which combustion could be appliedwas fairly narrow until the development of thesteam engine in the period from the late seven-teenth century to the early nineteenth century.The steam engine applied the combustion of coalto the production of heat for boiling water, whichin turn provided the power to run machinery. Bythe beginning of the twentieth century, combus-tion had found a new application in the internalcombustion engine, used to power automobiles.

EXPLOSIONS AND EXPLO-

SIVES. An internal combustion engine doesnot simply burn fuel; rather, by the combinedaction of the fuel injectors (in a modern vehicle),in concert with the pistons, cylinders, and sparkplugs, it actually produces small explosions in themolecules of gasoline. These produce the outputof power necessary to turn the crankshaft, andultimately the wheels.

An explosion, in simple terms, is a sped-upform of combustion. The first explosives wereinvented by the Chinese during the Middle Ages,and these included not only fireworks and explosive rockets, but gunpowder. Ironically,however, China rejected the use of gunpowder inwarfare for many centuries, while Europeanstook to it with enthusiasm. Needless to say, Euro-peans’ possession of firearms aided their con-quest of the Americas, as well as much of Africa,Asia, and the Pacific, during the period fromabout 1500 to 1900.

The late nineteenth and early twentieth cen-turies saw the development of new explosives,such as TNT or trinitrotoluene, a hydrocarbon.Then in the mid-twentieth century came themost fearsome explosive of all: the nuclear bomb.A nuclear explosion is not itself the result of anoxidation-reduction reaction, but of somethingmuch more complex—either the splitting ofatoms (fission) or the forcing together of atomicnuclei (fusion).

Nuclear bombs release far more energy thanany ordinary explosive, but the resulting blastalso causes plenty of ordinary combustion. Whenthe United States dropped atomic bombs on the


carbon jumps from 0 to 4, while that of oxygen isreduced to –2. As they burn, these two form car-bon dioxide or CO2, in which the two –2 chargesof the oxygen atoms cancel out the +4 charge ofthe carbon atom to yield a compound that iselectrically neutral.

COMBUSTION IN HUMAN EX-

PERIENCE. Combustion has been a signifi-cant part of human life ever since our prehistoricancestors learned how to harness the power offire to cook food and light their caves. We tend tothink of premodern times—to use the memo-rable title of a book by American historianWilliam Manchester, about the Middle Ages—asA World Lit Only By Fire. In fact, our modern ageis even more combustion-driven than that of ourforebears.

For centuries, burning animal fat—in torch-es, lamps, and eventually in candles—providedlight for humans. Wood fires supplied warmth, aswell as a means to cook meals. These were themain uses of combustion, aside from the occa-sional use of fire in warfare or for other purpos-es (including that ghastly medieval form of exe-cution, burning at the stake). One notable mili-

Oxidation-reduction reactions fuel thespace-shuttle at take-off. (Corbis. Reproduced bypermission.)




Japanese cities of Hiroshima and Nagasaki in

August 1945, those cities suffered not only the

effects of the immediate blast, but also massive

fires resulting from the explosion itself.

FUELING THE SPACE SHUT-

TLE. Oxidation-reduction reactions also fuel

the most advanced form of transportation

known today, the space shuttle. The actual

orbiter vehicle is relatively small compared to its

external power apparatus, which consists of two

solid rocket boosters on either side, along with anexternal fuel tank.

Inside the solid rocket boosters are ammoni-um perchlorate (NH4ClO4) and powdered alu-minum, which undergo an oxidation-reductionreaction that gives the shuttle enormousamounts of extra thrust. As for the larger singleexternal fuel tank, this contains the gases thatpower the rocket: hydrogen and oxygen.

Because these two are extremely explosive,they must be kept in separate compartments.

If not polished regularly, silver responds to oxidation by tarnishing—forming a surface ofsilver sulfide, or Ag2S. (Mimmo Jodice/Corbis. Reproduced by permission.)





When they react, they form water, of course, butin doing so, they also release vast quantities ofenergy. The chemical equation for this is: 2H2 +O2 42H2O + energy.

On January 28, 1986, something went terri-bly wrong with this arrangement on the spaceshuttle Challenger. Cold weather had fatigued theO-rings that sealed the hydrogen and oxygencompartments, and the gases fed straight into theflames behind the shuttle itself. This produced apowerful and uncontrolled oxidation-reductionreaction, an explosion that took the lives of allseven astronauts aboard the shuttle.

The Environment and Human Health

Combustion, though it can do much good, canalso do much harm. This goes beyond the obvi-ous: by burning fossil fuels or hydrocarbons,excess carbon (in the form of carbon dioxide andcarbon monoxide) is released to the atmosphere,with a damaging effect on the environment.

In fact, oxidation-reduction reactions areintimately connected with the functioning of thenatural environment. For example, photosynthe-sis, the conversion of light to chemical energy byplants, is a form of oxidation-reduction reactionthat produces two essentials of human life: oxy-gen and carbohydrates. Likewise cellular respira-tion, which along with photosynthesis is dis-cussed in the Carbon essay, is an oxidation-reduction reaction in which living things breakdown molecules of food to produce energy, car-bon dioxide, and water.

Enzymes in the human body regulate oxida-tion-reduction reactions. These complex pro-teins, of which several hundred are known, act ascatalysts, speeding up chemical processes in thebody. Oxidation-reduction reactions also takeplace in the metabolism of food for energy, withsubstances in the food broken down into compo-nents the body can use.

OXIDATION: SPOILING AND

AGING. At the same time, oxidation-reduc-tion reactions are responsible for the spoiling offood, the culprit here being the oxidation portionof the reaction. To prevent spoilage, manufactur-ers of food items often add preservatives, whichact as reducing agents.

Oxidation may also be linked with the effectsof aging in humans, as well as with other condi-

tions such as cancer, hardening of the arteries,and rheumatoid arthritis. It appears that oxygenmolecules and other oxidizing agents, alwayshungry for electrons, extract these from themembranes in human cells. Over time, this cancause a gradual breakdown in the body’simmune system.

To forestall the effects of oxidation, somedoctors and scientists recommend antioxi-dants—natural reducing agents such as vitaminC and vitamin E. The vitamin C in lemon juicecan be used to prevent oxidizing on the cut sur-face of an apple, to keep it from turning brown.Perhaps, some experts maintain, natural reduc-ing agents can also slow the pace of oxidation inthe human body.

Forming a New Surface on Metal

Clearly, oxidization can have a corrosive effect,and nowhere is this more obvious than in thecorrosion of metals by exposure to oxidizingagents—primarily oxygen itself. Most metalsreact with O2, and might corrode so quickly thatthey become useless, were it not for the forma-tion of a protective coating—an oxide.

Iron forms an oxide, commonly known asrust, but this in fact does little to protect it fromcorrosion, because the oxide tends to flake off,exposing fresh surfaces to further oxidation.Every year, businesses and governments devotemillions of dollars to protecting iron and steelfrom oxidation by means of painting and othermeasures, such as galvanizing with zinc. In fact,oxidation-reduction reactions virtually definethe world of iron. Found naturally only in ores,the element is purified by heating the ore withcoke (impure carbon) in the presence of oxygen,such that the coke reduces the iron.

COINAGE METALS. Copper, as wehave seen, responds to oxidation by corroding ina different way: not by rusting, but by changingcolor. A similar effect occurs in silver, which tar-nishes, forming a surface of silver sulfide, orAg2S. Copper and silver are two of the “coinagemetals,” so named because they have often beenused to mint coins. They have been used for thispurpose not only because of their beauty, butalso due to their relative resistance to corrosion.This resistance has, in fact, earned them the nick-name “noble metals.”



The third member of this mini-family isgold, which is virtually noncorrosive. Wonderfulas gold is in this respect, however, no one is like-ly to use it as a roofing material, or for any suchlarge-scale application involving its resistance tooxidation. Aside from the obvious expense, goldis soft, and not very good for structural uses, evenif it were much cheaper. Yet there is such a “won-der metal”: one that experiences virtually no cor-rosion, is cheap, and strong enough in alloys to be used for structural purposes. Its name isaluminum.

ALUMINUM. There was a time, in fact,when aluminum was even more expensive thangold. When the French emperor Napoleon IIIwanted to impress a dinner guest, he arranged forthe person to be served with aluminum utensils,while less distinguished personages had to settlefor “ordinary” gold and silver.

In 1855, aluminum sold for $100,000 apound, whereas in 1990, the going rate was about$0.74. Demand did not go down—in fact, itincreased exponentially—but rather, supplyincreased, thanks to the development of an inex-


ANION: An ion with a negative charge;

pronounced “AN-ie-un.”

BINARY COMPOUND: A compound

involving two elements.

CHEMICAL REACTION: A process

whereby the chemical properties of a sub-

stance are changed by a rearrangement of

the atoms in the substance.

CHEMICAL SPECIES: A generic term

used for any substance studied in chem-

istry—whether it be an element, com-

pound, mixture, atom, molecule, ion, and

so forth.

COVALENT BONDING: A type of

chemical bonding in which two atoms

share valence electrons.

ELECTROCHEMISTRY: The study of

the relationship between chemical and

electrical energy.

ELECTROLYSIS: The use of an elec-

tric current to produce a chemical change.

ELECTRONEGATIVITY: The relative

ability of an atom to attract valence

electrons.

CATION: An ion with a positive charge;

pronounced “KAT-ie-un.”

ION: An atom or group of atoms that

has acquired an electric charge due to the

loss or gain of electrons.

OXIDATION: A chemical reaction in

which a substance loses electrons. It is

always accompanied by reduction; hence

the term oxidation-reduction reaction.

OXIDATION NUMBER: A number

assigned to each atom in an oxidation-

reduction reaction as a means of keeping

track of the electrons involved. Another

term for oxidation number is “oxidation

state.”

OXIDATION-REDUCTION REACTION:

A chemical reaction involving the transfer

of electrons.

REDOX REACTION: Another name

for an oxidation-reduction reaction.

REDUCTION: A chemical reaction in

which a substance gains electrons. It is

always accompanied by oxidation; hence

the term oxidation-reduction reaction.

VALENCE ELECTRONS: Electrons

that occupy the highest energy levels in an

atom. These are the only electrons involved

in chemical bonding.

KEY TERMS



pensive aluminum-reduction process. Two men,one American and one French, discovered thisprocess at the same time: interestingly, their yearsof birth and death were the same.

Aluminum was once a precious metalbecause it proved extremely difficult to separatefrom oxygen. The Hall-Heroult process overcamethe problem by applying electrolysis—the use ofan electric current to produce a chemicalchange—as a way of reducing Al3+ ions (whichhave a high affinity for oxygen) to neutral alu-minum atoms. In the United States today, 4.5%of the total electricity output is used for the pro-duction of aluminum through electrolysis.

The foregoing statistic is staggering, consid-ering just how much electricity Americans use,and it indicates the importance of this once-pre-cious metal. Actually, aluminum oxidizes just likeany other metal—and does so quite quickly, as amatter of fact, by forming a coating of aluminumoxide (Al2O3). But unlike rust, the aluminumoxide is invisible, and acts as a protective coating.Chromium, nickel, and tin react to oxygen in asimilar way, but these are not as inexpensive asaluminum.

Electrochemistry and Batteries

Electrochemistry is the study of the relationshipbetween chemical and electrical energy. Amongits applications is the creation of batteries, whichuse oxidation-reduction reactions to produce anelectric current.

A basic battery can be pictured schematical-ly as two beakers of solution connected by a wire.In one solution is the oxidizing agent; in theother, a reducing agent. The wire allows electronsto pass back and forth between the two solutions,but to ensure that the flow goes both ways, thetwo solutions are also connected by a “saltbridge.” The salt bridge contains a gel or solutionthat permits ions to pass back and forth, but aporous membrane prevents the solutions fromactually mixing.

In the lead storage battery of an automobile,lead itself is the reducing agent, while lead (IV)

oxide (PbO2) acts as the oxidizing agent. A high-ly efficient type of battery, able to withstand wideextremes in temperature, the lead storage batteryhas been in use since 1915. Along the way, fea-tures have been altered, but the basic principleshave remained—a testament to the soundness ofits original design.

The batteries people use for powering allkinds of portable appliances, from flashlights toboom boxes, are called dry cell batteries. In con-trast to the model described above, using solu-tions, a dry cell (as its name implies) involves noliquid components. Instead, it utilizes variouselements in a range of combinations, includingzinc, magnesium, mercury, silver, nickel, andcadmium. The last two are applied in the nickel-cadmium battery, which is particularly usefulbecause it can be recharged over and over againby an external current. The current turns theproducts of the chemical reactions in the batteryback into reactants.

WHERE TO LEARN MORE

“Batteries.” Oregon State University Department of Chem-istry (Web site). <http://www.chem.orst.edu/ch411/scbatt.htm> (June 4, 2001).

“Batteries and Fuel Cells” (Web site). <http://vectorsite.com/ttfuelc.html> (June 4, 2001).

Borton, Paula and Vicky Cave. The Usborne Book of Bat-teries and Magnets. Tulsa, OK: EDC Publishing, 1995.

Craats, Rennay. The Science of Fire. Milwaukee, WI:Gareth Stevens Publishing, 2000.

Knapp, Brian J. Oxidation and Reduction. Danbury, CT:Grolier Educational, 1998.

“Oxidation Reduction Links” (Web site). <http://users.erols.com/merosen/redox.htm> (June 4, 2001).

“Oxidation-Reduction Reactions.” General Chemistry(Web site). <http://ull.chemistry.uakron.edu/genchem/11/> (June 4, 2001).

“Oxidation-Reduction Reactions: Redox.” UNC-ChapelHill Chemistry Fundamentals (Web site).<http://www.shodor.org/unchem/advanced/redox/>(June 4, 2001).

Yount, Lisa. Antoine Lavoisier: Founder of Modern Chem-istry. Springfield, NJ: Enslow Publishers, 1997.

Zumdahl, Steven S. Introductory Chemistry: A Founda-tion, fourth edition. Boston: Houghton Mifflin, 2000.




CHEM ICAL EQU I L IBR IUMChemical Equilibrium

CONCEPTReactions are the “verbs” of chemistry—theactivity that chemists study. Many reactionsmove to their conclusion and then stop, meaningthat the reactants have been completely trans-formed into products, with no means of return-ing to their original state. In some cases, the reac-tion truly is irreversible, as for instance whencombustion changes both the physical andchemical properties of a substance. There areplenty of other circumstances, however, in whicha reverse reaction is not only possible but anongoing process, as the products of the first reac-tion become the reactants in a second one. Thisdynamic state, in which the concentrations ofreactants and products remains constant, isreferred to as equilibrium. It is possible to predictthe behavior of substances in equilibriumthrough the use of certain laws, which are appliedin industries seeking to lower the costs of pro-ducing specific chemicals. Equilibrium is alsouseful in understanding processes that pre-serve—or potentially threaten—human health.

HOW IT WORKS

Chemical Reactions in Brief

What follows is a highly condensed discussion ofchemical reactions, and particularly the methodsfor writing equations to describe them. For amore detailed explanation of these principles, thereader is encouraged to consult the ChemicalReactions essay.

A chemical reaction is a process whereby thechemical properties of a substance are altered bya rearrangement of the atoms in the substance.

The changes produced by a chemical reaction arefundamentally different from physical changes,such as boiling or melting liquid water, changesthat alter the physical properties of water withoutaffecting its molecular structure.

INDICATIONS THAT A CHEMI-

CAL REACTION HAS OCCURRED.

Though chemical reactions are most effectivelyanalyzed in terms of molecular properties andbehaviors, there are numerous indicators thatsuggest to us when a chemical reaction hasoccurred. It is unlikely that all of these will resultfrom any one reaction, and in fact chances arethat a particular reaction will manifest only oneor two of these effects. Nonetheless, these offer ushints that a reaction has taken place:

Signs that a substance has undergone achemical reaction:

• Water is produced• A solid forms• Gases are produced• Bubbles are formed• There is a change in color• The temperature changes• The taste of a consumable substance

changes• The smell changesCHEMICAL CHANGES CON-

TRASTED WITH PHYSICAL CHANG-

ES OF TEMPERATURE. Many of theseeffects can be produced simply by changing thetemperature of a substance, but again, the mereact of applying heat from outside (or removingheat from the substance itself) does not consti-tute a chemical change. Water can be “produced”by melting ice, but the water was already there—it only changed form. By contrast, when an acid


ChemicalEquilibrium

and a base react to form water and a salt, that is atrue chemical reaction.

Similarly, the freezing of water forms a solid,but no new substance has been formed; in achemical reaction, by contrast, two liquids canreact to form a solid. When water boils throughthe application of heat, bubbles form, and a gasor vapor is produced; yet in chemical changes,these effects are not the direct result of applyingheat.

In this context, a change in temperature,noted as another sign that a reaction has takenplace, is a change of temperature from within thesubstance itself. Chemical reactions can be classi-fied as heat-producing (exothermic) or heat-absorbing (endothermic). In either case, thetransfer of heat is not accomplished simply bycreating a temperature differential, as wouldoccur if heat were transferred merely throughphysical means.

Why Do Chemical ReactionsOccur?

At one time, chemists could only study reactionsfrom “outside,” as it were—purely in terms ofeffects noticeable through the senses. Between

the early nineteenth and the early twentieth cen-turies, however, the entire character of chemistrychanged, as did the terms in which chemists dis-cussed reactions. Today, those reactions are ana-lyzed primarily in terms of subatomic, atomic,and molecular properties and activities.

Despite all this progress, however, chemistsstill do not know exactly what happens in achemical reaction—but they do have a goodapproximation. This is the collision model,which explains chemical reactions in terms ofcollisions between molecules. If the collision isstrong enough, it can break the chemical bondsin the reactants, resulting in a re-formation ofatoms within different molecules. The more themolecules collide, the more bonds are being bro-ken, and the faster the reaction.

Increase in the numbers of collisions can beproduced in two ways: either the concentrationsof the reactants are increased, or the temperatureis increased. By raising the temperature, thespeeds of the molecules themselves increase, andthe collisions possess more energy. A certainenergy threshold, the activation energy (symbol-ized Ea) must be crossed in order for a reaction tooccur. A temperature increase raises the likeli-hood that a given collision will cross the activa-tion-energy threshold, producing the energy tobreak the molecular bonds and promote thechemical reaction.

Raising the temperature and the concentra-tions of reactants can increase the energy andhasten the reactions, but in some cases it is notpossible to do either. Fortunately, the rate ofreaction can be increased in a third way, throughthe introduction of a catalyst, a substance thatspeeds up the reaction without participating in iteither as a reactant or product. Catalysts are thusnot consumed in the reaction. Many chemistrytextbooks discuss catalysts within the context ofequilibrium; however, because catalysts play suchan important role in human life, in this bookthey are the subject of a separate essay.

Chemical Equations InvolvingEquilibrium

A chemical equation, like a mathematical equa-tion, symbolizes an interaction between entitiesthat produces a particular result. In the case of achemical equation, the entities are not numbersbut reactants, and they interact with each othernot through addition or multiplication, but by


MANY MOUNTAIN CLIMBERS USE PRESSURIZED OXYGEN

AT VERY HIGH ALTITUDES TO MAINTAIN PROPER HEMO-GLOBIN-OXYGEN EQUILIBRIUM. (Galen Rowell/Corbis. Repro-

duced by permission.)


ChemicalEquilibrium

chemical reaction. Yet just as a product is theresult of multiplication in mathematics, a prod-uct in a chemical equation is the substance orsubstances that result from the reaction.

Instead of an equals sign between the reac-tants and the product, an arrow is used. Whenthe arrow points to the right, this indicates a for-ward reaction; conversely, an arrow pointing tothe left symbolizes a reverse reaction. In a reversereaction, the products of a forward reaction havebecome the reactants, and the reactants of theforward reaction are now the products. This isindicated by an arrow that points toward the left.

Chemical equilibrium, which occurs whenthe ratio between the reactants and products isconstant, and in which the forward and reversereactions take place at the same rate, is symbol-ized thus: 1. Note that the arrows, the upper onepointing right and the lower one pointing left,are of the same length. There may be certaincases, discussed below, in which it is necessary toshow these arrows as unequal in length as ameans of indicating the dominance of either theforward or reverse reaction.

Chemical equations usually include notationindicating the state or phase of matter for thereactants and products: (s) for a solid; (l) for aliquid; (g) for a gas. A fourth symbol, (aq), indi-cates a substance dissolved in water—that is, anaqueous solution. In the following paragraphs,we will apply a chemical equation to the demon-stration of equilibrium, but will not discuss thebalancing of equations. The reader is encouragedto consult the passage in the Chemical Reactionsessay that addresses that process, vital to therecording of accurate data.

A SIMPLE EQUILIBRIUM EQUA-

TION. Let us now consider a simple equation,involving the reaction between water and carbonmonoxide (CO) at high temperatures in a closedcontainer. The initial equation is written thus:H2O(g) + CO(g) 4H2(g) + CO2g). In plain Eng-lish, water in the gas phase (steam) has reactedwith carbon monoxide to produce hydrogen gasand carbon dioxide.

As the reaction proceeds, the amount ofreactants decreases, and the concentration ofproducts increases. At some point, however, therewill be a balance between the numbers of prod-ucts and reactants—a state of chemical equilibri-um represented by changing the right-pointingarrow to an equilibrium symbol: H2O(g) +

CO(g) ! H2(g) + CO2g). Assuming that the sys-tem is not disturbed (that is, that the container iskept closed and no outside substances are intro-duced), equilibrium will continue to be main-tained, because the reverse reaction is occurringat the same rate as the forward one.

Note what has been said here: reactions arestill occurring, but the forward and rearwardreactions balance one another. Thus equilibriumis not a static condition, but a dynamic one, andindeed, chemical equilibrium is sometimesreferred to as “dynamic equilibrium.” On theother hand, some chemists refer to chemicalequilibrium simply as equilibrium, but here thequalifier chemical has been used to distinguishthis from the type of equilibrium studied inphysics. Physical equilibrium, which involves fac-tors such as center of gravity, does help us tounderstand chemical equilibrium, but it is a dif-ferent phenomenon.


Homogeneous and Heteroge-neous Equilibria

It should be noted that the equation used aboveidentifies a situation of homogeneous equilibri-um, in which all the substances are in the samephase or state of matter—gas, in this case. It isalso possible to achieve chemical equilibrium in areaction involving substances in more than onephase of matter.

An example of such heterogeneous equilib-rium is the decomposition of calcium carbonatefor the production of lime, a process that involvesthe application of heat. Here the equation wouldbe written thus: CaCO3(s) ! CaO(s) + CO2(g).Both the calcium carbonate (CaCO3) and thelime (CaO) are solids, whereas the carbon diox-ide produced in this reaction is a gas.

The Equilibrium Constant

In 1863, Norwegian chemists Cato MaximilianGuldberg (1836-1902) and Peter Waage (1833-1900)—who happened to be brothers-in-law—formulated what they called the law of massaction. Today, this is called the law of chemicalequilibrium, which states that the direction takenby a reaction is dependant not merely on themass of the various components of the reaction,



ChemicalEquilibrium

but also upon the concentration—that is, themass present in a given volume.

This can be expressed by the formula aA +bB 1 cC +dD, where the capital letters representchemical species, and the italicized lowercase let-ters indicate their coefficients. The equation[C]c[D]d/[A]a[B]b yields what is called an equi-librium constant, symbolized K.

The above formula expresses the equilibri-um constant in terms of molarity, the amount ofsolute in a given volume of solution, but in thecase of gaseous reactants and products, the equi-librium constant can also be expressed in termsof partial pressures. In the reaction of water andcarbon monoxide to produce hydrogen mole-cules and carbon dioxide (H2O + CO ! H2 +CO2). In chemical reactions involving solids,however, the concentration of the solid—becauseit is considered to be invariant—does not appearin the equilibrium constant. In the reactiondescribed earlier, in which calcium carbonatewas in equilibrium with solid lime and gaseouscarbon dioxide, K = pressure of CO2.

We will not attempt here to explore the equi-librium constant in any depth, but it is importantto recognize its usefulness. For a particular reac-tion at a specific temperature, the ratio of con-centrations between reactants and products willalways have the same value—the equilibriumconstant, or K. Because it is not dependant on theamounts of reactants and products mixedtogether initially, K remains the same: the con-centrations themselves may vary, but the ratiosbetween the concentrations in a given situationdo not.

Le Châtelier’s Principle

Not all situations of equilibrium are alike:depending on certain factors, the position ofequilibrium may favor one side of the equationor the other. If a company is producing chemicalsfor sale, for example, its production managerswill attempt to influence reactions in such a wayas to favor the forward reaction. In such a situa-tion, it is said that the equilibrium position hasbeen shifted to the right. In terms of physicalequilibrium, mentioned above, this would beanalogous to what would happen if you wereholding your arms out on either side of yourbody, with a heavy lead weight in your left handand a much smaller weight in the right hand.

Your center of gravity, or equilibrium position,would shift to the left to account for the greaterforce exerted by the heavier weight.

A value of K significantly above 1 causes ashift to the right, meaning that at equilibrium,there will be more products than reactants. Thisis a situation favorable to a chemical company’smanagers, who desire to create more of the prod-uct from less of the reactants. However, natureabhors an imbalance, as expressed in Le Châte-lier’s principle. Named after French chemistHenri Le Châtelier (1850-1936), this principlemaintains that whenever a stress or change isimposed on a chemical system in equilibrium,the system will adjust the amounts of the varioussubstances to reduce the impact of that stress.

Suppose we add more of a particular sub-stance to increase the rate of the forward reac-tion. In an equation for this reaction, the equilib-rium symbol is altered, with a longer arrowpointing to the right to indicate that the forwardreaction is favored. Again, the equilibrium posi-tion has shifted to the right—just as one makesphysical adjustments to account for an imbal-anced weight. The system responds by workingto consume more of the reactant, thus adjustingto the stress that was placed on it by the additionof more of that substance. By the same token, ifwe were to remove a particular reactant or prod-uct, the system would shift in the direction of thedetached component.

Note that Le Châtelier’s principle is mathe-matically related to the equilibrium constant.Suppose we have a basic equilibrium equation ofA + B 1 C, with A and B each having molaritiesof 1, and C a molarity of 4. This tells us that K isequal to the molarity of C divided by that of Amultiplied by B = 4/(1 • 1). Suppose, now, thatenough of C were added to bring its concentra-tion up to 6. This would mean that the systemwas no longer at equilibrium, because C/(A • B)no longer equals 4. In order to return the ratio to4, the numerator (C) must be decreased, whilethe denominator (A • B) is increased. The reac-tion thus shifts from right to left.

CHANGES IN VOLUME AND

TEMPERATURE. If the volume of gases ina closed container is decreased, the pressureincreases. An equilibrium system will thereforeshift in the direction that reduces the pressure;but if the volume is increased, thus reducing thepressure, the system will respond by shifting to



increase pressure. Note, however, that not allincreases in pressure lead to a shift in the equilib-rium. If the pressure were increased by the addi-tion of a noble gas, the gas itself—since these ele-ments are noted for their lack of reactivity—would not be part of the reaction. Thus thespecies added would not be part of the equilibri-um constant expression, and there would be nochange in the equilibrium.

In any case, no change in volume alters theequilibrium constant K; but where changes intemperature are involved, K is indeed altered. Inan exothermic, or heat-producing reaction, theheat is treated as a product. Thus, when nitrogenand hydrogen react, they produce not onlyammonia, but a certain quantity of heat. If thissystem is at equilibrium, Le Châtelier’s principleshows that the addition of heat will induce a shiftin equilibrium to the left—in the direction thatconsumes heat or energy.

The reverse is true in an endothermic, orheat-absorbing reaction. As in the processdescribed earlier, the thermal decomposition ofcalcium carbonate produces lime and carbondioxide. Because heat is used to cause this reac-tion, the amount of heat applied is treated as areactant, and an increase in temperature willcause the equilibrium position to shift to theright.

Equilibrium and Health

Discussions of chemical equilibrium tend tobe rather abstract, as the foregoing sections onthe equilibrium constant and Le Châtelier’s prin-ciple illustrate. (The reader is encouraged to con-sult additional sources on these topics, whichinvolve a number of particulars that have beentouched upon only briefly here.) Despite thechallenges involved in addressing the subject ofequilibrium, the results of chemical equilibriumcan be seen in processes involving human health.

The cooling of food with refrigerators, alongwith means of food preservation that do notinvolve changes in temperature, maintains chem-ical equilibrium in the foods and thereby pre-vents or at least retards spoilage. Even moreimportant is the maintenance of equilibrium inreactions between hemoglobin and oxygen inhuman blood.

HEMOGLOBIN AND OXYGEN.

Hemoglobin, a protein containing iron, is the

material in red blood cells responsible for trans-porting oxygen to the cells. Each hemoglobinmolecule attaches to four oxygen atoms, and theequilibrium conditions of the hemoglobin-oxy-gen interaction can be expressed thus: Hb(aq) +4O2(g) ! Hb(O2)4(aq), where “Hb” stands forhemoglobin. As long as there is sufficient oxygenin the air, a healthy equilibrium is maintained; butat high altitudes, considerable changes occur.

At significant elevations above sea level, theair pressure is lowered, and thus it is more diffi-cult to obtain the oxygen one needs. The result,in accordance with Le Châtelier’s principle, is ashift in equilibrium to the left, away from theoxygenated hemoglobin. Without adequate oxy-gen fed to the body’s cells and tissues, a persontends to feel light-headed.

When someone not physically prepared forthe change is exposed to high altitudes, it may benecessary to introduce pressurized oxygen froman oxygen tank. This shifts the equilibrium to theright. For people born and raised at high alti-tudes, however, the body’s chemistry performsthe equilibrium shift—by producing morehemoglobin, which also shifts equilibrium to theright.

HEMOGLOBIN AND CARBON

MONOXIDE. When someone is exposed tocarbon monoxide gas, a frightening variation onthe normal hemoglobin-oxygen interactionoccurs. Carbon monoxide “fools” hemoglobininto mistaking it for oxygen because it also bondsto hemoglobin in groups of four, and the equi-librium expression thus becomes: Hb(aq) +4CO(g) ! Hb(CO)4(aq). Instead of hemoglobin,what has been produced is called carboxyhemo-globin, which is even redder than hemoglobin.Therefore, one sign of carbon monoxide poison-ing is a flushed face.

The bonds between carbon monoxide andhemoglobin are about 300 times as strong asthose between hemoglobin and oxygen, and thismeans a shift in equilibrium toward the right sideof the equation—the carboxyhemoglobin side. Italso means that K for the hemoglobin-carbonmonoxide reaction is much higher than for thehemoglobin-oxygen reaction. Due to the affinityof hemoglobin for carbon monoxide, the hemo-globin puts a priority on carbon monoxidebonds, and hemoglobin that has bonded withcarbon monoxide is no longer available to carryoxygen.


ChemicalEquilibrium



ACTIVATION ENERGY: The minimal

energy required to convert reactants into

products, symbolized Ea.

AQUEOUS SOLUTION: A mixture of

water and a substance that is dissolved in it.

CATALYST: A substance that speeds up

a chemical reaction without participating

in it, either as a reactant or product.

Catalysts are thus not consumed in the

reaction.

CHEMICAL EQUATION: A represen-

tation of a chemical reaction in which the

chemical symbols on the left stand for the

reactants, and those on the right are the

product or products.

CHEMICAL EQUILIBRIUM: A situa-

tion in which the ratio between the reac-

tants and products in a chemical reaction is

constant, and in which the forward reac-

tions and reverse reactions take place at the

same rate.









so forth.

COEFFICIENT: a number used to indi-

cate the presence of more than one unit—

typically, more than one molecule—of a

chemical species in a chemical equation.

COLLISION MODEL: The theory that

chemical reactions are the result of colli-

sions between molecules that are strong

enough to break bonds in the reactants,

resulting in a reformation of atoms.

ENDOTHERMIC: A term describing a

chemical reaction in which heat is

absorbed or consumed.

KEY TERMSChemicalEquilibrium

Carbon monoxide in small quantities cancause headaches and dizziness, but larger con-centrations can be fatal. To reverse the effects ofthe carbon monoxide, pure oxygen must beintroduced to the body. It will react with the car-boxyhemoglobin to produce properly oxygenat-ed hemoglobin, along with carbon monoxide:Hb(CO)4(aq) + 4O2(g) ! Hb(O2)4(aq) +4CO(g). The gaseous carbon monoxide thus pro-duced is dissipated when the person exhales.

WHERE TO LEARN MORE

“Catalysts” (Web site). <http://edie.cprost.sfu.ca/~rhlogan/catalyst.html> (June 9, 2001).

Challoner, Jack. The Visual Dictionary of Chemistry. NewYork: DK Publishing, 1996.

“Chemical Equilibrium.” Davidson College Department ofChemistry (Web site). <http://www.chm.davidson.edu/ronutt/che115/EquKin.htm> (June 9, 2001).

“Chemical Equilibrium in the Gas Phase.” Virginia TechChemistry Department (Web site). <http://www.chem.vt.edu/RVGS/ACT/notes/chem-eqm.html>(June 9, 2001).

“Chemical Sciences: Mechanism of Catalysis.” University ofAlberta Department of Chemistry (Web site).<http://www.chem.ualberta.ca/~plambeck/che/p102/p02174.htm> (June 9, 2001).

Ebbing, Darrell D.; R. A. D. Wentworth; and James P.Birk. Introductory Chemistry. Boston: Houghton Mif-flin, 1995.

Hauser, Jill Frankel. Super Science Concoctions: 50 Myste-rious Mixtures for Fabulous Fun. Charlotte, VT:Williamson Publishing, 1996.

“Mark Rosen’s Chemical Equilibrium Links” (Web site).<http://users.erols.com/merosen/equilib.htm> (June9, 2001).


ChemicalEquilibrium

Oxlade, Chris. Chemistry. Illustrated by Chris Fair-clough. Austin, TX: Raintree Steck-Vaughn, 1999.

Zumdahl, Steven S. Introductory Chemistry: A Founda-tion, 4th ed. Boston: Houghton Mifflin, 2000.


EXOTHERMIC: A term describing a

chemical reaction in which heat is

produced.

FORWARD REACTION: A chemical

reaction symbolized by a chemical equa-

tion in which the reactants and product are

separated by an arrow that points to the

right, toward the products.

HETEROGENEOUS EQUILIBRIUM:

Chemical equilibrium in which the sub-

stances involved are in different phases of

matter (solid, liquid, gas.)

HOMOGENEOUS EQUILIBRIUM:

Chemical equilibrium in which the sub-

stances involved are in the same phase of

matter.

LE CHÂTELIER’S PRINCIPLE: A

statement, formulated by French chemist

Henri Le Châtelier (1850-1936), which

holds that whenever a stress or change is

imposed on a system in chemical equilibri-

um, the system will adjust the amounts of

the various substances in such a way as to

reduce the impact of that stress.

PRODUCT: The substance or sub-

stances that result from a chemical

reaction.

REACTANT: A substance that interacts

with another substance in a chemical reac-

tion, resulting in a product.

REVERSE REACTION: A chemical

reaction symbolized by a chemical equa-

tion in which the products of a forward

reaction have become the reactants, and

the reactants of the forward reaction are

now the products. This is indicated by an

arrow that points toward the left.

SYSTEM: In chemistry and other sci-

ences, the term “system” usually refers to

any set of interactions isolated from the

rest of the universe. Anything outside of

the system, including all factors and forces

irrelevant to a discussion of that system, is

known as the environment.

KEY TERMS CONT INUED



C ATALY S T SCatalysts

CONCEPTIn most of the processes studied within the phys-ical sciences, the lesson again and again is thatnature provides no “free lunch”; in other words,it is not possible to get something for nothing. Achemical reaction, for instance, involves the cre-ation of substances different from those thatreacted in the first place, but the number ofatoms involved does not change. In view ofnature’s inherently conservative tendencies, then,the idea of a catalyst—a substance that speeds upa reaction without being consumed—seemsalmost like a magic trick. But catalysts are veryreal, and their presence in the human body helpsto sustain life. Similarly, catalysts enable the synthesis of foods, and catalytic converters inautomobiles protect the environment from dan-gerous exhaust fumes. Yet the presence of oneparticular catalyst in the upper atmosphere posessuch a threat to Earth’s ozone layer that produc-tion of certain chemicals containing that sub-stance has been banned.

HOW IT WORKS

Reactions and Collisions

In a chemical reaction, substances known asreactants interact with one another to create newsubstances, called products. In the present con-text, our concern is not with the reactants andproducts themselves, but with an additional enti-ty, an agent that enables the reaction to move for-ward at faster rates and lower temperatures.

According to the collision model generallyaccepted by chemists, chemical reactions are theresult of collisions between molecules. Collisions

that are sufficiently energetic break the chemicalbonds that hold molecules together; as a result,the atoms in those molecules are free to recom-bine with other atoms to form new molecules.Hastening of a chemical reaction can be pro-duced in one of three ways. If the concentrationsof the reactants are increased, this means thatmore molecules are colliding, and potentiallymore bonds are being broken. Likewise if thetemperature is increased, the speeds of the mole-cules themselves increase, and their collisionspossess more energy.

Energy is an important component in thechemical reaction because a certain threshold,called the activation energy (Ea), must be crossedbefore a reaction can occur. A temperatureincrease raises the energy of the collisions,increasing the likelihood that the activation-energy threshold will be crossed, resulting in thebreaking of molecular bonds.

Catalysts and Catalysis

It is not always feasible or desirable, however, toincrease the concentration of reactants, or thetemperature of the system in which the reactionis to occur. Many of the processes that take placein the human body, for instance,“should” requirehigh temperatures—temperatures too high tosustain human life. But fortunately, our bodiescontain proteins called enzymes, discussed laterin this essay, that facilitate the necessary reactionswithout raising temperatures or increasing theconcentrations of substances.

An enzyme is an example of a catalyst, a sub-stance that speeds up a reaction without partici-pating in it either as a reactant or product. Cata-lysts are thus not consumed in the reaction. The


Catalysts

catalyst does its work—catalysis—by creating adifferent path for the reaction, and though themeans whereby it does this are too complex todiscuss in detail here, the process of catalyst canat least be presented in general terms.

Imagine a graph whose x-axis is labeled“reaction progress,” while the y-axis bears thelegend “energy.” There is some value of y equal tothe normal activation energy, and in the courseof experiencing the molecular collisions that leadto a reaction, the reactants reach this level. In acatalyzed reaction, however, the level of activa-tion energy necessary for the reaction is repre-sented by a lower y-value on the graph. The cat-alyzed substances do not need to have as muchenergy as they do without a catalyst, and there-


fore the reaction can proceed more quickly—without changing the temperature or concentra-tions of reactants.


A Brief History of Catalysis

Long before chemists recognized the existence ofcatalysts, ordinary people had been using theprocess of catalysis for a number of purposes:making soap, for instance, or fermenting wine tocreate vinegar, or leavening bread. Early in thenineteenth century, chemists began to take noteof this phenomenon.

CATALYTIC CONVERTERS EMPLOY A CATALYST TO FACILITATE THE TRANSFORMATION OF POLLUTION-CAUSING EXHAUSTS

TO LESS HARMFUL SUBSTANCES. (Ian Harwood; Ecoscene/Corbis. Reproduced by permission.)


Catalysts

In 1812, Russian chemist Gottlieb Kirchhofwas studying the conversion of starches to sugarin the presence of strong acids when he noticedsomething interesting. When a suspension ofstarch in water was boiled, Kirchhof observed, nochange occurred in the starch. However, when headded a few drops of concentrated acid beforeboiling the suspension (that is, particles of starchsuspended in water), he obtained a very differentresult. This time, the starch broke down to formglucose, a simple sugar, while the acid—whichclearly had facilitated the reaction—underwentno change.

Around the same time, English chemist SirHumphry Davy (1778-1829) noticed that in cer-tain organic reactions, platinum acted to speedalong the reaction without undergoing anychange. Later on, Davy’s star pupil, the greatBritish physicist and chemist Michael Faraday(1791-1867), demonstrated the ability of plat-inum to recombine hydrogen and oxygen thathad been separated by the electrolysis of water.The catalytic properties of platinum later foundapplication in catalytic converters, as we shall see.

AN IMPROVED DEFINITION. In1835, Swedish chemist Jons Berzelius (1779-1848) provided a name to the process Kirchhofand Davy had observed from very different per-spectives: catalysis, derived from the Greek words

kata (“down”) and lyein (“loosen.”) As Berzeliusdefined it, catalysis involved an activity quite dif-ferent from that of an ordinary chemical reac-tion. Catalysis induced decomposition in sub-stances, resulting in the formation of new com-pounds—but without the catalyst itself actuallyentering the compound.

Berzelius’s definition assumed that a catalystmanages to do what it does without changing atall. This was perfectly adequate for describingheterogeneous catalysis, in which the catalyst andthe reactants are in different phases of matter. Inthe platinum-catalyzed reactions that Davy andFaraday observed, for instance, the platinum is asolid, while the reaction itself takes place in agaseous or liquid state. However, homogeneouscatalysis, in which catalyst and reactants are inthe same state, required a different explanation,which English chemist Alexander WilliamWilliamson (1824-1904) provided in an 1852study.

In discussing the reaction observed byKirchhof, of liquid sulfuric acid with starch in anaqueous solution, Williamson was able to showthat the catalyst does break down in the course ofthe reaction. As the reaction takes place, it formsan intermediate compound, but this too is bro-ken down before the reaction ends. The catalystthus emerges in the same form it had at thebeginning of the reaction.

Enzymes: Helpful Catalystsin the Body

In 1833, French physiologist Anselme Payen(1795-1871) isolated a material from malt thataccelerated the conversion of starch to sugar, asfor instance in the brewing of beer. Payen gavethe name “diastase” to this substance, and in1857, the renowned French chemist Louis Pas-teur (1822-1895) suggested that lactic acid fer-mentation is caused by a living organism.

In fact, the catalysts studied by Pasteur arenot themselves separate organisms, as Germanbiochemist Eduard Buchner (1860-1917) showedin 1897. Buchner isolated the catalysts that bringabout the fermentation of alcohol from livingyeast cells—what Payen had called “diastase,” andPasteur “ferments.” Buchner demonstrated thatthese are actually chemical substances, notorganisms. By that time, German physiologistWilly Kahne had suggested the name “enzyme”for these catalysts in living systems.


FRITZ HABER. (Austrian Archives/Corbis. Reproduced by per-

mission.)


A DANGEROUS CATALYST IN

THE ATMOSPHERE. Around the sametime that automakers began rolling out modelsequipped with catalytic converters, scientists andthe general public alike became increasingly con-cerned about another threat to the environment.In the upper atmosphere of Earth are traces ofozone, a triatomic (three-atom) molecular formof oxygen which protects the planet from theSun’s ultraviolet rays. During the latter part ofthe twentieth century, it became apparent that ahole had developed in the ozone layer overAntarctica, and many chemists suspected a cul-prit in chlorofluorocarbons, or CFCs.

CFCs had long been used in refrigerants andair conditioners, and as propellants in aerosolsprays. Because they were nontoxic and noncor-rosive, they worked quite well for such purposes,but the fact that they are chemically unreactivehad an extremely negative side-effect. Instead ofreacting with other substances to form new com-pounds, they linger in Earth’s atmosphere, even-tually drifting to high altitudes, where ultravioletlight decomposes them. The real trouble beginswhen atoms of chlorine, isolated from the CFC,encounter ozone.

Chlorine acts as a catalyst to transform theozone to elemental oxygen, which is not nearly aseffective as ozone for shielding Earth from ultra-violet light. It does so by interacting also withmonatomic, or single-atom oxygen, with which itproduces ClO, or the hypochlorite ion. The endresult of reactions between chlorine, monatomicoxygen, hypochlorite, and ozone is the produc-tion of chlorine, hypochlorite, and diatomic oxy-gen—in other words, no more ozone. It is esti-mated that a single chlorine atom can destroy upto 1 million ozone molecules per second.

Due to concerns about the danger to theozone layer, an international agreement calledthe Montreal Protocol, signed in 1996, bannedthe production of CFCs and the coolant Freonthat contains them. But people still need coolantsfor their homes and cars, and this has led to the creation of substitutes—most notablyhydrochlorofluorocarbons (HCFCs), organiccompounds that do not catalyze ozone.

Other Examples of Catalysts

Catalysts appear in a number of reactions, bothnatural and artificial. For instance, catalysts areused in the industrial production of ammonia,


CatalystsEnzymes are made up of amino acids, whichin turn are constructed from organic compoundscalled proteins. About 20 amino acids make upthe building blocks of the many thousands ofknown enzymes. The beauty of an enzyme is thatit speeds up complex, life-sustaining reactions inthe human body—reactions that would be tooslow at ordinary body temperatures. Rather thanforce the body to undergo harmful increases intemperature, the enzyme facilitates the reactionby opening up a different reaction pathway thatallows a lower activation energy.

One example of an enzyme is cytochrome,which aids the respiratory system by catalyzingthe combination of oxygen with hydrogen with-in the cells. Other enzymes facilitate the conver-sion of food to energy, and make possible a vari-ety of other necessary biological functions.

Because numerous interactions are requiredin their work of catalysis, enzymes are very large,and may have atomic mass figures as high as 1million amu. However, it should be noted thatreactions are catalyzed at very specific loca-tions—called active sites—on an enzyme. Thereactant molecule fits neatly into the active siteon the enzyme, much like a key fitting in a lock;hence the name of this theory, the “lock-and-model.”

Catalysis and the Environment

The exhaust from an automobile contains manysubstances that are harmful to the environment.As a result of increased concerns regarding thepotential damage to the atmosphere, the federalgovernment in the 1970s mandated the adoptionof catalytic converters, devices that employ a cat-alyst to transform pollutants in the exhaust toless harmful substances.

Platinum and palladium are favored materi-als for catalytic converters, though some non-metallic materials, such as ceramics, have beenused as well. In any case, the function of a cat-alytic converter is to convert exhausts throughoxidation-reduction reactions. Nitric oxide isreduced to molecular oxygen and nitrogen; at thesame time, the hydrocarbons in petroleum, alongwith carbon monoxide, are oxidized to form car-bon dioxide and water. Sometimes a reducingagent, such as ammonia, is used to make thereduction process more effective.



ACTIVATION ENERGY: The minimal

energy required to convert reactants into

products, symbolized Ea

AQUEOUS SOLUTIONS: A mixture

of water and a substance that is dis-

solved in it.

CATALYST: A substance that speeds up

a chemical reaction without participat-

ing in it, either as a reactant or product.

Catalysts are thus not consumed in the

reaction.





COLLISION MODEL: The theory that

chemical reactions are the result of colli-

sions between molecules strong enough to

break bonds in the reactants, resulting in a

reformation of atoms.

HETEROGENEOUS CATALYSIS: A

reaction in which the catalyst and the reac-

tants are in different phases of matter.

HOMOGENEOUS CATALYSIS: A

reaction in which catalyst and reactants are

in the same phase of matter.

PRODUCT: The substance or sub-

stances that result from a chemical

reaction.



tion, resulting in a product.

SYSTEM: In chemistry and other sci-

ences, the term “system” usually refers to

any set of interactions isolated from the

rest of the universe. Anything outside of

the system, including all factors and forces

irrelevant to a discussion of that system, is

known as the environment.

KEY TERMSCatalysts nitric acid (produced from ammonia), sulfuric

acid, and other substances. The ammoniaprocess, developed in 1908 by German chemistFritz Haber (1868-1934), is particularly notewor-thy. Using iron as a catalyst, Haber was able tocombine nitrogen and hydrogen under pressureto form ammonia—one of the world’s mostwidely used chemicals.

Eighteen ninety-seven was a good year forcatalysts. In that year, it was accidentally discov-ered that mercury catalyzes the reaction by whichindigo dye is produced; also in 1897, Frenchchemist Paul Sabatier (1854-1941) found thatnickel catalyzes the production of edible fats.Thanks to Sabatier’s discovery, nickel is used totransform inedible plant oils to margarine andshortening.

Another good year for catalysts—particular-ly those involved in the production of poly-mers—was 1953. That was the year when German chemist Karl Ziegler (1898-1973) dis-covered a resin catalyst for the production ofpolyethylene, which produced a newer, tougherproduct with a much higher melting point thanpolyethylene as it was produced up to that time.Also in 1953, Italian chemist Giulio Natta (1903-1979) adapted Ziegler’s idea, and developed anew type of plastic he called “isotactic” polymers.These could be produced easily, and in abun-dance, through the use of catalysts.

One of the lessons of chemistry, or indeed ofany science, is that there are few things chemistscan do that nature cannot achieve on a far morewondrous scale. No artificial catalyst can com-pete with enzymes, and no use of a catalyst in alaboratory can compare with the grandeur ofthat which takes place on the Sun. As German-American physicist Hans Bethe (1906-) showedin 1938, the reactions of hydrogen that form heli-um on the surface of the Sun are catalyzed bycarbon—the same element, incidentally, foundin all living things on Earth.

WHERE TO LEARN MORE

“Bugs in the News: What the Heck Is an Enzyme?” Univer-sity of Kansas (Web site). <http://falcon.cc.ukans.edu/~jbrown/enzyme.html> (June 9, 2001).

“Catalysis.” University of Idaho Department of Chemistry(Web site). <http://www.chem.uidaho.edu/~honors/rate4.html> (June 9, 2001).


Catalysts“Catalysts” (Web site). <http://edie.cprost.sfu.ca/~rhlogan/catalyst.html> (June 9, 2001).


“Enzymes.” Strategis (Web site). <http://strategis.ic.gc.ca/SSG/tc00048e.html> (June 9, 2001).

“Enzymes: Classification, Structure, Mechanism.” TheHebrew University (Web site). <http://www.md.huji.ac.il/MedChem/Mechanism-Chymotrypsin/> (June9, 2001).


“Ozone Depletion” (Web site). <http://www.energy.rochester.edu/iea/1992/p1/2-3.htm> (June 9, 2001).

“University Chemistry: Chemical Kinetics: Catalysis.” Uni-versity of Alberta Department of Chemistry (Web site).<http://www.chem.ualberta.ca/courses/plambeck/p102/p0217x.htm> (June 9, 2001).





A C IDS AND BASESAcids and Bases

CONCEPTThe name “acid” calls to mind vivid sensoryimages—of tartness, for instance, if the acid inquestion is meant for human consumption, aswith the citric acid in lemons. On the other hand,the thought of laboratory- and industrial-strength substances with scary-sounding names,such as sulfuric acid or hydrofluoric acid, carrieswith it other ideas—of acids that are capable ofdestroying materials, including human flesh. Thename “base,” by contrast, is not widely known inits chemical sense, and even when the older termof “alkali” is used, the sense-impressions pro-duced by the word tend not to be as vivid as thosegenerated by the thought of “acid.” In theirindustrial applications, bases too can be highlypowerful. As with acids, they have many house-hold uses, in substances such as baking soda oroven cleaners. From a taste standpoint, (as any-one who has ever brushed his or her teeth withbaking soda knows), bases are bitter rather thansour. How do we know when something is anacid or a base? Acid-base indicators, such as lit-mus paper and other materials for testing pH,offer a means of judging these qualities in vari-ous substances. However, there are larger struc-tural definitions of the two concepts, whichevolved in three stages during the late nineteenthand early twentieth centuries, that provide amore solid theoretical underpinning to theunderstanding of acids and bases.

HOW IT WORKS

Introduction to Acids and Bases

Prior to the development of atomic and molecu-lar theory in the nineteenth century, followed by

the discovery of subatomic structures in the latenineteenth and early twentieth centuries,chemists could not do much more than makemeasurements and observations. Their defini-tions of substances were purely phenomenologi-cal—that is, the result of experimentation andthe collection of data. From these observations,they could form general rules, but they lackedany means of “seeing” into the atomic andmolecular structures of the chemical world.

The phenomenological distinctions betweenacids and bases, gathered by scientists fromancient times onward, worked well enough formany centuries. The word “acid” comes from theLatin term acidus, or “sour,” and from an earlyperiod, scientists understood that substancessuch as vinegar and lemon juice shared a com-mon acidic quality. Eventually, the phenomeno-logical definition of acids became relativelysophisticated, encompassing such details as thefact that acids produce characteristic colors incertain vegetable dyes, such as those used in mak-ing litmus paper. In addition, chemists realizedthat acids dissolve some metals, releasing hydro-gen in the process.

WHY “BASE” AND NOT “ALKA-

LI”? The word “alkali” comes from the Arabical-qili, which refers to the ashes of the seawortplant. The latter, which typically grows in marshyareas, was often burned to produce soda ash,used in making soap. In contrast to acids, bases—caffeine, for example—have a bitter taste, andmany of them feel slippery to the touch. Theyalso produce characteristic colors in the vegetabledyes of litmus paper, and can be used to promotecertain chemical reactions. Note that todaychemists use the word “base” instead of “alkali,”


Acids andBases


the reason being that the latter term has a nar-rower meaning: all alkalies are bases, but not allbases are alkalies.

Originally, “alkali” referred only to the ashesof burned plants, such as seawort, that containedeither sodium or potassium, and from which theoxides of sodium and potassium could beobtained. Eventually, alkali came to mean the sol-uble hydroxides of the alkali and alkaline earthmetals. This includes sodium hydroxide, theactive ingredient in drain and oven cleaners;magnesium hydroxide, used for instance in milkof magnesia; potassium hydroxide, found insoaps and other substances; and other com-pounds. Broad as this range of substances is, itfails to encompass the wide array of materialsknown today as bases—compounds which reactwith acids to form salts and water.

Toward a Structural Definition

The reaction to form salts and water is, in fact,one of the ways that acids and bases can bedefined. In an aqueous solution, hydrochloricacid and sodium hydroxide react to form sodiumchloride—which, though it is suspended in anaqueous solution, is still common table salt—along with water. The equation for this reactionis HCl(aq) + NaOH(aq) 4H2O + NaCl(aq). Inother words, the sodium (Na) ion in sodiumhydroxide switches places with the hydrogen ionin hydrochloric acid, resulting in the creation ofNaCl (salt) along with water.

But why does this happen? Useful as this def-inition regarding the formation of salts andwater is, it is still not structural—in other words,it does not delve into the molecular structure andbehavior of acids and bases. Credit for the firsttruly structural definition of the difference goesto the Swedish chemist Svante Arrhenius (1859-1927). It was Arrhenius who, in his doctoral dis-sertation in 1884, introduced the concept of anion, an atom possessing an electric charge.

His understanding was particularly impres-sive in light of the fact that it was 13 more yearsbefore the discovery of the electron, the sub-atomic particle responsible for the creation ofions. Atoms have a neutral charge, but when anelectron or electrons depart, the atom becomes apositive ion or cation. Similarly, when an elec-tron or electrons join a previously uncharged

atom, the result is a negative ion or anion. Notonly did the concept of ions greatly influence thefuture of chemistry, but it also provided Arrhe-nius with the key necessary to formulate his dis-tinction between acids and bases.

The Arrhenius Definition

Arrhenius observed that molecules of certaincompounds break into charged particles whenplaced in liquid. This led him to the Arrheniusacid-base theory, which defines an acid as anycompound that produces hydrogen ions (H+)when dissolved in water, and a base as any com-pound that produces hydroxide ions (OH-) whendissolved in water.

This was a good start, but two aspects ofArrhenius’s theory suggested the need for a defi-nition that encompassed more substances. Firstof all, his theory was limited to reactions in aque-ous solutions. Though many acid-base reactionsdo occur when water is the solvent, this is notalways the case.

Second, the Arrhenius definition effectivelylimited acids and bases only to those ionic com-pounds, such as hydrochloric acid or sodiumhydroxide, which produced either hydrogen or

SVANTE ARRHENIUS. (Hulton-Deutsch Collection/Corbis. Repro-

duced by permission.)


Acids andBases

hydroxide ions. However, ammonia, or NH3, actslike a base in aqueous solutions, even though itdoes not produce the hydroxide ion. The same istrue of other substances, which behave like acidsor bases without conforming to the Arrheniusdefinition.

These shortcomings pointed to the need fora more comprehensive theory, which arrivedwith the formulation of the Brønsted-Lowry definition by English chemist Thomas Lowry(1874-1936) and Danish chemist J. N. Brønsted(1879-1947). Nonetheless, Arrhenius’s theoryrepresented an important first step, and in 1903,he was awarded the Nobel Prize in Chemistryfor his work on the dissociation of moleculesinto ions.

The Brønsted-Lowry Definition

The Brønsted-Lowry acid-base theory defines anacid as a proton (H+) donor, and a base as a pro-ton acceptor, in a chemical reaction. Protons arerepresented by the symbol H+, and in represent-ing acids and bases, the symbols HA and A-,respectively, are used. These symbols indicatethat an acid has a proton it is ready to give away,while a base, with its negative charge, is ready toreceive the positively charged proton.

Though it is used here to represent a proton,it should be pointed out that H+ is also thehydrogen ion—a hydrogen atom that has lost itssole electron and thus acquired a positive charge.


THE TARTNESS OF LEMONS AND LIMES IS A FUNCTION OF THEIR ACIDITY—SPECIFICALLY THE WAY IN WHICH CITRIC

ACID MOLECULES FIT INTO THE TONGUE’S “SWEET” RECEPTORS. (Orion Press/Corbis. Reproduced by permission.)


Acids andBases

It is thus really nothing more than a lone proton,but this is the one and only case in which anatom and a proton are exactly the same thing. Inan acid-base reaction, a molecule of acid is“donating” a proton, in the form of a hydrogenion. This should not be confused with a far morecomplex process, nuclear fusion, in which anatom gives up a proton to another atom.

AN ACID-BASE REACTION IN

BRØNSTED-LOWRY THEORY. Themost fundamental type of acid-base reaction inBrønsted-Lowry theory can be symbolized thusHA(aq) + H2O(l) 4H3O+(aq) + A-(aq). The firstacid shown—which, like three of the four “play-ers” in this equation, is dissolved in an aqueoussolution—combines with water, which can serveas either an acid or a base. In the present context,it functions as a base.

Water molecules are polar, meaning that thenegative charges tend to congregate on one endof the molecule with the oxygen atom, while thepositive charges remain on the other end with thehydrogen atoms. The Brønsted-Lowry modelemphasizes the role played by water, which pullsthe proton from the acid, resulting in the cre-ation of H3O+, known as the hydronium ion.

The hydronium ion produced here is anexample of a conjugate acid, an acid formedwhen a base accepts a proton. At the same time,the acid has lost its proton, becoming A–, a con-jugate base—that is, the base formed when anacid releases a proton. These two products of thereaction are called a conjugate acid-base pair, aterm that refers to two substances related to oneanother by the donating of a proton.

Brønsted and Lowry’s definition representsan improvement over that of Arrhenius, becauseit includes all Arrhenius acids and bases, as wellas other chemical species not encompassed inArrhenius theory. An example, mentioned earlier, is ammonia. Though it does not produceOH– ions, ammonia does accept a proton from awater molecule, and the reaction between thesetwo (with water this time serving the function ofacid) produces the conjugate acid-base pair ofNH4

+ (an ammonium ion) and OH-. Note thatthe latter, the hydroxide ion, was not produced byammonia, but is the conjugate base that resultedwhen the water molecule lost its H+ atom or proton.

The Lewis Definition

Despite the progress offered to chemists by theBrønsted-Lowry model, it was still limited todescribing compounds that contain hydrogen. AsAmerican chemist Gilbert N. Lewis (1875-1946)recognized, this did not encompass the full rangeof acids and bases; what was needed, instead, wasa definition that did not involve the presence of ahydrogen atom.

Lewis is particularly noted for his work inthe realm of chemical bonding. The bonding ofatoms is the result of activity on the part of thevalence electrons, or the electrons at the “out-side” of the atom. Electrons are arranged in dif-ferent ways, depending on the type of bonding,but they always bond in pairs.

According to the Lewis acid-base theory, anacid is the reactant that accepts an electron pairfrom another reactant in a chemical reaction,while a base is the reactant that donates an electron pair to another reactant. As with theBrønsted-Lowry definition, the Lewis definitionis reaction-dependant, and does not define acompound as an acid or base in its own right.Instead, the manner in which the compoundreacts with another serves to identify it as an acidor base.

AN IMPROVEMENT OVER ITS

PREDECESSORS. The beauty of theLewis definition lies in the fact that it encom-passes all the situations covered by the others—and more. Just as Brønsted-Lowry did not dis-prove Arrhenius, but rather offered a definitionthat covered more substances, Lewis expandedthe range of substances beyond those covered byBrønsted-Lowry. In particular, Lewis theory canbe used to differentiate the acid and base inbond-producing chemical reactions where ionsare not produced, and in which there is no pro-ton donor or acceptor. Thus it represents animprovement over Arrhenius and Brønsted-Lowry respectively.

An example is the reaction of boron trifluo-ride (BF3) with ammonia (NH3), both in the gasphases, to produce boron trifluoride ammoniacomplex (F3BNH3). In this reaction, boron triflu-oride accepts an electron pair and is therefore aLewis acid, while ammonia donates the electronpair and is thus a Lewis base. Though hydrogen isinvolved in this particular reaction, Lewis theoryalso addresses reactions involving no hydrogen.



Acids andBases


pH and Acid-Base Indicators

Though chemists apply the sophisticated struc-tural definitions for acids and bases that we havediscussed, there are also more “hands-on” meth-ods for identifying a particular substance(including complex mixtures) as an acid or base.Many of these make use of the pH scale, devel-oped by Danish chemist Søren Sørensen (1868-1939) in 1909.

The term pH stands for “potential of hydro-gen,” and the pH scale is a means of determiningthe acidity or alkalinity of a substance. (Though,as noted, the term “alkali” has been replaced by“base,” alkalinity is still used as an adjectival termto indicate the degree to which a substance dis-plays the properties of a base.) There are theoret-ically no limits to the range of the pH scale, butfigures for acidity and alkalinity are usually givenwith numerical values between 0 and 14.

THE MEANING OF pH VALUES.

A rating of 0 on the pH scale indicates a sub-stance that is virtually pure acid, while a 14 rat-ing represents a nearly pure base. A rating of 7indicates a neutral substance. The pH scale islogarithmic, or exponential, meaning that thenumbers represent exponents, and thus anincreased value of 1 represents not a simplearithmetic addition of 1, but an increase of 1power. This, however, needs a little further expla-nation.

The pH scale is actually based on negativelogarithms for the values of H3O+ (the hy-dronium ion) or H+ (protons) in a given sub-stance. The formula is thus pH = -log[H3O+] or -log[H+], and the presence of hydronium ions orprotons is measured according to their concen-tration of moles per liter of solution.

pH VALUES OF VARIOUS SUB-

STANCES. The pH of a virtually pure acid,such as the sulfuric acid in car batteries, is 0, andthis represents 1 mole (mol) of hydronium perliter (l) of solution. Lemon juice has a pH of 2,equal to 10-2 mol/l. Note that the pH value of 2translates to an exponent of -2, which, in thiscase, results in a figure of 0.01 mol/l.

Distilled water, a neutral substance with apH of 7, has a hydronium equivalent of 10-7

mol/l. It is interesting to observe that most of the

fluids in the human body have pH values in theneutral range blood (venous, 7.35; arterial, 7.45);urine (6.0—note the higher presence of acid);and saliva (6.0 to 7.4).

At the alkaline end of the scale is borax, witha pH of 9, while household ammonia has a pHvalue of 11, or 10-11 mol/l. Sodium hydroxide, orlye, an extremely alkaline chemical with a pH of14, has a value equal to 10-14 moles of hydroniumper liter of solution.

LITMUS PAPER AND OTHER

INDICATORS. The most precise pH meas-urements are made with electronic pH meters,which can provide figures accurate to 0.001 pH.However, simpler materials are also used. Bestknown among these is litmus paper (made froman extract of two lichen species), which turnsblue in the presence of bases and red in the pres-ence of acids. The term “litmus test” has becomepart of everyday language, referring to a make-or-break issue—for example, “views on abortionrights became a litmus test for Supreme Courtnominees.”

Litmus is just one of many materials used formaking pH paper, but in each case, the change ofcolor is the result of the neutralization of thesubstance on the paper. For instance, paper coat-ed with phenolphthalein changes from colorlessto pink in a pH range from 8.2 to 10, so it is use-ful for testing materials believed to be moderate-ly alkaline. Extracts from various fruits and veg-etables, including red cabbages, red onions, andothers, are also applied as indicators.

Some Common Acids and Bases

The tables below list a few well-known acids and bases, along with their formulas and a fewapplications

Common Acids

• Acetic acid (CH3COOH): vinegar, acetate• Acetylsalicylic acid (HOOCC6H4OOCCH3):

aspirin• Ascorbic acid (H2C6H6O6): vitamin C• Carbonic acid (H2CO3): soft drinks, seltzer

water• Citric acid (C6H8O7): citrus fruits, artificial

flavorings• Hydrochloric acid (HCl): stomach acid• Nitric acid (HNO3): fertilizer, explosives• Sulfuric acid (H2SO4): car batteries



Common Bases

• Aluminum hydroxide (Al[OH]3): antacids,deodorants

• Ammonium hydroxide (NH4OH): glasscleaner

• Calcium hydroxide (Ca[OH]2): causticlime, mortar, plaster

• Magnesium hydroxide (Mg[OH]2): laxa-tives, antacids

• Sodium bicarbonate/sodium hydrogen car-bonate (NaHCO3): baking soda

• Sodium carbonate (Na2CO3): dish detergent• Sodium hydroxide (NaOH): lye, oven and

drain cleaner• Sodium hypochlorite (NaClO): bleach

Of course these represent only a few of themany acids and bases that exist. Selected sub-stances listed above are discussed briefly below.

Acids

ACIDS IN THE HUMAN BODY

AND FOODS. As its name suggests, citricacid is found in citrus fruits—particularlylemons, limes, and grapefruits. It is also used as aflavoring agent, preservative, and cleaning agent.Produced commercially from the fermentationof sugar by several species of mold, citric acidcreates a taste that is both tart and sweet. Thetartness, of course, is a function of its acidity, ora manifestation of the fact that it produceshydrogen ions. The sweetness is a more complexbiochemical issue relating to the ways that citricacid molecules fit into the tongue’s “sweet”receptors.

Citric acid plays a role in one famous stom-ach remedy, or antacid. This in itself is interest-ing, since antacids are more generally associatedwith alkaline substances, used for their ability toneutralize stomach acid. The fizz in Alka-Seltzer,however, comes from the reaction of citric acids(which also provide a more pleasant taste) withsodium bicarbonate or baking soda, a base. Thisreaction produces carbon dioxide gas. As a pre-servative, citric acid prevents metal ions fromreacting with, and thus hastening the degrada-tion of, fats in foods. It is also used in the pro-duction of hair rinses and low-pH shampoos andtoothpastes.

The carboxylic acid family of hydrocarbonderivatives includes a wide array of substances—not only citric acids, but amino acids. Amino

acids combine to make up proteins, one of theprincipal components in human muscles, skin,and hair. Carboxylic acids are also applied indus-trially, particularly in the use of fatty acids formaking soaps, detergents, and shampoos.

SULFURIC ACID. There are plenty ofacids found in the human body, includinghydrochloric acid or stomach acid—which, inlarge quantities, causes indigestion, and the needfor neutralization with a base. Nature also pro-duces acids that are toxic to humans, such as sul-furic acid.

Though direct exposure to sulfuric acid isextremely dangerous, the substance has numer-ous applications. Not only is it used in car batter-ies, but sulfuric acid is also a significant compo-nent in the production of fertilizers. On the otherhand, sulfuric acid is damaging to the environ-ment when it appears in the form of acid rain.Among the impurities in coal is sulfur, and thisresults in the production of sulfur dioxide andsulfur trioxide when the coal is burned. Sulfurtrioxide reacts with water in the air, creating sul-furic acid and thus acid rain, which can endangerplant and animal life, as well as corrode metalsand building materials.

Bases

The alkali metal and alkaline earth metal familiesof elements are, as their name suggests, bases. Anumber of substances created by the reaction ofthese metals with nonmetallic elements are takeninternally for the purpose of settling gastric trou-ble or clearing intestinal blockage. For instance,there is magnesium sulfate, better known asEpsom salts, which provide a powerful laxativealso used for ridding the body of poisons.

Aluminum hydroxide is an interesting base,because it has a wide number of applications,including its use in antacids. As such, it reactswith and neutralizes stomach acid, and for thatreason is found in commercial antacids such asDi-Gel™, Gelusil™, and Maalox™. Aluminumhydroxide is also used in water purification, indyeing garments, and in the production of cer-tain kinds of glass. A close relative, aluminumhydroxychloride or Al2(OH)5Cl, appears in manycommercial antiperspirants, and helps to closepores, thus stopping the flow of perspiration.

SODIUM HYDROGEN CAR-

BONATE (BAKING SODA). Bakingsoda, known by chemists both as sodium bicar-


Acids andBases


Acids andBases


bonate and sodium hydrogen carbonate, isanother example of a base with multiple purpos-es. As noted earlier, it is used in Alka-Seltzer™,with the addition of citric acid to improve theflavor; in fact, baking soda alone can perform the

function of an antacid, but the taste is rather un-pleasant.

Baking soda is also used in fighting fires,because at high temperatures it turns into carbondioxide, which smothers flames by obstructing

ACID: A substance that, in its edible

form, is sour to the taste, and in non-edible

forms, is often capable of dissolving met-

als. Acids and bases react to form salts and

water. These are all phenomenological def-

initions, however, in contrast to the three

structural definitions of acids and bases—

the Arrhenius, BrØnsted-Lowry, and Lewis

acid-base theories.

ALKALI: A term referring to the soluble

hydroxides of the alkali and alkaline earth

metals. Once “alkali” was used for the class

of substances that react with acids to form

salts; today, however, the more general

term base is preferred.

ALKALINITY: An adjectival term used

to identify the degree to which a substance

displays the properties of a base.

ANION: The negatively charged ion

that results when an atom gains one or

more electrons. “Anion” is pronounced

“AN-ie-un”.

AQUEOUS SOLUTION: A substance

in which water constitutes the solvent. A

large number of chemical reactions take

place in an aqueous solution.

ARRHENIUS ACID-BASE THEORY:

The first of three structural definitions of

acids and bases. Formulated by Swedish

chemist Svante Arrhenius (1859-1927), the

Arrhenius theory defines acids and bases

according to the ions they produce in an

aqueous solution: an acid produces hydro-

gen ions (H+), and a base hydroxide ions

(OH–).

BASE: A substance that, in its edible

form, is bitter to the taste. Bases tend to be

slippery to the touch, and in reaction with

acids they produce salts and water. Bases

and acids are most properly defined, how-

ever, not in these phenomenological terms,

but by the three structural definitions of

acids and bases—the Arrhenius, BrØnsted-

Lowry, and Lewis acid-base theories.

BRØNSTED-LOWRY ACID-BASE THE-

ORY: The second of three structural

definitions of acids and bases. Formulated

by English chemist Thomas Lowry (1874-

1936) and Danish chemist J. N. BrØnsted

(1879-1947), BrØnsted-Lowry theory

defines an acid as a proton (H+) donor, and

a base as a proton acceptor.

CATION: The positively charged ion

that results when an atom loses one or

more electrons. “Cation” is pronounced

“KAT-ie-un”.





so forth.

CONJUGATE ACID: An acid formed

when a base accepts a proton (H+).

KEY TERMS


Acids andBases

the flow of oxygen to the fire. Of course, bakingsoda is also used in baking, when it is combinedwith a weak acid to make baking powder. Thereaction of the acid and the baking soda pro-duces carbon dioxide, which causes dough and

batters to rise. In a refrigerator or cabinet, bakingsoda can absorb unpleasant odors, and addition-ally, it can be applied as a cleaning product.

SODIUM HYDROXIDE (LYE) .

Another base used for cleaning is sodium


CONJUGATE ACID-BASE PAIR:

The acid and base produced when an acid

donates a single proton to a base. In the

reaction that produces this pair, the acid

and base switch identities. By donating a

proton, the acid becomes a conjugate base,

and by receiving the proton, the base

becomes a conjugate acid.

CONJUGATE BASE: A base formed

when an acid releases a proton.

ION: An atom or atoms that has lost or

gained one or more electrons, and thus has

a net electric charge. There are two types of

ions: anions and cations.

IONIC BONDING: A form of chemical

bonding that results from attractions

between ions with opposite electric

charges.

IONIC COMPOUND: A compound in

which ions are present. Ionic compounds

contain at least one metal and nonmetal

joined by an ionic bond.

LEWIS ACID-BASE THEORY: The

third of three structural definitions of

acids and bases. Formulated by American

chemist Gilbert N. Lewis (1875-1946),

Lewis theory defines an acid as the reactant

that accepts an electron pair from another

reactant in a chemical reaction, and a base

as the reactant that donates an electron

pair to another reactant.

PH SCALE: A logarithmic scale for

determining the acidity or alkalinity of a

substance, from 0 (virtually pure acid) to 7

(neutral) to 14 (virtually pure base).

PHENOMENOLOGICAL: A term

describing scientific definitions based

purely on experimental phenomena. These

convey only part of the picture, however—

primarily, the part a chemist can perceive

either through measurement or through

the senses, such as sight. A structural defi-

nition is therefore usually preferable to a

phenomenological one.



tion, resulting in the creation of a product.

SALTS: Ionic compounds formed by

the reaction between an acid and a base. In

this reaction, one or more of the hydrogen

ions of an acid is replaced with another

positive ion. In addition to producing salts,

acid-base reactions produce water.

SOLUTION: A homogeneous mixture

in which one or more substances (the

solute) is dissolved in one or more other

substances (the solvent)—for example,

sugar dissolved in water.

SOLVENT: A substance that dissolves

another, called a solute, in a solution.

STRUCTURAL: A term describing sci-

entific definitions based on aspects of

molecular structure and behavior rather

than purely phenomenological data.



Acids andBases

hydroxide, known commonly as lye or causticsoda. Unlike baking soda, however, it is not to betaken internally, because it is highly damaging tohuman tissue—particularly the eyes. Lye appearsin drain cleaners, such as Drano™, and ovencleaners, such as Easy-Off™, which make use ofits ability to convert fats to water-soluble soap.

In the process of doing so, however, relative-ly large amounts of lye may generate enough heatto boil the water in a drain, causing the water toshoot upward. For this reason, it is not advisableto stand near a drain being treated with lye. In aclosed oven, this is not a danger, of course; andafter the cleaning process is complete, the con-verted fats (now in the form of soap) can be dis-solved and wiped off with a sponge.

WHERE TO LEARN MORE“Acids and Bases Frequently Asked Questions.” General

Chemistry Online (Web site). <http://antoine.fsu.umd.edu/chem/senese/101/acidbase/faq.shtml>(June 7, 2001).

“Acids, Bases, and Salts.” Chemistry Coach (Web site).<http://www.chemistrycoach.com/acids.htm> (June7, 2001).

“Acids, Bases, and Salts.” University of Akron, Departmentof Chemistry (Web site). <http://ull.chemistry.uakron.edu/genobc/Chapter_09/title.html> (June 7,2001).

ChemLab. Danbury, CT: Grolier Educational, 1998.


Haines, Gail Kay. What Makes a Lemon Sour? Illustratedby Janet McCaffery. New York: Morrow, 1977.

Oxlade, Chris. Acids and Bases. Chicago: HeinemannLibrary, 2001.

Patten, J. M. Acids and Bases. Vero Beach, FL: RourkeBook Company, 1995.

Walters, Derek. Chemistry. Illustrated by Denis Bishopand Jim Robins. New York: F. Watts, 1982.

Zumdahl, Steven S. Introductory Chemistry A Founda-tion, 4th ed. Boston: Houghton Mifflin, 2000.


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A C ID -BASE REACT IONSAcid-Base Reactions

CONCEPTTo an extent, acids and bases can be defined interms of factors that are apparent to the senses:edible acids taste sour, for instance, while basesare bitter-tasting and slippery to the touch. Thebest way to understand these two types of sub-stances, however, is in terms of their behavior inchemical reactions. Not only do the reactions ofacids and bases result in the creation of salts andwater, but acids and bases can be defined by theways in which they participate in a reaction—forinstance, by donating or accepting electron pairs.The reaction of acids and bases to form waterand salts is called neutralization, and it has a widerange of applications, including the promotionof plant growth in soil and the treatment ofheartburn in the human stomach. Neutralizationalso makes it possible to test substances for theirpH level, a measure of the degree to which thesubstance is acidic or alkaline.

HOW IT WORKS

Phenomenological Defini-tions of Acids and Bases

Before studying the reactions of acids and bases,it is necessary to define exactly what each is. Thisis not as easy as it sounds, and the Acids andBases essay discusses in detail a subject coveredmore briefly here: the arduous task chemistsfaced in developing a workable distinction. Let usstart with the phenomenological differencesbetween the two—that is, aspects relating tothings that can readily be observed without refer-ring to the molecule properties and behaviors ofacids and bases.

Acids are fairly easy to understand on thephenomenological level: the name comes fromthe Latin term acidus, or “sour,” and many soursubstances from daily life—lemons, for instance,or vinegar—are indeed highly acidic. In fact,lemons and most citrus fruits contain citric acid(C6H8O7), while the acidic quality of vinegarcomes from acetic acid (CH3COOH). In addi-tion, acids produce characteristic colors in cer-tain vegetable dyes, such as those used in makinglitmus paper.

The word “base,” as it is used in this context,may be a bit more difficult to appreciate on a sen-sory level. It helps, perhaps, if the older term“alkali” is used, though even so, people tend tothink of alkaline substances primarily in contrastto acids. “Alkali,” which serves to indicate thebasic quality of both the alkali metal and alkalineearth metal families of elements, comes from theArabic al-qili. The latter refers to the ashes of theseawort plant, which usually grows in marshyareas and, in the past, was often burned to pro-duce soda ash for making soap.

The reason chemists of today use the word“base” instead of “alkali” is that the latter termhas a narrower meaning: all alkalies are bases, butnot all bases are alkalies. Originally referring onlyto the ashes of burned plants containing eithersodium or potassium, alkali was eventually usedto designate the soluble hydroxides of the alkaliand alkaline earth metals. Among these are sodi-um hydroxide or lye; magnesium hydroxide(found in milk of magnesia); potassium hydrox-ide, which appears in soaps; and other com-pounds. Because these represent only a few ofthe substances that react with acids in the waysdiscussed in this essay, the term “base” is preferred.


Acid-BaseReactions

THE FORMATION OF SALTS. Aschemistry evolved, and physical scientistsbecame aware of the atomic and molecular sub-structures that make up the material world, theydeveloped more fundamental distinctionsbetween acids and bases. By the early twentiethcentury, chemists had applied structural distinc-tions between acids and bases—that is, defini-tions based on the molecular structures andbehaviors of those substances.

An important intermediary step occurred aschemists came to the conclusion that reactions ofacids and bases form salts and water. Forinstance, in an aqueous solution, hydrochloricacid or HCl(aq) reacts with the base sodiumhydroxide, designated as NaOH(aq), to formsodium chloride, or common table salt(NaCl[aq]) and H2O. What happens is that thesodium (Na) ion (an atom with an electriccharge) in sodium hydroxide switches placeswith the hydrogen ion in hydrochloric acid,resulting in the creation of NaCl and water.

Ions themselves had yet to be defined in1803, when the great Swedish chemist JonsBerzelius (1779-1848) added another piece to thefoundation for a structural definition. Acids andbases, he suggested, have opposite electriccharges. In this, he was about eight decades ahead

of his time: only in 1884 did his countrymanSvante Arrhenius (1859-1927) introduce theconcept of the ion. This, in turn, enabled Arrhe-nius to formulate the first structural distinctionbetween acids and bases.

The Arrhenius Acid-Base Theory

Arrhenius acid-base theory defines the two sub-stances with regard to their behavior in an aque-ous solution: an acid is any compound that pro-duces hydrogen ions (H+), and a base is one thatproduces hydroxide ions (OH–) when dissolvedin water. This occurred, for instance, in the reac-tion discussed above: the hydrochloric acid pro-duced a hydrogen ion, while the sodium hydrox-ide produced a hydroxide ion, and these two ionsbonded to form water.

Though it was a good start, Arrhenius’s the-ory was limited to reactions in aqueous solu-tions. In addition, it confined its definition ofacids and bases only to those ionic compounds,such as hydrochloric acid or sodium hydroxide,that produced either hydrogen or hydroxide ions.But ammonia, or NH3, acts like a base in aqueoussolutions, even though it does not produce thehydroxide ion. These shortcomings pointed tothe need for a more comprehensive theory, whichcame with the formulation of the Brønsted-Lowry definition.

The Brønsted-Lowry Acid-Base Theory

Developed by English chemist Thomas Lowry(1874-1936) and Danish chemist J. N. Brønsted(1879-1947), the Brønsted-Lowry acid-base the-ory defines an acid as a proton (H+) donor, and abase as a proton acceptor, in a chemical reaction.Protons are represented by the symbol H+, acation (positively charged ion) of hydrogen.

Elemental hydrogen, called protium to dis-tinguish it from its isotopes, has just one protonand one electron—no neutrons. Therefore, thehydrogen cation, which has to lose its sole elec-tron to gain a positive charge, is essentially noth-ing but a proton. It is thus at once an atom, anion, and a proton, but the ionization of hydrogenconstitutes the only case in which this is possible.

Thus when the term “proton donor” or “pro-ton acceptor” is used, it does not mean that aproton is splitting off from an atom or joining


GILBERT N. LEWIS.


Acid-BaseReactions

another, as in a nuclear reaction. Rather, when anacid behaves as a proton donor, this means thatthe hydrogen proton/ion/atom is separatingfrom an acidic compound; conversely, when abase acts as a proton acceptor, the positivelycharged hydrogen ion is bonding with the basiccompound.

REACTIONS IN BRØNSTED-

LOWRY ACID-BASE THEORY. Inrepresenting Brønsted-Lowry acids and bases,the symbols HA and A–, respectively, are used.These appear in the equation representing themost fundamental type of Brønsted-Lowry acid-base reaction: HA(aq) + H2O(l) 4H3O+(aq) +A–(aq). The symbols (aq), (l), and 4are explainedin the Chemical Reactions essay. In plain English,this equation states that when an acid in an aque-ous solution reacts with liquid water, the result isthe creation of H3O+, known as the hydroniumion, along with a base. Both products of the reac-tion are dissolved in an aqueous solution.

Because water molecules are polar, the nega-tive charges tend to congregate on one end of themolecule with the oxygen atom, while the posi-tive charges remain on the other end with thehydrogen atoms. The Brønsted-Lowry modelemphasizes the role played by water, which pullsthe proton from the acid, resulting in the cre-ation of the hydronium ion.

The hydronium ion, in this equation, is anexample of a conjugate acid, an acid formedwhen a base accepts a proton. At the same time,the acid has lost its proton, becoming A–, a con-jugate base—that is, the base formed when anacid releases a proton. These two products of thereaction are called a conjugate acid-base pair, aterm that refers to two substances related to oneanother by the donating of a proton.

Brønsted and Lowry’s definition includes allArrhenius acids and bases, as well as other chem-ical species not encompassed in Arrhenius theo-ry. As mentioned earlier, ammonia is a base, yet itdoes not produce OH– ions; however, it doesaccept a proton from a water molecule. Water canserve either as an acid or base; in this instance, itis an acid, and in reaction with ammonia, it pro-duces the conjugate acid-base pair of NH4

+ (anammonium ion) and OH–. Ammonia did notproduce the hydroxide ion here; rather, OH– isthe conjugate base that resulted when the watermolecule lost its H+ atom (i.e., a proton.)

The Lewis Acid-Base Theory

The Brønsted-Lowry model still had its limita-tions, in that it only described compounds con-taining hydrogen. American chemist Gilbert N.Lewis (1875-1946), however, developed a theoryof acids and bases that makes no reference to thepresence of hydrogen. Instead, it relates to some-thing much more fundamental: the fact thatchemical bonding always involves pairs of elec-trons.

Lewis acid-base theory defines an acid as thereactant that accepts an electron pair fromanother reactant in a chemical reaction, while a base is the reactant that donates an electronpair to another reactant. Note that, as with theBrønsted-Lowry definition, the Lewis definitionis reaction-dependant. Instead of defining acompound as an acid or base in its own right, itidentifies these in terms of how the compoundreacts with another.

The Lewis definition encompasses all the sit-uations covered by the others, as well as manyother reactions not described in the theories ofeither Arrhenius or Brønsted-Lowry. In particu-lar, Lewis theory can be used to differentiate theacid and base in chemical reactions where ionsare not produced, something that takes it farbeyond the scope of Arrhenius theory. Also,Lewis theory addresses situations in which thereis no proton donor or acceptor, thus offering animprovement over Brønsted-Lowry.

When boron trifluoride (BF3) and ammonia(NH3), both in the gas phases, react to produceboron trifluoride ammonia complex (F3BNH3),boron trifluoride accepts an electron pair. There-fore, it is a Lewis acid, while ammonia—whichdonates the electron pair—can be defined as aLewis base. This particular reaction involveshydrogen, but since the operative factor in Lewistheory relates to electron pairs and not hydrogen,the theory can be used to address reactions inwhich that element is not present.


Dissociation

Dissociation is the separation of a molecule intoions, and it is a key factor for evaluating the“strength” of acids and bases. The more a sub-stance is prone to dissociation, the better it can



Acid-BaseReactions

conduct an electric current, because the separa-tion of charges provides a “pathway” for the cur-rent’s flow. A substance that dissociates com-pletely, or almost completely, is called a strongelectrolyte, whereas one that dissociates onlyslightly (or not at all) is designated as a weakelectrolyte.

The terms “weak” and “strong” are alsoapplied to acids and bases. For instance, vinegaris a weak acid, because it dissociates only slightly,and therefore conducts little electric current. Bycontrast, hydrochloric acid (HCl) is a strong acid,because it dissociates almost completely intopositively charged hydrogen ions and negativelycharged chlorine ones. Represented symbolically,this is: HCl 4H+ + Cl–.

A REACTION INVOLVING A

STRONG ACID. It may seem a bit back-ward that a strong acid or base is one that “fallsapart,” while the weak one stays together. Tounderstand the difference better, let us return tothe reaction described earlier, in which an acid inaqueous solution reacts with water to produce abase in aqueous solution, along with hydronium:HA(aq) + H2O(l) 4H3O+(aq) + A–(aq). Instead ofusing the generic symbols HA and A–, however, letus substitute hydrochloric acid (HCl) and chlo-ride (Cl–) respectively.

The reaction HCl(aq) + H2O(l) 4H3O+(aq)+ Cl–(aq) is a reversible one, and for that reason,the symbol for chemical equilibrium (!) can beinserted in place of the arrow pointing to theright. In other words, the substances on the rightcan just as easily react, producing the substanceson the left. In this reverse reaction, the reactantsof the forward reaction would become products,and the products of the forward reaction serve asthe reactants.

However, the reaction described here is notperfectly reversible, and in fact the most properchemical symbolism would show a longer arrowpointing to the right, with a shorter arrow point-ing to the left. Due to the presence of a strongelectrolyte, there is more forward “thrust” to thisreaction.

Because it is a strong acid, the hydrogenchloride in solution is not a set of molecules, buta collection of H+ and Cl– ions. In the reaction,the weak Cl– ions to the right side of the equilib-rium symbol exert very little attraction for the H+

ions. Instead of bonding with the chloride, these

hydrogen ions join the water (a stronger base) toform hydronium.

The chloride, incidentally, is the conjugatebase of the hydrochloric acid, and this illustratesanother principal regarding the “strength” ofelectrolytes: a strong acid produces a relativelyweak conjugate base. Likewise, a strong base pro-duces a relatively weak conjugate acid.

THE STRONG ACIDS AND

BASES. There are only a few strong acids andbases, which are listed below:

Strong Acids

• Hydrobromic acid (HBr)• Hydrochloric acid (HCl)• Hydroiodic acid (HI)• Nitric acid (HNO3)• Perchloric acid (HClO4)• Sulfuric acid (H2SO4)

Strong Bases

• Barium hydroxide (Ba[OH]2)• Calcium hydroxide (Ca[OH]2)• Lithium hydroxide (LiOH)• Potassium hydroxide (KOH)• Sodium hydroxide (NaOH)• Strontium hydroxide (Sr[OH]2)

Virtually all others are weak acids or bases,meaning that only a small percentage of mole-cules in these substances ionize by dissociation.The concentrations of the chemical speciesinvolved in the dissociation of weak acids andbases are mathematically governed by the equi-librium constant K..

Neutralization

Neutralization is the process whereby an acid andbase react with one another to form a salt andwater. The simplest example of this occurs in thereaction discussed earlier, in which hydrochloricacid or HCl(aq) reacts with the base sodiumhydroxide, designated as NaOH(aq), in an aqueous solution. The result is sodium chlo-ride, or common table salt (NaCl[aq]) and H2O.This equation is written thus: HCl(aq) +NaOH(aq) 4NaCl(aq) + H2O.

The human stomach produces hydrochloricacid, commonly known as “stomach acid.” It isgenerated in the digestion process, but when aperson eats something requiring the stomach towork overtime in digesting it—say, a pizza—thestomach may generate excess hydrochloric acid,



and the result is “heartburn.” When this happens,people often take antacids, which contain a basesuch as aluminum hydroxide (Al[OH]3) or mag-nesium hydroxide (Mg[OH]2).

When a person takes an antacid, the reactionleads to the creation of a salt, but not the saltwith which most people are familiar—NaCl. Asshown above, that particular salt is the product ofa reaction between hydrochloric acid and sodiumhydroxide, but a person who ingested sodiumhydroxide (a substance used to unclog drains andclean ovens) would have much worse heartburnthan before! In any case, the antacid reacts withthe stomach acid to produce a salt, as well aswater, and thus the acid is neutralized.

When land formerly used for mining isreclaimed, the acidic water in the area must beneutralized, and the use of calcium oxide (CaO)as a base is one means of doing so. Acidic soil,too, can be neutralized by the introduction ofcalcium carbonate (CaCO3) or limestone, alongwith magnesium carbonate (MgCO3). If soil istoo basic, as for instance in areas where there hasbeen too little precipitation, acid-like substancessuch as calcium sulfate or gypsum (SaSO4) canbe used. In either case, neutralization promotesplant growth.

TITRATION AND pH. One of themost important applications of neutralization isin titration, the use of a chemical reaction todetermine the amount of a chemical substance ina sample of unknown purity. In a typical form ofneutralization titration, a measured amount ofan acid is added to a solution containing anunknown amount of a base. Once enough of theacid has been added to neutralize the base, it ispossible to determine how much base exists inthe solution.

Titration can also be used to measure pH(“power of hydrogen”) level by using an acid-base indicator. The pH scale assigns values rang-ing from 0 (a virtually pure acid) to 14 (a virtu-ally pure base), with 7 indicating a neutral substance. An acid-base indicator such as litmus paper changes color when it neutralizesthe solution.

The transition interval (the pH at which thecolor of an indicator changes) is different for dif-ferent types of indicators, and thus various indi-cators are used to measure substances in specificpH ranges. For instance, methyl red changes

from red to yellow across a pH range of 4.4 to 6.2,so it is most useful for testing a substance sus-pected of being moderately acidic.

BUFFERED SOLUTIONS. Abuffered solution is one that resists a change inpH even when a strong acid or base is added to it.This buffering results from the presence of aweak acid and a strong conjugate base, and it canbe very important to organisms whose cells canendure changes only within a limited range ofpH values. Human blood, for instance, containsbuffering systems, because it needs to be at pHlevels between 7.35 and 7.45.

The carbonic acid-bicarbonate buffer sys-tem is used to control the pH of blood. The mostimportant chemical equilibria (that is, reactionsinvolving chemical equilibrium) for this systemare: H+ + HCO3

– ! H2CO3 ! H20 + CO2. Inother words, the hydrogen ion (H+) reacts withthe hydrogen carbonate ion (HCO3

– ) to producecarbonic acid (H2CO3). The latter is in equi-librium with the first set of reactants, as well aswith water and carbon dioxide in the forwardreaction.

The controls the pH level by changing theconcentration of carbon dioxide by exhalation.In accordance with Le Châtelier’s principle, thisshifts the equilibrium to the right, consuming H+

ions. In normal blood plasma, the concentrationof HCO3

– is about 20 times as great as that ofH2CO3, and this large concentration of hydrogencarbonate gives the buffer a high capacity to neu-tralize additional acid. The buffer has a muchlower capacity to neutralize bases because of themuch smaller concentration of carbonic acid.

Water: Both Acid and Base

Water is an amphoteric substance; in otherwords, it can serve either as an acid or a base.When water experiences ionization, one watermolecule serves as a Brønsted-Lowry acid,donating a proton to another water molecule—the Brønsted-Lowry base. This results in the pro-duction of a hydroxide ion and a hydronium ion:H2O(l) + H2O(l) ! H3O+(aq) + OH–(aq).

This equilibrium equation is actually one inwhich the tendency toward the reverse reaction ismuch greater; therefore the equilibrium symbol,if rendered in its most proper form, would showa much shorter arrow pointing toward the right.In water purified by distillation, the concentra-


Acid-BaseReactions


Acid-BaseReactions


ACID: A substance that in its edible

form is sour to the taste, and in non-edible

forms is often capable of dissolving metals.

Acids and bases react to form salts and

water. These are all phenomenological def-

initions, however, in contrast to the three

structural definitions of acids and bases—

the Arrhenius, Brønsted-Lowry, and Lewis

acid-base theories.

ALKALI: A term referring to the soluble

hydroxides of the alkali and alkaline earth

metals. Once “alkali” was used for the class

of substances that react with acids to form

salts; today, however, the more general

term base is preferred.

ALKALINITY: An adjectival term used

to identify the degree to which a substance

displays the properties of a base.

AMPHOTERIC: A term describing a

substance that can serve either as an acid or

a base. Water is the most significant

amphoteric substance.

AQUEOUS SOLUTION: A substance

in which water constitutes the solvent. A

large number of chemical reactions take

place in an aqueous solution.

ARRHENIUS ACID-BASE THEORY:

The first of three structural definitions of

acids and bases. Formulated by Swedish

chemist Svante Arrhenius (1859-1927), the

Arrhenius theory defines acids and bases

according to the ions they produce in an

aqueous solution an acid produces hydro-

gen ions (H+), and a base hydroxide ions

(OH–).

BASE: A substance that in its edible

form is bitter to the taste. Bases tend to be

slippery to the touch, and in reaction with

acids they produce salts and water. Bases

and acids are most properly defined, how-

ever, not in these phenomenological terms,

but by the three structural definitions of

acids and bases—the Arrhenius, Brønsted-

Lowry, and Lewis acid-base theories.

BASIC: In the context of acids and

bases, the word is the counterpart to

“acidic,” identifying the base-like quality of

a substance.

BRØNSTED-LOWRY ACID-BASE THE-

ORY: The second of three structural

definitions of acids and bases. Formulated

by English chemist Thomas Lowry (1874-

1936) and Danish chemist J. N. Brønsted

(1879-1947), Brønsted-Lowry theory

defines an acid as a proton (H+) donor, and

a base as a proton acceptor.





so forth.

CONJUGATE ACID: An acid formed

when a base accepts a proton (H+).

CONJUGATE ACID-BASE PAIR:

The acid and base produced when an acid

donates a single proton to a base. In the

reaction that produces this pair, the acid

and base switch identities. By donating a

proton, the acid becomes a conjugate base,

and by receiving the proton, the base

becomes a conjugate acid.

CONJUGATE BASE: A base formed

when an acid releases a proton.

DISSOCIATION: The separation of

molecules into ions.

KEY TERMS


Acid-BaseReactions


ION: An atom or atoms that has lost or

gained one or more electrons, and thus has

a net electric charge. There are two types of

ions: anions and cations.

IONIC BONDING: A form of chemical

bonding that results from attractions

between ions with opposite electric

charges.

IONIC COMPOUND: A compound in

which ions are present. Ionic compounds

contain at least one metal and nonmetal

joined by an ionic bond.

LEWIS ACID-BASE THEORY: The

third of three structural definitions of

acids and bases. Formulated by American

chemist Gilbert N. Lewis (1875-1946),

Lewis theory defines an acid as the reactant

that accepts an electron pair from another

reactant in a chemical reaction, and a base

as the reactant that donates an electron

pair to another reactant.

NEUTRALIZATION: The process

whereby an acid and base react with one

another to form a salt and water.

PH SCALE: A logarithmic scale for

determining the acidity or alkalinity of a

substance, from 0 (virtually pure acid) to 7

(neutral) to 14 (virtually pure base).




only convey part of the picture, however—








tion, resulting in the creation of a product.

SALTS: Ionic compounds formed by

the reaction between an acid and a base. In

this reaction, one or more of the hydrogen

ions of an acid is replaced with another

positive ion. In addition to producing salts,

acid-base reactions produce water.



solute) is dissolved in another substance

(the solvent)—for example, sugar dis-

solved in water.

SOLVENT: A substance that dissolves

another, called a solute, in a solution.

STRONG ELECTROLYTE: A substance

highly prone to dissociation. The terms

“strong acid” or “strong base” refers to those

acids or bases which readily dissociate.



molecular structure and behavior rather

than purely phenomenological data.

TITRATION: The use of a chemical

reaction to determine the amount of a

chemical substance in a sample of

unknown purity. Testing pH levels is an

example of titration.

TRANSITION INTERVAL: The pH

level at which the color of an acid-base

indicator changes.

WEAK ELECTROLYTE: A substance

that experiences little or no dissociation.

The terms “weak acid” or “weak base” re-

fer to those acids or bases not prone to

dissociation.



Acid-BaseReactions

tions of hydronium (H3O+) and hydroxide(OH–) ions are equal. When multiplied by oneanother, these yield the constant figure 1.0 • 10–14,which is the equilibrium constant for water. Infact, this constant—denoted as Kw—is called theion-product constant for water.

Because the product of these two concentra-tions is always the same, this means that if one ofthem goes up, the other one must go down inorder to yield the same constant. This explainsthe fact, noted earlier, that water can serve eitheras an acid or base—or, if the concentrations ofhydronium and hydroxide ions are equal—as aneutral substance. In situations where the con-centration of hydronium is higher, and thehydroxide concentration automatically decreas-es, water serves as an acid. Conversely, when thehydroxide concentration is high, the hydroniumconcentration decreases correspondingly, andthe water is a base.

WHERE TO LEARN MORE

“Acids and Bases.” Vision Learning (Web site).<http://www.visionlearning.com/library/science/chemistry-2/CHE2.2-acid_base.htm> (June 7, 2001).

“Acids, Bases, and Chemical Reactions” Open Access Col-lege (Web site). <http://oac.schools.sa.edu.au/8-10science/acids.htm> (June 7, 2001).

“Acids, Bases, pH.” About.com (Web site). <http://chemistry.about.com/science/chemistry/cs/acidsbasesph/ (June 7, 2001).

“Acids, Bases, and Salts” (Web site). <http://edie.cprost.sfu.ca/~rhlogan/ionic_eq.html> (June 7, 2001).

“Junior Part: Acids, Bases, and Salts” (Web site).<http://www.rjclarkson.demon.co.uk/junior/junior4.htm> (June 7, 2001).

Knapp, Brian J. Acids, Bases, and Salts. Danbury, CT:Grolier Educational, 1998.

Moje, Steven W. Cool Chemistry: Great Experiments withSimple Stuff. New York: Sterling Publishing Compa-ny, 1999.

Walters, Derek. Chemistry. Illustrated by Denis Bishopand Jim Robins. New York: F. Watts, 1982.




327

SC IENCE OF EVERYDAY TH INGSr e a l - l i f e CHEMISTRY

SOLUT IONS ANDM IX TURES

SOLUT IONS ANDM IX TURES

MIXTURES

SOLUT IONS

OSMOS IS

D IST I L LAT ION AND F I LTRAT ION



M I X TURESMixtures

CONCEPTElements and compounds are pure substances,but much of the material around us—includingair, wood, soil, and even (in most cases) water—appears in the form of a mixture. Unlike a puresubstance, a mixture has variable composition: inother words, it cannot be reduced to a single typeof atom or molecule. Mixtures can be eitherhomogeneous—that is, uniform in appearance,and having only one phase—or heterogeneous,meaning that the mixture is separated into vari-ous regions with differing properties. Mosthomogeneous mixtures are also known as solu-tions, and examples of these include air, coffee,and even metal alloys. As for heterogeneous mix-tures, these occur, for instance, when sand or oilare placed in water. Oil can, however, be dis-persed in water as an emulsion, with the aid of asurfactant or emulsifier, and the result is analmost homogeneous mixture. Another interest-ing example of a heterogeneous mixture occurswhen colloids are suspended in fluid, whetherthat fluid be air or water.

HOW IT WORKS

Mixtures and Pure Substances

A mixture is a substance with a variable compo-sition, meaning that it is composed of moleculesor atoms of differing types. These may haveintermolecular bonds, or bonds between mole-cules, but such bonds are not nearly as strong asthose formed when elements bond to form acompound.

Examples of mixtures include milk, coffee,tea, and soft drinks—indeed, they include virtu-ally any drink except for pure water, which is ararity, because water is highly soluble and tendsto contain minerals and other impurities. Air,too, is a mixture, because it contains oxygen,nitrogen, noble gases, carbon dioxide, and watervapor. Likewise metal alloys such as bronze,brass, and steel are mixtures.

STRUCTURAL AND PHENOME-

NOLOGICAL DISTINCTIONS. Be-fore chemists accepted the atomic theory of mat-ter, it was often difficult to distinguish a mixture,such as air or steel, from pure substances, whichhave the same composition throughout. Puresubstances are either elements such as oxygen oriron, or compounds—for example, carbon diox-ide or iron oxide. An element is made up of onlyone type of atom, while a compound can bereduced to one type of bonded chemicalspecies—usually either a molecule or a forma-tion of ions.

A distinction between mixtures and puresubstances based on atomic and molecular struc-tures is called, fittingly enough, a structural defi-nition. Prior to the development of atomic theo-ry by English chemist John Dalton (1766-1844),and the subsequent identification of moleculesby Italian physicist Amedeo Avogadro (1776-1856), chemists defined compounds and mix-tures on the basis of phenomenological data garnered through observation and measure-ment. This could and did lead to incorrect suppositions.

PROUST AND THE DEBATE

OVER CONSTANT COMPOSITION.

Around the time of Dalton and Avogadro, French


Mixtures


chemist Antoine Lavoisier (1743-1794) correctlyidentified an element as a substance that couldnot be broken down into a simpler substance.Later, his countryman Joseph-Louis Proust(1754-1826) clarified the definition of a com-pound, stating it in much the same terms that wehave used: the composition of a compound is thesame throughout. But with the very rudimentaryknowledge of atoms and molecules that chemistsof the early nineteenth century possessed, thedistinctions between compounds and mixturesremained elusive.

Proust’s contemporary and intellectual rivalClaude Louis Berthollet (1748-1822) observedthat when metals such as iron are heated, theyform oxides in which the percentage of oxygenincreases with temperature. If constant composi-tion is a fact, he challenged Proust, then how isthis possible? Proust maintained that the propor-tions of elements in a compound are always thesame, a hypothesis Berthollet countered byobserving that metals can form variable alloys.Bronze, for instance, is an alloy of tin and copperat a ratio of about 25:75, but if the ratio werechanged to, say, 30:70, it would still be calledbronze.

At the time, the equipment—both in termsof knowledge and tools for analysis—simply did

not exist whereby Berthollet’s assertions could besuccessfully proven wrong. Indeed, Berthollet, astudent of Lavoisier who contributed to progressin the study of acids and reactants, was no anti-scientific villain: he was simply responding to theevidence as he saw it. Only with the discovery ofsubatomic particles in the late nineteenth andearly twentieth centuries, discoveries thatchanged the entire character of chemistry as adiscipline, did it truly become possible to answerBerthollet’s challenges.

Chemists now understand that the oxideformed on the surface of a metal is a compoundseparate from the metal itself, and that alloys(discussed later in this essay) are not compoundsat all: they are mixtures.

Mixtures and Compounds

With our modern knowledge of atomic andmolecular properties, we can more fully distin-guish between a compound and a mixture. First,a mixture can exist in virtually any proportionamong its constituent parts, whereas a com-pound has a definite and constant composition.Coffee, whether weak or strong, is still coffee, butif a second oxygen atom chemically bonds to theoxygen in carbon monoxide (CO), the resulting

THE WONDROUS INVENTION KNOWN AS SOAP IS A MIXTURE WITH SURFACTANT QUALITIES. (Chad Weckler/Corbis. Reproduced

by permission.)


Mixtures


compound is not simply “strong carbon monox-ide” or even “oxygenated carbon monoxide”: it iscarbon dioxide (CO2), an entirely different sub-stance. The difference is enormous: carbon diox-ide is a part of the interactions between plant andanimal life that sustains the natural environment,whereas carbon monoxide is a toxic gas pro-duced, for instance, by automobiles.

Second, the parts of a mixture retain most oftheir properties when they join together, but ele-ments lose their elemental characteristics oncethey form a compound. Sugar is still sweet,regardless of the substance into which it is dis-solved. In coffee or another drink, it may nolonger be dry and granular, though it could bereturned to that state by evaporating the coffee.In any case, dryness and granularity are physicalproperties, whereas sweetness is a chemical one.On the other hand, when elemental carbonbonds with hydrogen and oxygen to form sugaritself, the resulting compound is nothing like theelements from which it is made.

Third, a mixture can usually be created sim-ply by the physical act of stirring items together,and/or by applying heat. As discussed below, rel-atively high temperatures are sometimes neededto dissolve sugar in tea, for instance; likewise,alloys are usually formed by heating metals to

much higher temperatures. On the other hand,the formation of a compound involves a chemi-cal reaction, a vastly more complex interactionthan the one required to create a mixture. Car-bon, a black solid found in coal, can be mixedwith hydrogen and oxygen—both colorless andodorless gases—and the resulting mess would besomething; but it would not be sugar. Only thechemical reaction between the three elements, inspecific proportions and under specific condi-tions, results in their chemical bonding to formtable sugar, known to chemists as sucrose.

Homogeneous and Heteroge-neous Mixtures

HOMOGENEOUS MIXTURES

AND SOLUTIONS. As we have seen, thecomposition of a mixture is variable. Coffee, forinstance, can be weak or strong, and milk can be“whole” or low-fat. Beer can be light or dark incolor, as well as “light” in terms of calorie con-tent; furthermore, its alcohol content can be var-ied, as with all alcoholic drinks. Though theirmolecular composition is variable, each of themixtures described here is the same throughout:in other words, there is no difference betweenone region and another in a glass of beer. Such amixture is described as homogeneous.

BRONZE—AN ALLOY OF TIN AND COPPER—WAS ONE OF THE MOST IMPORTANT INVENTIONS IN HUMAN HISTORY. THIS

STONE CARVING IS FROM THE PERIOD KNOWN AS THE BRONZE AGE. (Kevin Schafer/Corbis. Reproduced by permission.)


Mixtures A homogeneous mixture is one that is thesame throughout, but this is not the same as say-ing that a compound has a constant composi-tion. Rather, when we say that a homogeneousmixture is the same in every region, we are speak-ing phenomenologically rather than structurally.From the standpoint of ordinary observation, aglass of milk is uniform, but as we shall see, milkis actually a type of emulsion, in which one sub-stance is dispersed in another. There are no milk“molecules,” and structural analysis would revealthat milk does not have a constant molecularcomposition.

Coffee is an example of a solution, a specifictype of homogeneous mixture. Actually, mosthomogeneous mixtures can be considered solu-tions, but since a solution is properly defined as ahomogeneous mixture in which one or moresubstances is dissolved in another substance, a50:50 homogeneous mixture is technically not asolution. When particles are perfectly dissolvedin a solution, the composition is uniform, butagain, this is not a compound, because that com-position can always be varied.

HETEROGENEOUS MIXTURES.

Anyone who has ever made iced tea has observedthat it is easy to sweeten when it is hot, becausetemperature affects the ability of a solvent suchas water—in this case, water with particles fromtea leaves filtered into it—to dissolve a solute,such as sugar. Cold tea, on the other hand, ismuch harder to sweeten with ordinary sugar.Usually what happens is that, instead of obtain-ing a homogeneous mixture, the result is hetero-geneous.

Whereas every region within a homoge-neous mixture is more or less the same as everyother region, heterogeneous mixtures containregions that differ from one another. In theexample we have used, a glass of cold tea withundissolved sugar at the bottom is a heteroge-neous mixture: the tea at the top is unsweetenedor even bitter, whereas at the bottom, there is anoverly sweet sludge of tea and sugar.

FROM HOMOGENEOUS TO

HETEROGENEOUS AND BACK

AGAIN. The distinction between homoge-neous and heterogeneous solutions only serves tofurther highlight the difference between com-pounds and mixtures. Whereas the compositionof a compound is definite and quantitative (somany atoms of element x bonded with so many

atoms of element y in such-and-such a struc-ture), there is less of a sharp dividing linebetween homogeneous and heterogeneous solutions.

When too much of a solute is added to thesolvent, the solvent becomes saturated, and is nolonger truly homogeneous. Such is the case whenthe sugar drifts to the bottom of the tea, or whencoffee grounds form in the bottom of a coffee potcontaining an otherwise uniform mixture of cof-fee and water.

Milk comes to us as a homogeneous sub-stance, thanks to the process of homogenization,but when it comes out of the cow, it is a moreheterogeneous mixture of milk fat and water. Infact, milk is an example of an emulsion, the resultof a process whereby two substances that wouldotherwise form a heterogeneous mixture aremade to form a homogeneous one.


Colloids

In 1827, Scottish naturalist Robert Brown (1773-1858) was studying pollen grains under a micro-scope when he noticed that the grains underwenta curious zigzagging motion in the water. At first,he assumed that the motion had a biologicalexplanation—in other words, that it resultedfrom life processes within the pollen—but later,he discovered that even pollen from long-deadplants behaved in the same way.

What Brown had observed was the suspen-sion of a colloid—a particle intermediate in sizebetween a molecule and a speck of dust—in thewater. When placed in water or another fluid(both liquids and gases are called “fluids” in thephysical sciences), a colloid forms a mixture sim-ilar both to a solution and a suspension. As witha solution, the composition remains homoge-neous—the particles never settle to the bottomof the container. At the same time, however, theparticles remain suspended rather than dis-solved, rendering a cloudy appearance to the dispersion.

Almost everyone, especially as a child, hasbeen fascinated by the colloidal dispersion ofdust particles in a beam of sunlight. They seem tobe continually in motion, as indeed they are, andthis movement is called “Brownian motion” in



Mixtureshonor of the man who first observed it. YetBrown did not understand what he was seeing;only later did scientists recognize that Brownianmotion is the result of movement on the part ofmolecules in a fluid. Even though the moleculesare much smaller than the colloid, there areenough of them producing enough collisions tocause the colloid to be in constant motion.

Another remarkable aspect of dust particlesfloating in sunlight is the way that they cause acolumn of sunlight to become visible. This phe-nomenon, called the Tyndall effect after Englishphysicist John Tyndall (1820-1893), makes itseem as though we are actually seeing a beam oflight. In fact, what we are seeing is the reflectionof light off the colloidal dispersion. Anotherexample of a colloidal dispersion occurs when apuff of smoke hangs in the air; furthermore, aswe shall see, milk is a substance made up of col-loids—in this case, particles of fat and proteinsuspended in water.

Emulsions

Miscibility is a qualitative term identifying therelative ability of two substances to dissolve inone another. Generally, water and water-basedsubstances have high miscibility with regard toone another, as do oil and oil-based substanceswith one another. Oil and water, on the otherhand (or substances based in either) have verylow miscibility.

The reason is that water molecules are polar,meaning that the positive electric charge is in oneregion of the molecule, while the negative chargeis in another region. Oil, on the other hand, isnonpolar, meaning that charges are more evenlydistributed throughout the molecule. Trying tomix oil and water is thus rather like attempting touse a magnet to attract a nonmagnetic metalsuch as gold.

HOW SURFACTANTS AID THE

EMULSION PROCESS. An emulsion isa mixture of two immiscible liquids such as oiland water—we will use these simple terms, withthe idea that these stand for all oil- and water-soluble substances—by dispersing microscopicdroplets of one liquid in another. In order toachieve this dispersion, there needs to be a “mid-dle man,” known as an emulsifier or surfactant.The surfactant, made up of molecules that areboth water- and oil-soluble, acts as an agent forjoining other substances in an emulsion.

All liquids have a certain degree of surfacetension. This is what causes water on a hard sur-face to bead up instead of lying flat. (Mercury hasan extremely high surface tension.) Surfactantsbreak down this surface tension, enabling themarriage of two formerly immiscible liquids.

In an emulsion, millions of surfactants sur-round the dispersed droplets (known as theinternal phase), shielding them from the otherliquid (the external phase). Supposing oil is the internal phase, then the oil-soluble end of thesurfactant points toward the oils, while the water-soluble side joins with the water of theexternal phase.

PHYSICAL CHARACTERISTICS

OF AN EMULSION. The resulting emul-sion has physical (as opposed to chemical) prop-erties different from those of the substances thatmake it up. Water and most oils are transparent,whereas emulsions are typically opaque and mayhave a lustrous, pearly gleam. Oil and water flowfreely, but emulsions can be made to form thick,non-flowing creams.

Emulsions are inherently unstable: they are,as it were, only temporarily homogeneous. Agood example of this is an oil-and-vinegar saladdressing, which has to be shaken before putting iton salad; otherwise, what comes out of the bottleis likely to be mainly oil, which floats to the topof the water-based vinegar. It is characteristic ofemulsions that eventually gravity pulls themapart, so that the lighter substances float to thetop. This process may take only a few minutes, aswith the salad dressing; on the other hand, it cantake thousands of years.

APPLICATIONS. Surfactants them-selves are often used in laundry or dish detergent,because most stains on plates or clothes are oil-based, whereas the detergent itself is applied tothe clothes in an aqueous solution. As for emul-sions, these are found in a wide variety of prod-ucts, from cosmetics to paint to milk.

In the pharmaceutical industry, for instance,emulsions are used as a means of delivering theactive ingredients of drugs, many of which arenot water soluble, in a medium that is. Pharma-ceutical manufacturers also use emulsions tomake medicines more palatable (easier to take);to control dosage and improve effectiveness; andto improve the usefulness of topical drugs such asointments, which must contend with both oilyand water-soluble substances on the skin.



Mixtures The oils and other conditioning agents usedin hair-care products would leave hair limp andsticky if applied by themselves. Instead, theseproducts are applied in emulsions that dilute theoils in other materials. Emulsions are also used incolor photography, as well as in the productionof pesticides and fungicides.

Paints and inks, too, are typically emulsifiedsubstances, and these sometimes come in theform of dispersions akin to those of colloids. Aswe have seen, in a dispersion fine particles ofsolids are suspended in a liquid. Surfactants areused to bond the particles to the liquid, thoughpaint must be shaken, usually with the aid of ahigh-speed machine, before it is consistentenough to apply.

As mentioned earlier, milk is a form ofemulsion that contains particles of milk fat orcream dispersed in water. Light strikes the micro-scopic fat and protein colloids, resulting in thefamiliar white color. Other examples of emulsi-fied foods include not only salad dressings, butgravies and various sauces, peanut butter, icecream, and other items. Not only does emulsionaffect the physical properties of foods, but it mayenhance the taste by coating the tongue withemulsified oils.

Soap

The wondrous invention called soap, which hasmade the world a cleaner place, exhibits several ofthe properties we have discussed. It is a mixturewith surfactant qualities, and in powdered form,it can form something close to a true solutionwith water. Discovered probably by the Phoeni-cians around 600 B.C., it was made in ancienttimes by boiling animal fat (tallow) or vegetableoils with an alkali or base of wood ashes.

This was a costly process, and for many cen-turies, only the wealthy could afford soap—a factthat contributed to the notorious lack of person-al hygiene that prevailed among Europeans of theMiddle Ages. In fact, the situation did not changeuntil late in the eighteenth century, when thework of two French chemists yielded a more eco-nomical manufacturing method.

In 1790, Nicholas Leblanc (1742-1806)developed a process for creating sodium hydrox-ide (called caustic soda at the time) from sodiumchloride, or ordinary table salt. This made for aninexpensive base or alkali, with which soap man-ufacturers could combine natural fats and oils.

Then in 1823, Michel Eugène Chevreul (1786-1889) showed that fat is a compound of glycerolwith organic acids, a breakthrough that led to theuse of fatty acids in producing soaps.

THE CHEMISTRY OF SOAP.

Soap as it is made today comes from a salt of analkali metal, such as sodium or potassium, com-bined with a mixture of carboxylic acids (“fattyacids”). These carboxylic acids are the result of areaction called saponification, involving triglyc-erides and a base, such as sodium hydroxide.Saponification breaks the triglycerides into theircomponent fatty acids; then, the base neutralizesthese to salts. This reaction produces glycerin, ahydrocarbon bonded to a hydroxyl (-OH) group.(Hydrocarbons are discussed in the essays onOrganic Chemistry; Polymers.)

In general, the formula for soap is RCOOX,where X stands for the alkali metals, and R for thehydrocarbon chain. Because it is a salt (meaningthat it is formed from the reaction of an acid witha base), soap partially separates into its compo-nent ions in water. The active ion is RCOO-,whose two ends behave in different fashions,making it a surfactant. The hydrocarbon end (R-) is said to be lipophilic, or “oil-loving,” whichis appropriate, since most oils are hydrocarbons.On the other hand, the carboxylate end (-COO-)is hydrophilic, or “water-loving.” As a result, soapcan dissolve in water, but can also clean greasystains.

When soap is mixed with water, it does notform a true homogeneous mixture or solution dueto the presence of hydrocarbons. These attract oneanother, forming spherical aggregates calledmicelles. The lipophilic “tails” of the hydrocarbonsare turned toward the interior of the micelle, whilethe hydrophilic heads remain facing toward thewater that forms the external phase.

Alloys

We have primarily discussed liquid mixtures, butin fact mixtures can be gaseous or even solid—asfor example in the case of an alloy, a mixture oftwo or more metals. Alloys are usually created bymelting the component metals, then mixingthem together in specific proportions, but analloy can also be created by bonding metal powders.

The structure of an elemental metal includestight “electron sea” bonds, which are discussed inthe essay on Metals. These, combined with the



Mixtures


ALLOY: A mixture of two or more

metals.

AQUEOUS: A term describing a solu-

tion in which a substance or substances are

dissolved in water.

ATOM: The smallest particle of an

element.





so forth.

COMPOUND: A substance made up of

atoms of more than one element, which are

chemically bonded and usually joined in

molecules. The composition of a com-

pound is always the same, unless it is

changed chemically.

DISPERSION: A term describing the

distribution of particles in a fluid.

ELEMENT: A substance made up of

only one kind of atom. Unlike compounds,

elements cannot be chemically broken

down into other substances.

EMULSIFIER: A surfactant.

EMULSION: A mixture of two immis-

cible liquids such as oil and water, made by

dispersing microscopic droplets of one liq-

uid in another. In creating an emulsion,

surfactants act as bridges between the two

liquids.

FLUID: A substance with the ability to

flow. In physical sciences such as chemistry

and physics, “fluid” refers both to liquids

and gases.

HETEROGENEOUS: A term describ-

ing a mixture that is not the same through-

out; rather, it has various regions possess-

ing different properties. An example of a

mixture is sand in a container of water.

Rather than dissolving to form a homoge-

neous mixture, as sugar would, the sand

sinks to the bottom.

HOMOGENEOUS: A term describing

a mixture that is the same throughout, as

for example when sugar is fully dissolved

in water. A solution is a homogenous

mixture.

IMMISCIBLE: Possessing a negligible

value of miscibility.

ION: An atom or group of atoms that

has lost or gained one or more electrons,

and thus has a net electric charge.

MISCIBILITY: A qualitative term iden-

tifying the relative ability of two substances

to dissolve in one another.

MIXTURE: A substance with a variable

composition, meaning that it is com-

posed of molecules or atoms of differing

types. Compare with pure substance or

compound.

MOLECULE: A group of atoms, usual-

ly but not always representing more

than one element, joined in a structure.

Compounds are typically made of up

molecules.




only convey part of the picture, however—






KEY TERMS


Mixtures


metal’s crystalline structure, create a situation inwhich internal bonding is very strong, butnondirectional. As a result, most metals formalloys, but again, this is not the same as a com-pound: the elemental metals retain their identityin these metal composites, forming characteristicfibers, beads, and other shapes.

SOME FAMOUS ALLOYS. One ofthe most important alloys in early human histo-ry was a 25:75 mixture of tin and copper thatgave its name to an entire stage of technologicaldevelopment: the Bronze Age (c. 3300-1200 B.C.).This combination formed a metal much strongerthan either copper or tin, and bronze remaineddominant until the discovery of new iron-smelt-ing methods ushered in the Iron Age in about1200 B.C.

Another important alloy of the ancientworld was brass, which is about one-third zinc totwo-thirds copper. Introduced in the period c.1400-c. 1200 B.C, brass was one of the metals thatdefined the world in which the Bible was written.Biblical references to it abound, as in the famouspassage from I Corinthians 13 (read at manyweddings), in which the Apostle Paul proclaimsthat “without love, I am as sounding brass.” Somebiblical scholars, however, maintain that some ofthe references to brass in the Old Testament areactually mistranslations of “bronze.”

Tin, copper, and antimony form pewter, asoft mixture that can be molded when cold andbeaten regularly without turning brittle. Thoughused in Roman times, it became most popular inEngland from the fourteenth to the eighteenthcenturies, when it offered a cheap substitute forsilver in plates, cups, candelabra, and pitchers.The pewter turned out by colonial Americanmetalsmiths is still admired for its beauty andfunctionality.

As for iron, it appears in nature as an impureore, but even when purified, it is typically alloyedwith other elements. Among the well-knownforms of iron are cast iron, a variety of mixturescontaining carbon and/or silicon; wrought iron,which contains small amounts of various otherelements, such as nickel, cobalt, copper, chromi-um, and molybdenum; and steel. Steel is a mix-ture of iron with manganese and chromium,purified with a blast of hot air. Steel is also some-times alloyed with aluminum in a one-third/two-thirds mixture to make duraluminum, developedfor the superstructures of German zeppelins dur-ing World War I.

WHERE TO LEARN MORE“All About Colloids.” Synthashield (Web site).

<http://www.synthashield.net/vault/colloids.html>(June 6, 2001).

Carona, Phillip B. Magic Mixtures: Alloys and Plastics.Englewood Cliffs, NJ: Prentice Hall, 1963.

PURE SUBSTANCE: A substance that

has the same composition throughout.

Pure substances are either elements or

compounds, and should not be confused

with mixtures. A homogeneous mixture is

not the same as a pure substance, because

although it has an unvarying composition,

it cannot be reduced to a single type of

atom or molecule.


in which one or more substances is dis-

solved in another substances—for exam-

ple, sugar dissolved in water.



molecular structure rather than purely

phenomenological data.

SURFACTANT: A substance made up

of molecules that are both water- and oil-

soluble, which acts as an agent for joining

other substances in an emulsion.

SUSPENSION: A term that refers to a

mixture in which solid particles are sus-

pended in a fluid.



MixturesChemLab. Danbury, CT: Grolier Educational, 1998.

“Corrosion of Alloys.” Corrosion Doctors (Web site).<http://www.corrosion-doctors.org/MatSelect/corralloys.htm> (June 6, 2001).

“Emulsions.” Pharmweb (Web site). <http://pharmweb.usc.edu/phar306/Handouts/emulsions/> (June 6,2001).


Knapp, Brian J. Elements, Compounds, and Mixtures.Danbury, CT: Grolier Educational, 1998.

Maton, Anthea. Exploring Physical Science. Upper SaddleRiver, NJ: Prentice Hall, 1997.

Patten, J. M. Elements, Compounds, and Mixtures. VeroBeach, FL: Rourke Book Company, 1995.

The Soap and Detergent Association (Web site).<http://www.sdahq.org/> (June 6, 2001).




SOLUT IONSSolutions

CONCEPTWe are most accustomed to thinking of solutionsas mixtures of a substance dissolved in water, butin fact the meaning of the term is broader thanthat. Certainly there is a special place in chem-istry for solutions in which water—”the univer-sal solvent”—provides the solvent medium. Thisis also true in daily life. Coffee, tea, soft drinks,and even water itself (since it seldom appears inpure form) are solutions, but the meaning of theterm is not limited to solutions involving water.Indeed, solutions do not have to be liquid; theycan be gaseous or solid as well. One of the mostimportant solutions in the world, in fact, is theair we breathe, a combination of nitrogen, oxy-gen, noble gases, carbon dioxide, and watervapor. Nonetheless, aqueous or water-based sub-stances are the focal point of study where solu-tions are concerned, and reactions that take placein an aqueous solution provide an importantarea of study.

HOW IT WORKS

Mixtures

Solutions belong to the category of substancesidentified as mixtures, substances with variablecomposition. This may seem a bit confusing,since among the defining characteristics of asolution are its uniformity and consistency. Butthis definition of a solution involves externalqualities—the things we can observe with oursenses—whereas a mixture can be distinguishedfrom a pure substance not only in terms of exter-nal characteristics, but also because of its internaland structural properties.

At one time, chemists had a hard time dif-ferentiating between mixtures and pure sub-stances (which are either elements or com-pounds), primarily because they had little or noknowledge of those “internal and structuralproperties.” Externally, a mixture and compoundcan be difficult to distinguish, but the “variablecomposition” mentioned above is a key factor.Coffee can be almost as weak as water, or sostrong it seems to galvanize the throat as it goesdown—yet in both cases, though its compositionhas varied, it is still coffee.

By contrast, if a compound is altered by thevariation of just one atom in its molecular struc-ture, it becomes something else entirely. When anoxygen atom bonds with a carbon and oxygenatom, carbon monoxide—a poisonous gas pro-duced by the burning of fossil fuels—is turnedinto carbon dioxide, an essential component ofthe interaction between plants and animals in thenatural environment.

MIXTURES VS. COMPOUNDS:

THREE DISTINCTIONS. Air (a mix-ture) can be distinguished from a compound,such as carbon dioxide, in three ways. First, amixture can exist in virtually any proportionsamong its constituent parts, whereas a com-pound has a definite and constant composition.Air can be oxygen-rich or oxygen-poor, but if wevary the numbers of oxygen atoms chemicallybonded to form a compound, the result is anentirely different substance.

Second, the parts of a mixture retain most oftheir properties when they join together, but ele-ments—once they form a compound—lose theirelemental characteristics. If pieces of carbon werefloating in the air, their character would be quite


Solutions


different from that of the carbon in carbon diox-ide. Carbon, in its natural form, is much like coalor graphite (graphite is pure carbon, and coal animpure form of the element), but when a carbonatom combines with an oxygen atom, the twoform a gas. Pieces of carbon in the air, on theother hand, would be solid, as carbon itself is.

Third, a mixture can usually be created sim-ply by the physical act of stirring items together,and/or by applying heat. Certainly this is true ofmany solutions, such as brewed coffee. On theother hand, a compound such as carbon dioxidecan only be formed by the chemical reaction ofatoms in specific proportions and at specifictemperatures. Some such reactions consumeheat, whereas others produce it. These interac-tions of heat are highly complex; on the otherhand, in making coffee, heat is simply producedby an external source (the coffee maker) andtransferred to the coffee in the brewing process.

HETEROGENEOUS AND HOMO-

GENEOUS MIXTURES. There are twobasic types of mixtures. One of these occurs, forinstance, when sand is added to a beaker ofwater: the sand sinks to the bottom, and the com-position of the sand-water mixture cannot be

said to be the same throughout. It is thus a het-erogeneous mixture. The same is true of cold teawhen table sugar (sucrose) is added to it: thesugar drifts to the bottom, and as a result, thefirst sip of tea from the glass is not nearly as sweetas the last.

Wood is a mixture that typically appears inheterogeneous form, a fact that can be easily con-firmed by studying wood paneling. There areknots, for instance—areas where branches oncegrew, and where the concentrations of sap arehigher. Striations of color run through theboards of paneling, indicating complex varia-tions in the composition of the wood. Much thesame is true of soil, with a given sample varyingwidely depending on a huge array of factors: theconcentrations of sand, rock, or decayed vegeta-tion, for instance, or the activities of earthwormsand other organisms in processing the soil.

In a homogeneous mixture, by contrast,there is no difference between one area of themixture and another. If coffee has been brewedproperly, and there are no grounds at the bottomof the pot, it is homogeneous. If sweetener isadded to tea while it is hot, it too should yield ahomogeneous mixture.

WATER AND OIL DROPLETS EXPOSED TO POLARIZED LIGHT. WATER AND OIL HAVE VERY LITTLE MISCIBILITY WITH

REGARD TO ONE ANOTHER, MEANING THAT THEY TEND TO REMAIN SEPARATE WHEN COMBINED. (Science Pictures

Limited/Corbis. Reproduced by permission.)


SolutionsMISCIBILITY AND SOLUBILI-

TY. Miscibility is a term describing the relativeability of two substances to dissolve in oneanother. Generally, water and water-based sub-stances have high miscibility with regard to oneanother, as do oil and oil-based substances. Oiland water, on the other hand (or substancesbased in either) have very low miscibility. Thereasons for this will be discussed below.

Miscibility is a qualitative term like “fast” or“slow”; on the other hand, solubility—the maxi-mum amount of a substance that dissolves in agiven amount of solvent at a specific tempera-ture—can be either qualitative or quantitative. Ina general sense, the word is qualitative, referringto the property of being soluble, or able to dis-solve in a solution.

At some points in this essay, “solubility” willbe used in this qualitative sense; however, withinthe realm of chemistry, it also has a quantitativemeaning. In other words, instead of involving animprecise quality such as “fast” or “slow,” solubil-ity in this sense relates to precise quantities morealong the lines of “100 MPH (160.9 km/h).” Sol-ubility is usually expressed in grams (g) (0.0022lb) of solute per 100 g (0.22 lb) of solvent.

OTHER QUANTITATIVE TERMS.

Another quantitative means of describing a solu-tion is in terms of mass percent: the mass of thesolute divided by the mass of the solution, andmultiplied by 100%. If there are 25 g of solute ina solution of 200 g, for instance, 25 would bedivided by 200 and multiplied by 100% to yield a12.5% figure of solution composition.

Molarity also provides a quantitative meansof showing the concentration of solute to solu-tion. Whereas mass percent is, as its name indi-cates, a comparison of the mass of the solute tothat of the solvent, molarity shows the amount ofsolute in a given volume of solution. This ismeasured in moles of solute per liter of solution,abbreviated mol/l.


Saturation and Dilution

The quantitative terms for describing solubilitythat we have reviewed are useful to a profession-al chemist, or to anyone performing work in alaboratory. For the most part, however, qualita-


Solutions and Solubility

Virtually all varieties of homogeneous mixturescan be classified as a solution—that is, a homo-geneous mixture in which one or more sub-stances is dissolved in another substance. If twoor more substances were combined in exactlyequal proportions, this would technically not be a solution; hence the distinction betweensolutions and the larger class of homogeneousmixtures.

A solution is made up of two parts: thesolute, the substance or substances that are dis-solved; and the solvent, the substance that dis-solves the solute. The solvent typically deter-mines the physical state of the solution: in otherwords, if the solvent is a liquid, the solution willmost likely be a liquid. Likewise if the solvent is asolid such as a metal, melted at high tempera-tures to form a metallic solution called an alloy,then when the solution is cooled to its normaltemperature, it will be solid again.

WATER AND ETHANOL ARE COMPLETELY MISCIBLE,WHETHER THE SOLUTION BE 99% WATER AND 1%ETHANOL, 50% OF BOTH, OR 99% ETHANOL AND 1%WATER. SHOWN HERE IS A GLASS OF BEER—AN

EXAMPLE OF A WATER-ETHANOL SOLUTION. (Owen

Franken/Corbis. Reproduced by permission.)


Solutionstive expressions will suffice for the present dis-cussion. Among the most useful qualitative termsis saturation.

When it is possible to dissolve solute in a sol-vent, the solution is said to be unsaturated—rather like a sponge that has not been filled tocapacity with liquid. By contrast, a solution thatcontains as much solute as it can at a given tem-perature is like a sponge that has been filled withall the liquid it can possibly contain. It can nolonger absorb more of the liquid; rather, it canonly push liquid particles along, and the spongeis said to be saturated. In the same way, if tea ishot, it is easier to introduce sugar to it, but whenthe temperature is low, sugar will not dissolve aseasily.

SATURATION AND TEMPERA-

TURE. With tea and many other substances,higher temperatures mean that a greater amountof solute can be added before the solution is fullysaturated. The reason for this relationshipbetween saturation and temperature, accordingto generally accepted theory, is that heated sol-vent particles move more quickly than cold ones,and as a result, create more space into which thesolvent can fit. Indeed, “space” is a prerequisitefor a solution: the molecules of solute need tofind a “hole” between the molecules of solventinto which they can fit. Thus the molecules in asolution can be compared to a packed crowd: if acrowd is suddenly dispersed, it is easier to walkthrough it.

Not all substances respond to rises in tem-perature in the manner described, however. As arule, gases (with the exception of helium) aremore soluble at lower temperatures than at high-er ones. Higher temperatures mean an increasein kinetic energy, with more molecules collidingwith one another, and this makes it easier for the gases to escape the liquid in which they aredissolved.

Carbonated soft drinks get their “fizz” fromcarbon dioxide gas dissolved, along with sugarand other flavorings, in a solution of water. Whenthe soft-drink container is opened, much of thecarbon dioxide quickly departs, while a certainproportion stays dissolved in the cola. Theremaining carbon dioxide will eventually escapeas well, but it will do so more quickly if left in awarm environment rather than a cold one, suchas the inside of a refrigerator.

DILUTION AND CONCENTRA-

TION. Another useful set of qualitative termsfor solutions also relates to the relative amount ofsolute that has been dissolved. But whereas satu-ration is an absolute condition at a certain tem-perature (in other words, the solution is definite-ly saturated at such-and-such a temperature),concentration and dilution are much more rela-tive terms.

When a solution contains a relatively smallamount of solute, it is said to be dilute; on theother hand, a solution with a relatively largeamount of solute is said to be concentrated. Forinstance, hydrogen peroxide (H2O2) as sold overthe counter in drug stores, to be used in homes asa disinfectant of bleaching agent, is a dilute 3%solution in water. In higher concentrations,hydrogen peroxide can cause burns to humanskin, and is used to power rockets.

Chemists often keep on hand in their labo-ratories concentrated versions of various fre-quently used solutions. Maintaining them inconcentrated form saves space; when more of asolution is needed, the chemist dilutes it in wateraccording to desired molarity values. To do so,the chemist determines how much water needs tobe added to a particular “stock solution” (as theseconcentrated solutions in a laboratory are called)to create a solution of a particular concentration.In the dilution with water, the number of molesof the solute does not change; rather, the particlesof solute are dispersed over a larger volume.

“Like Dissolves Like”

In the molecules of a compound, the atoms areheld together by powerful attractions. The mole-cules within a compound, however, also have anintermolecular attraction, but this is much weak-er than the interatomic bonds in a molecule. It isthus quite possible for the molecules in a solu-tion to exert a greater attractive force than theone holding the molecules of solute togetherwith one another. When this happens, the solutedissolves.

A useful rule of thumb when studying solu-tions is “like dissolves like.” In other words, waterdissolves water-based solutes, whereas oil dis-solves oily substances. Or to put it another way,water and water-based substances are misciblewith one another, as are oil and oil-based sub-stances. Water and oil together, however, arelargely immiscible.



SolutionsPOLAR WATER, NONPOLAR

OIL. Water and oil and immiscible due to theirrespective molecule structures, and hence theirinherent characteristics of intermolecular bond-ing. Water molecules are polar, meaning that thepositive electric charge is at one end of the mole-cule, while the negative charge is at the other end.Oil, on the other hand, is nonpolar—charges are more evenly distributed throughout the molecule.

The oxygen atom in a water molecule has amuch greater electronegativity than the hydro-gen atoms, so it pulls negatively charged elec-trons to its end of the molecule. Conversely, mostoils are composed of carbon and hydrogen,which have similar values of electronegativity. Asa result, positive and negative charges are distrib-uted evenly throughout the oil molecule. Where-as a water molecule is like a magnet with a northand south pole, an oil molecule is like a nonmag-netic metal. Thus, as most people know, “oil andwater don’t mix.”

The immiscible quality of oil and water inrelation to one another explains a number ofphenomena from the everyday world. It is easyenough to wash mud from your hands underrunning water, because the water and the mud(which, of course, contains water) are highly mis-cible. But motor oil or oil-based paint will leave afilm on the hands, as these substances bond tooily molecules in the skin itself, and no amountof water will wash the film off. To remove it, oneneeds mineral spirits, paint thinner, or soapmanufactured to bond with the oils.

Everyone has had the experience of eatingspicy foods and then trying to cool down themouth with cold water—and just about everyonehas discovered that this does not work. This isbecause most spicy substances contain emulsi-fied oils that coat the tongue, and these oils arenot attracted to water. A much better “solution”(in more ways than one) is milk, and in particu-lar, whole milk. Though a great deal of milk iscomposed of water, it also contains tiny dropletsof fats and proteins that join with the oils in spicysubstances, thus cooling down the mouth.

Crossing the Oil/Water Barrier

The active ingredient in a dry-cleaning chemicalusually has nonpolar molecules, because thetoughest stains are made by oils and greases. But

what about ordinary soap, which we use in waterto wash both water- and oil-based substancesfrom our bodies? Though, as noted earlier, cer-tain kinds of oil stains require special cleaningsolutions, ordinary soap is fine for washing offthe natural oils secreted by the skin. Likewise, itcan wash off small concentrations of oil thatcome from the environment and attach to theskin—for instance, from working over a deepfryer in a fast-food restaurant.

How does the soap manage to “connect”both with oils and water? Likewise, how doesmilk—which is water-soluble and capable ofdilution in water—wash away the oils associatedwith spicy food? To answer these questions, weneed to briefly consider a subject examined inmore depth within the Mixtures essay: emul-sions, or mixtures of two immiscible liquids.

EMULSIFIERS AND SURFAC-

TANTS. The dispersion of two substances inan emulsion is achieved through the use of anemulsifier or surfactant. Made up of moleculesthat are both water- and oil-soluble, an emulsifi-er or surfactant acts as an agent for joining othersubstances in an emulsion. The two words arevirtually synonymous, but “emulsifier” is usedtypically in reference to foods, whereas “surfac-tant” most often refers to an ingredient in deter-gents and related products.

In an emulsion, millions of surfactants sur-round the dispersed droplets of solute, known asthe internal phase, shielding them from the sol-vent, or external phase. Surfactants themselvesare often used in laundry or dish detergent,because most stains on plates or clothes are oil-based, whereas the detergent itself is applied tothe clothes in a water-based external phase. Theemulsifiers in milk help to bond oily particles ofmilk fat (cream) and protein to the externalphase of water that comprises the majority ofmilk’s volume.

As for soap, it is a mixture of an acid and abase—specifically, carboxylic acids joined with abase such as sodium hydroxide. Carboxylic acids,chains of hydrogen and carbon atoms, are justsome of many hydrocarbons that form the chem-ical backbone of a vast array of organic sub-stances. Because it is a salt, meaning that it isformed from the reaction of an acid with a base,soap partially separates into its component ionsin water. The active ion is RCOO- (R stands for



Solutionsthe hydrocarbon chain), whose two ends behavein different fashions, making it a surfactant.

The hydrocarbon end (R-) is said to belipophilic, or “oil-loving”; on the other hand, thecarboxylate end (-COO-) is hydrophilic, or“water-loving.” As a result, soap can dissolve inwater, but can also clean greasy stains. When soapis mixed with water, it does not form a true solu-tion, due to the presence of the hydrocarbons,which attract one another to form sphericalaggregates called micelles. The lipophilic “tails”of the hydrocarbons are turned toward the inte-rior of the micelle, while the hydrophilic “heads”remain facing toward the water that forms theexternal phase.

ETHANOL. When a hydrocarbon joinswith other substances in one of the hydrocarbonfunctional groups, these form numerous hydro-carbon derivatives. Among the hydrocarbonderivatives is alcohol, and within this grouping isethyl alcohol or ethanol, which includes a hydro-carbon bonded to the –OH functional group.The formula for ethanol is C2H6O, though this issometimes rendered as C2H5OH to show that thealcohol functional group (–OH) is bonded to thehydroxyl radical (C2H5).

Ethanol, the same alcohol found in alcoholicbeverages, is obviously miscible with water. Beer,after all, is mostly water, and if ethanol and waterwere not miscible, it would be impossible to mixalcohol with water or a water-based substance tomake Scotch and water or a Bloody Mary (vodkaand tomato juice.) In fact, water and ethanol arecompletely miscible, whether the solution be99% water and 1% ethanol, 50% of both, or 99%ethanol and 1% water.

How can this be, given the fact that ethanolis a hydrocarbon derivative—in other words, acousin of petroleum? The addition of the term“derivative” gives us a clue: note that ethanolcontains oxygen, just as water does. As in water,the oxygen and hydrogen form a polar bond, giv-ing that portion of the ethanol molecule a highaffinity for water. Molecules of water and thealcohol functional group are joined through anintermolecular force called hydrogen bonding.

Aqueous Solutions

The term aqueous refers to water, and becausewater has extraordinary solvent qualities (dis-cussed in the Osmosis essay) it serves as themedium for numerous solutions. Furthermore,

reactions in aqueous solutions—though we canonly touch on them briefly here—constitute alarge and significant body of reactions studied bychemists.

Most of the solutions we have discussed upto this point are aqueous. We have referred, atleast in an external or phenomenological way, tothe means by which sugar dissolves in an aque-ous solution, such as tea. Now let us consider,from a molecular or structural standpoint, themeans by which salt does so as well.

SALTWATER. Salt is formed of posi-tively charged sodium ions, firmly bonded tonegatively charged chlorine ions. To break theseionic bonds and dissolve the sodium chloride,water must exert a strong attraction. However,salt is not really composed of molecules but sim-ply repeating series of sodium and chlorineatoms joined together in a simple face-centeredcubic lattice which has each each sodium ion(Na+) surrounded by six chlorine ions (Cl–); atthe same time, each chlorine ion is surroundedby six sodium ions.

In dissolving the salt, water molecules sur-round ions that make up the salt, holding themin place through the electrical attraction ofopposite charges. Due to the differences in electronegativity that give the water molecule itshigh polarity, the hydrogen atoms are more positively charged, and thus these attract the negatively charged chlorine ion. Similarly,the negatively charged oxygen end of the watermolecule attracts the positively charged sodiumion. As a result, the salt is “surrounded” or dissolved.

PLASMA AND AQUEOUS-SO-

LUTION REACTIONS IN THE

BODY. Whereas saltwater is an aqueous solu-tion that can eventually kill someone who drinksit, plasma is an essential component of the lifeprocess—and in fact, it contains a small quantityof sodium chloride. Not to be confused with thephase of matter also called plasma, this plasma isthe liquid portion of blood. Blood itself is about55% plasma, with red and white blood cells andplatelets suspended in it.

Plasma is in turn approximately 90% water,with a variety of other substances suspended ordissolved in it. Prominent among these sub-stances are proteins, of which there are about 60in plasma, and these serve numerous importantfunctions—for instance, as antibodies. Plasma



Solutions


also contains ions, which prevent red blood cellsfrom taking up excess water in osmosis. Promi-nent among these ions are those of salt, or sodi-um chloride. Plasma also transports nutrientssuch as amino acids, glucose, and fatty acids, aswell as waste products such as urea and uric acid,which it passes on to the kidneys for excretion.

Much of the activity that sustains life in thebody of a living organism can be characterized asa chemical reaction in an aqueous solution.When we breathe in oxygen, it is taken to thelungs and fed into the bloodstream, where it

associates with the iron-containing hemoglobinin red blood cells and is transported to theorgans. In the stomach, various aqueous-solu-tion reactions process food, turning part of itinto fuel that the blood carries to the cells, wherethe oxygen engages in complex reactions withnutrients.

A Miscellany of Solutions

Aqueous-solution reactions can lead to the for-mation of a solid, as when a solution of potassi-

AQUEOUS: A term describing a solu-

tion in which a substance or substances are

dissolved in water.

COMPOUND: A substance made up of

atoms of more than one element, which are

chemically bonded and usually joined in

molecules. The composition of a com-

pound is always the same, unless it is

changed chemically.

CONCENTRATED: A qualitative term

describing a solution with a relatively large

ratio of solute to solvent.

DILUTE: A qualitative term describing

a solution with a relatively small ratio of

solute to solvent.

EMULSIFIER: A surfactant. (Usually

the term “emulsifier” is applied to foods,

and “surfactant” to detergents and other

inedible products.)

EMULSION: A mixture of two immis-

cible liquids, such as oil and water, made by

dispersing microscopic droplets of one liq-

uid in another. In creating an emulsion,

surfactants act as bridges between the two

liquids.




ing different properties. One example of

this is sand in a container of water. Rather

than dissolving to form a homogeneous

mixture as sugar does, the sand sinks to the

bottom.





mixture.

IMMISCIBLE: Possessing a negligible

value of miscibility.

MASS PERCENT: A quantitative

means of measuring solubility in terms of

the mass of the solute divided by the mass

of the solution, multiplied by 100%.

MISCIBILITY: A term identifying the

relative ability of two substances to dissolve

in one another. Miscibility is qualitative,

like “fast” or “slow”; solubility, on the other

hand, can be a quantitative term.


composition, composed of molecules or

atoms of differing types. Compare with

pure substance.

KEY TERMS


Solutions


um chromate (K2CrO4) is added to an aqueoussolution of barium nitrate (Ba[NO3]2 to formsolid barium chromate (BaCrO4) and a solutionof potassium nitrate (KNO3). This reaction isdescribed in the essay on Chemical Reactions.

We have primarily discussed liquid solu-tions, and in particular aqueous solutions. Itshould be stressed, however, that solutions canalso exist in the gaseous or solid phases. The airwe breathe is a solution, not a compound: inother words, there is no such thing as an “airmolecule.” Instead, it is made up of diatomic ele-

ments (those in which two atoms join to form amolecule of a single element); monatomic ele-ments (those elements that exist as single atoms);one element in a triatomic molecule; and twocompounds.

The “solvent” in air is nitrogen, a diatomicelement that accounts for 78% of Earth’s atmos-phere. Oxygen, also diatomic, constitutes anadditional 21%. Argon, which like all noble gasesis monatomic, ranks a distant third, with 0.93%.The remaining 0.07% is made up of traces ofother noble gases; the two compounds men-

MOLARITY: A quantitative means of

showing the concentration of solute to

solution. This is measured in moles of

solute per liter of solution, abbreviated

mol/L.

PURE SUBSTANCE: A substance—

either an element or compound—that has

an unvarying composition. This means

that by changing the proportions of atoms,

the result would be an entirely different

substance. Compare with mixture.

QUALITATIVE: Involving a compari-

son between qualities that are not precisely

defined, such as “fast” and “slow” or

“warm” and “cold.”

QUANTITATIVE: Involving a compari-

son between precise quantities—for

instance, 10 lb vs. 100 lb, or 50 MPH vs. 120

MPH.

SATURATED: A qualitative term

describing a solution that contains as

much solute as it can dissolve at a given

temperature.

SOLUBILITY: In a broad, qualitative

sense, solubility refers to the property of

being soluble. Chemists, however, usually

apply the word in a quantitative sense, to

indicate the maximum amount of a sub-

stance that dissolves in a given amount of

solvent at a specific temperature. Solubility

is usually expressed in grams of solute per

100 g of solvent.

SOLUBLE: Capable of dissolving in a

solvent.

SOLUTE: The substance or substances

that are dissolved in a solvent to form a

solution.



solute) is dissolved in a solvent—for exam-


SOLVENT: The substance or sub-

stances that dissolve a solute to form a

solution.

SURFACTANT: A substance made up

of molecules that are both water- and oil-

soluble, which acts as an agent for joining

other substances in an emulsion.

UNSATURATED: A term describing a

solution that is capable of dissolving

additional solute, if that solute is intro-

duced to it.



Solutions tioned, carbon dioxide and water (in vaporform); and, high in the atmosphere, the triatom-ic form of oxygen known as ozone (O3).

The most significant solid solutions arealloys of metals, discussed in the essay on Mix-tures, as well as in essays on various metal fami-lies, particularly the Transition Metals. Somewell-known alloys include bronze (three-quar-ters copper, one-quarter tin); brass (two-thirdscopper, one-third zinc); pewter (a mixture of tinand copper with traces of antimony); andnumerous alloys of iron—particularly steel—aswell as alloys involving other metals.

WHERE TO LEARN MORE

“Aqueous Solutions” (Web site). <http://www.tannerm.com/aqueous.htm> (June 6, 2001).

Gibson, Gary. Making Things Change. Brookfield, CT:Copper Beech Books, 1995.


“Introduction to Solutions” (Web site). <http://edie.cprost.sfu.ca/~rhlogan/solintro.html> (June 6, 2001).

Knapp, Brian J. Air and Water Chemistry. Illustrated byDavid Woodroffe. Danbury, CT: Grolier Educational,1998.

Seller, Mick. Elements, Mixtures, and Reactions. NewYork: Shooting Star Press, 1995.

“Solubility.” Spark Notes (Web site). <http://www.sparknotes.com/chemistry/solutions/solubility/terms.html> (June 6, 2001).

“Solutions and Solubility” (Web site). <http://educ.queensu.ca/~science/main/concept/chem/c10/c101amo1.htm> (June 6, 2001).

Watson, Philip. Liquid Magic. Illustrated by ElizabethWood and Ronald Fenton. New York: Lothrop, Lee,and Shepard Books, 1982.





OSMOS ISOsmosis

CONCEPTThe term osmosis describes the movement of asolvent through a semipermeable membranefrom a less concentrated solution to a more con-centrated one. Water is sometimes called “theperfect solvent,” and living tissue (for example, ahuman being’s cell walls) is the best example of asemipermeable membrane. Osmosis has a num-ber of life-preserving functions: it assists plantsin receiving water, it helps in the preservation offruit and meat, and is even used in kidney dialy-sis. In addition, osmosis can be reversed toremove salt and other impurities from water.

HOW IT WORKSIf you were to insert a hollow tube of a certaindiameter into a beaker of water, the water wouldrise inside the tube and reach the same level asthe water outside it. But suppose you sealed thebottom end of the tube with a semipermeablemembrane, then half-filled the tube with saltwater and again inserted it into the beaker. Overa period of time, the relative levels of the saltwater in the tube and the regular water in thebeaker would change, with the fresh water grad-ually rising into the beaker.

This is osmosis at work; however, beforeinvestigating the process, it is necessary to under-stand at least three terms. A solvent is a liquidcapable of dissolving or dispersing one or moreother substances. A solute is the substance that isdissolved, and a solution is the resulting mixtureof solvent and solute. Hence, when you mix apacket of sugar into a cup of hot coffee, the cof-fee—which is mostly water—acts as a solvent forthe sugar, a solute, and the resulting sweetenedcoffee is a solution. (Indeed, people who need a

cup of coffee in the morning might say that it isa “solution” in more ways than one!) The relativeamount of solute in the solution determineswhether it can be described as more or less con-centrated.

Water and Oil: MolecularDifferences

In the illustrations involving the beaker and thehollow tube, water plays one of its leading roles,as a solvent. It is possible to use a number ofother solvents for osmosis, but most of the onesthat will be discussed here are water-based sub-stances. In fact, virtually everything people drinkis either made with water as its central compo-nent (soft drinks, coffee, tea, beer and spirits), orcomes from a water-based plant or animal lifeform (fruit juices, wine, milk.) Then of coursethere is water itself, still the world’s most populardrink.

By contrast, people are likely to drink an oilyproduct only in extreme circumstances: forinstance, to relieve constipation, holistic-healthpractitioners often recommend a mixture ofolive oil and other compounds for this purpose.Oil, unlike water, has a tendency to pass straightthrough a person’s system, without largeamounts of it being absorbed through osmosis.In fact, oil and water differ significantly at themolecular level.

Water is the best example of a polar mole-cule, sometimes called a dipole. As everyoneknows, water is a name for the chemical H2O, inwhich two relatively small hydrogen atoms bondwith a large atom of oxygen. You can visualize awater molecule by imagining oxygen as a basket-ball with hydrogen as two baseballs fused to the


Osmosis


basketball’s surface. Bonded together as they are,the oxygen tends to pull electrons from thehydrogen atoms, giving it a slight negative chargeand the hydrogen a slight positive charge.

As a result, one end of a water molecule hasa positive electrical charge, and the other end anegative charge. This in turn causes the positiveend of one molecule to attract the negative sideof its neighbor, and vice versa. Though the elec-tromagnetic force is weak, even in relative terms,it is enough to bond water molecules tightly toone another.

By contrast, oily substances—whether theoil is animal-, vegetable-, or petroleum-based—are typically nonpolar, meaning that the positiveand negative charges are distributed evenly

across the surface of the molecule. Hence, thebond between oil molecules is much less tightthan for water molecules. Clean motor oil in acar’s crankshaft behaves as though it were madeof millions of tiny ball-bearings, each rollingthrough the engine without sticking. Water, onthe other hand, has a tendency to stick to sur-faces, since its molecules are so tightly bonded toone another.

This tight bond gives water highly unusualproperties compared to other substances close toits molecular weight. Among these are its highboiling point, its surprisingly low density whenfrozen, and the characteristics that make osmosispossible. Thanks to its intermolecular structure,water is not only an ideal solvent, but its closely

A SEMIPERMEABLE MEMBRANE PERMITS ONLY CERTAIN COMPONENTS OF SOLUTIONS, USUALLY THE SOLVENT, TO PASS

THROUGH.


Osmosis


packed structure enables easy movement, as, forinstance, from an area of low concentration to anarea of high concentration.

In the beaker illustration, the “pure” water isalmost pure solvent. (Actually, because of its sol-vent qualities, water seldom appears in a purestate unless one distills it: even water flowingthrough a “pure” mountain stream carries allsorts of impurities, including microscopic parti-cles of the rocks over which it flows.) In any case,the fact that the water in the beaker is almostpure makes it easy for it to flow through thesemipermeable membrane in the bottom of thetube. By contrast, the solute particles in the salt-water solution have a much harder time passingthrough, and are much more likely to block theopenings in the membrane. As a result, the move-ment is all in one direction: water in the beakermoves through the membrane, and into the tube.

A few points of clarification are in orderhere. A semipermeable membrane is anythingwith a structure somewhere between that of, say,plastic on the one hand and cotton on the other.Were the tube in the beaker covered with Saranwrap, for instance, no water would pass through.On the other hand, if one used a piece of cottonin the bottom of the tube, the water would passstraight through without osmosis taking place. Incontrast to cotton, Gore-tex is a fabric containing

a very thin layer of plastic with billions of tinypores which let water vapor flow out withoutallowing liquid water to seep in. This accountsfor the popularity of Gore-tex for outdoorgear—it keeps a person dry without holding intheir sweat. So Gore-tex would work well as asemipermeable membrane.

Also, it is important to consider the possibil-ities of what can happen during the process ofosmosis. If the tube were filled with pure salt, orsalt with only a little water in it, osmosis wouldreach a point and then stop due to osmotic pres-sure within the substance. Osmotic pressureresults when a relatively concentrated substancetakes in a solvent, thus increasing its pressureuntil it reaches a point at which the solution willnot allow any more solvent to enter.


Cell Behavior and Salt Water

Cells in the human body and in the bodies of allliving things behave like microscopic bags ofsolution housed in a semipermeable membrane.The health and indeed the very survival of a per-son, animal, or plant depends on the ability of

OSMOSIS EQUALIZES CONCENTRATION.


Osmosis


the cells to maintain their concentration ofsolutes.

Two illustrations involving salt waterdemonstrate how osmosis can produce disas-trous effects in living things. If you put a carrot insalty water, the salt water will “draw” the waterfrom inside the carrot—which, like the humanbody and most other forms of life, is mostlymade up of water. Within a few hours, the carrotwill be limp, its cells shriveled.

Worse still is the process that occurs when aperson drinks salt water. The body can handle alittle bit, but if you were to consume nothing butsalt water for a period of a few days, as in the caseof being stranded on the proverbial desert island,the osmotic pressure would begin drawing waterfrom other parts of your body. Since a humanbody ranges from 60% water (in an adult male)to 85% in a baby, there would be a great deal ofwater available—but just as clearly, water is theessential ingredient in the human body. If youcontinued to ingest salt water, you would eventu-ally experience dehydration and die.

How, then, do fish and other forms ofmarine life survive in a salt-water environment?In most cases, a creature whose natural habitat isthe ocean has a much higher solute concentra-tion in its cells than does a land animal. Hence,

for them, salt water is an isotonic solution, or onethat has the same concentration of solute—andhence the same osmotic pressure—as in theirown cells.

Osmosis in Plants

Plants depend on osmosis to move water fromtheir roots to their leaves. The further toward theedge or the top of the plant, the greater the soluteconcentration, which creates a difference inosmotic pressure. This is known as osmoticpotential, which draws water upward. In addi-tion, osmosis protects leaves against losing waterthrough evaporation.

Crucial to the operation of osmosis in plantsare “guard cells,” specialized cells dispersed alongthe surface of the leaves. Each pair of guard cellssurrounds a stoma, or pore, controlling its abili-ty to open and thus release moisture.

In some situations, external stimuli such assunlight may cause the guard cells to draw inpotassium from other cells. This leads to anincrease in osmotic potential: the guard cellbecomes like a person who has eaten a dry bis-cuit, and is now desperate for a drink of water towash it down. As a result of its increased osmot-ic potential, the guard cell eventually takes on

IN THIS 1966 PHOTO, A GIRL IS UNDERGOING KIDNEY DIALYSIS, A CRUCIAL MODERN MEDICAL APPLICATION THAT

RELIES ON OSMOSIS. (Bettmann/Corbis. Reproduced by permission.)


Osmosiswater through osmosis. The guard cells thenswell with water, opening the stomata andincreasing the rate of gas exchange throughthem. The outcome of this action is an increasein the rate of photosynthesis and plant growth.

When there is a water shortage, however,other cells transmit signals to the guard cells thatcause them to release their potassium. Thisdecreases their osmotic potential, and waterpasses out of the guard cells to the thirsty cellsaround them. At the same time, the resultantshrinkage in the guard cells closes the stomata,decreasing the rate at which water transpiresthrough them and preventing the plant fromwilting.

Osmosis and Medicine

Osmosis has several implications where medicalcare is concerned, particularly in the case of thestorage of vitally important red blood cells. Theseare normally kept in a plasma solution which isisotonic to the cells when it contains specific pro-portions of salts and proteins. However, if redblood cells are placed in a hypotonic solution, orone with a lower solute concentration than in thecells themselves, this can be highly detrimental.

Hence water, a life-giving and life-preservingsubstance in most cases, is a killer in this context.If red blood cells were stored in pure water,osmosis would draw the water into the cells,causing them to swell and eventually burst. Sim-ilarly, if the cells were placed in a solution with ahigher solute concentration, or hypertonic solu-tion, osmosis would draw water out of the cellsuntil they shriveled.

In fact, the plasma solution used by mosthospitals for storing red blood cells is slightlyhypertonic relative to the cells, to prevent themfrom drawing in water and bursting. Physiciansuse a similar solution when injecting a drugintravenously into a patient. The active ingredi-ent of the drug has to be suspended in some kindof medium, but water would be detrimental forthe reasons discussed above, so instead the doc-tor uses a saline solution that is slightly hyper-tonic to the patient’s red blood cells.

One vital process closely linked to osmosis isdialysis, which is critical to the survival of manyvictims of kidney diseases. Dialysis is the processby which an artificial kidney machine removeswaste products from a patients’ blood—per-forming the role of a healthy, normally function-

ing kidney. The openings in the dialyzing mem-brane are such that not only water, but salts andother waste dissolved in the blood, pass throughto a surrounding tank of distilled water. The redblood cells, on the other hand, are too large toenter the dialyzing membrane, so they return tothe patient’s body.

Preserving Fruits and Meats

Osmosis is also used for preserving fruits andmeats, though the process is quite different forthe two. In the case of fruit, osmosis is used todehydrate it, whereas in the preservation of meat,osmosis draws salt into it, thus preventing theintrusion of bacteria.

Most fruits are about 75% water, and thismakes them highly susceptible to spoilage. Topreserve fruit, it must be dehydrated, which—asin the case of the salt in the meat— presents bac-teria with a less-than-hospitable environment.Over the years, people have tried a variety ofmethods for drying fruit, but most of these havea tendency to shrink and harden the fruit. Thereason for this is that most drying methods, suchas heat from the Sun, are relatively quick anddrastic; osmosis, on the other hand, is slower,more moderate—and closer to the behavior ofnature.

Osmotic dehydration techniques, in fact,result in fruit that can be stored longer than fruitdehydrated by other methods. This in turn makesit possible to provide consumers with a widervariety of fruit throughout the year. Also, thefruit itself tends to maintain more of its flavorand nutritional qualities while keeping outmicroorganisms.

Because osmosis alone can only removeabout 50% of the water in most ripe fruits, how-ever, the dehydration process involves a second-ary method as well. First the fruit is blanched, orplaced briefly in scalding water to stop enzymat-ic action. Next it is subjected to osmotic dehy-dration by dipping it in, or spreading it with, aspecially made variety of syrup whose sugardraws out the water in the fruit. After this, airdrying or vacuum drying completes the process.The resulting product is ready to eat; can be pre-served on a shelf under most climatic conditions;and may even be powdered for making confec-tionery items.

Whereas osmotic dehydration of fruit is cur-rently used in many parts of the world, the salt-



Osmosis

curing of meat in brine is largely a thing of thepast, due to the introduction of refrigeration.Many poorer families, even in the industrializedworld, however, remained without electricitylong after it spread throughout most of Europeand North America. John Steinbeck’s Grapes ofWrath (1939) offers a memorable scene in whicha contemporary family, the Joads, kill and cure apig before leaving Oklahoma for California. Anda Web site for Walton Feed, an Idaho companyspecializing in dehydrated foods, offers reminis-cences by Canadians whose families were stillsalt-curing meats in the middle of the twentiethcentury. Verla Cress of southern Alberta, forinstance, offered a recipe from which the follow-ing details are drawn.


First a barrel is filled with a solution con-taining 2 gal (7.57 l) of hot water and 8 oz (.2268kg) of salt, or 32 parts hot water to one part salt,as well as a small quantity of vinegar. The pig orcow, which would have just been slaughtered,should then be cut up into what Cress called“ham-sized pieces (about 10-15 lb [5-7 kg])each.” The pieces are then soaked in the brinebarrel for six days, after which the meat isremoved, dried, “and put... in flour or gunnysacks to keep the flies away. Then hang it up in acool dry place to dry. It will keep like this for per-haps six weeks if stored in a cool place during theSummer. Of course, it will keep much longer inthe Winter. If it goes bad, you’ll know it!”

Cress offered another method, one still usedon ham today. Instead of salt, sugar is used in amixture of 32 oz (.94 l) to 3 gal (11.36 l) of water.After being removed, the meat is smoked—thatis, exposed to smoke from a typically aromaticwood such as hickory, in an enclosed barn—forthree days. Smoking the meat tends to make itlast much longer: four months in the summer,according to Cress.

The Walton Feeds Web page included anoth-er brine-curing recipe, this one used by thewomen of the Stirling, Alberta, Church of theLatter-Day Saints in 1973. Also included werereminiscences by Glenn Adamson (born 1915):“...When we butchered a pig, Dad filled a wood-en 45-gal (170.34 l) barrel with salt brine. We cutup the pig into maybe eight pieces and put it inthe brine barrel. The pork soaked in the barrelfor several days, then the meat was taken out, andthe water was thrown away.... In the hot summerdays after they [the pieces of meat] had dried,they were put in the root cellar to keep them cool.The meat was good for eating two or threemonths this way.”

For thousands of years, people used salt tocure and preserve meat: for instance, the sailingships that first came to the New World carried onboard barrels full of cured meat, which fed sailorson the voyage over. Meat was not the only type offood preserved through the use of salt or brine,which is hypertonic—and thus lethal—to bacte-ria cells. Among other items packed in brine werefish, olives, and vegetables.

Even today, some types of canned fish cometo the consumer still packed in brine, as do olives.Another method that survives is the use of

HYPERTONIC: Of higher osmotic

pressure.

HYPOTONIC: Of lower osmotic

pressure.

ISOTONIC: Of equal osmotic pressure.

OSMOSIS: The movement of a solvent

through a semipermeable membrane from

a less concentrated solution to a more con-

centrated one.

OSMOTIC POTENTIAL: A difference

in osmotic pressure that draws water from

an area of less osmotic pressure to an area

of greater osmotic pressure.

OSMOTIC PRESSURE: The pressure

that builds in a substance as it experiences

osmosis, and eventually stops that process.

SOLVENT: A liquid capable of dis-

solving or dispersing one or more other

substances.

SOLUTE: A substance capable of being

dissolved in a solvent.

SOLUTION: A mixture of solvent and

solute.

KEY TERMS


Osmosissugar—which can be just as effective as salt forkeeping out bacteria—to preserve fruit in jam.

Reverse Osmosis

Given the many ways osmosis is used for pre-serving food, not to mention its many interac-tions with water, it should not be surprising todiscover that osmosis can also be used for desali-nation, or turning salt water into drinking water.Actually, it is not osmosis, strictly speaking, butrather reverse osmosis that turns salt water fromthe ocean—97% of Earth’s water supply—intowater that can be used for bathing, agriculture,and in some cases even drinking.

When you mix a teaspoon of sugar into acup of coffee, as mentioned in an earlier illustra-tion, this is a non-reversible process. Short ofsome highly complicated undertaking—forinstance, using ultrasonic sound waves—it would be impossible to separate solute and solvent.

Osmosis, on the other hand, can be reversed.This is done by using a controlled external pres-sure of approximately 60 atmospheres, an atmos-phere being equal to the air pressure at sealevel—14.7 pounds-per-square-inch (1.013 !105 Pa.) In reverse osmosis, this pressure isapplied to the area of higher solute concentra-tion—in this case, the seawater. As a result, thepressure in the seawater pushes water moleculesinto a reservoir of pure water.

If performed by someone with a few rudi-mentary tools and a knowledge of how to pro-vide just the right amount of pressure, it is possi-ble that reverse osmosis could save the life of ashipwreck victim stranded in a location withouta fresh water supply. On the other hand, a personin such a situation may be able to absorb suffi-cient water from fruits and plant life, as TomHanks’s character did in the 2001 film Cast Away.

Companies such as Reverse Osmosis Sys-tems in Atlanta, Georgia, offer a small unit forhome or business use, which actually performsthe reverse-osmosis process on a small scale. Theunit makes use of a process called crossflow,which continually cleans the semipermeablemembrane of impurities that have been removedfrom the water. A small pump provides the pres-sure necessary to push the water through themembrane. In addition to an under-the-sinkmodel, a reverse osmosis water cooler is alsoavailable.

Not only is reverse osmosis used for makingwater safe, it is also applied to metals in a varietyof capacities, not least of which is its use in treat-ing wastewater from electroplating. But there areother metallurgical methods of reverse osmosisthat have little to do with water treatment: metalfinishing, as well as recycling of metals andchemicals. These processes are highly complicat-ed, but they involve the same principle of remov-ing impurities that governs reverse osmosis.

WHERE TO LEARN MORE

Francis, Frederick J., editor-in-chief. Encyclopedia of FoodScience and Technology. New York: Wiley, 2000.

Gardner, Robert. Science Project Ideas About KitchenChemistry. Berkeley, N.J.: Enslow Publishers, 2002.

Laschish, Uri. “Osmosis, Reverse Osmosis, and OsmoticPressure: What They Are” (Web site). <http://members.tripod.com/~urila/> (February 20, 2001).

“Lesson 5: Osmosis” (Web site). <http://www.biologylessons.sdsu.edu/classes/lab5/semnet/> (Feb-ruary 20, 2001).

Rosenfeld, Sam. Science Experiments with Water. Illus-trated by John J. Floherty, Jr. Irvington-on-Hudson,NY: Harvey House, 1965.

“Salt-Curing Meat in Brine.” Walton Feed (Web site).<http://waltonfeed.com/old/brine.html> (February20, 2001).




D I S T I L L AT ION AND F I LTR AT IONDistillation and Filtration

CONCEPTWhen most people think of chemistry, they thinkabout joining substances together. Certainly, thebonding of elements to form compoundsthrough chemical reactions is an integral compo-nent of the chemist’s study; but chemists are alsoconcerned with the separation of substances.Some forms of separation, in which compoundsare returned to their elemental form, or in whichatoms split off from molecules to yield a com-pound and a separated element, are complexphenomena that require chemical reactions. Butchemists also use simpler physical methods,distillation and filtration, to separate mixtures.Distillation can be used to purify water, makealcohols, or separate petroleum into variouscomponents that range from natural gas to gaso-line to tar. Filtration also makes possible the sep-aration of gases or liquids from solids, andsewage treatment—a form of filtration—sepa-rates liquids, solids, and gases through a series ofchemical and physical processes.

HOW IT WORKS

Mixtures: Homogeneous andHeterogeneous

In contrast to a pure substance, a class thatincludes only elements and compounds, mix-tures constitute a much broader category. Usual-ly, a mixture consists of many compounds mixedtogether, but it is possible to have a mixture (airis an example) in which elemental substances arecombined with compounds. Elements alone canbe combined in a mixture, as when copper andzinc are alloyed to make brass. It is also possibleto have a mixture of mixtures, as for example

when milk (a particular type of mixture called anemulsion) is added to coffee.

It is sometimes difficult, without studyingthe chemical aspects involved, to recognize thedifference between a mixture and a pure sub-stance. A mixture, however, can be defined as asubstance with a variable composition, meaningthat if more of one component is added, it doesnot change the essential character of the mixture.People unconsciously recognize this when theyjoke, “Would you like a little coffee with yourmilk?”

This comment, made when someone puts agreat quantity of milk into a cup of coffee,implies that we generally regard coffee and milkas a solution in which the coffee dissolves themilk. But even if someone made a cup of coffeethat was half milk, so that the resulting substancehad a vanilla-like color, we would still call it cof-fee. If the components of a pure substance arealtered, however, it becomes something entirelydifferent.

Two hydrogen atoms bonded to an oxygenatom create water, but when two hydrogen atomsbond to two oxygen atoms, this is hydrogen per-oxide, an altogether different substance. Water atordinary temperatures does not bubble, whereashydrogen peroxide does. Hydrogen peroxideboils at a temperature more than 1.5 times theboiling point of water, and potatoes boiled inhydrogen peroxide would be nothing like pota-toes boiled in water. Eating them, in fact, couldvery well be fatal.

HOMOGENEOUS VS. HETERO-

GENEOUS. Unlike a compound or element,a mixture can never be broken down to a singletype of molecule or atom, yet there are mixtures


Distillationand

Filtration

that appear to be the same throughout. These arecalled homogeneous mixtures, in which theproperties and characteristics are the samethroughout the mixture. In a heterogeneous mix-ture, on the other hand, the composition is notthe same throughout, and in fact the mixture isseparated into regions with varying properties.

Chemists of the past sometimes had difficul-ty distinguishing between homogeneous mix-tures—virtually all of which can be described assolutions—and pure substances. Air, for exam-ple, appears to be a pure substance; yet there is nosuch thing as an “air molecule.” Instead, air iscomposed primarily of nitrogen and oxygen,which appear in molecular rather than atomicform, along with separated atoms of noble gasesand two important compounds: carbon dioxideand water. (Water exists in air as a vapor.)

There is less probability of confusing a het-erogeneous mixture with a pure substance,because one of the defining aspects of a hetero-geneous substance is the fact that it is clearly amixture. When sand is added to a container ofwater, and the sand sinks to the bottom, it isobvious that there is no “sand-and-water” mole-cule pervading the entire mixture. Clearly, theupper portion of the container is mostly water,and the lower portion mostly sand.

Separating Mixtures

There are two basic processes for separating mix-tures, distillation and filtration. In general, theseare applied for the separation of homogeneousand heterogeneous mixtures, respectively. Distil-lation is the use of heat to separate the compo-nents of a liquid and/or gas, while filtration is theseparation of solids from a fluid (either a gas or aliquid) by allowing the fluid to pass through a filter.

In a solution such as salt water, there are twocomponents: the solvent (the water) and thesolute (the salt). These can be separated by distil-lation in a laboratory using a burner placedunder a beaker containing the salt water. As thewater is heated, it passes out of the beaker in theform of steam, and travels through a tube cooledby a continual flow of cold water. Inside the tube,the steam condenses to form liquid water, whichpasses into a second beaker. Eventually, all of thesolvent will be distilled from the first beaker,leaving behind the salt that constituted the soluteof the original solution.

Distillation may seem like a chemicalprocess, but in fact it is purely physical, becausethe composition of neither compound—salt orwater—has been changed. As for filtration, it isclearly a physical process, just as heterogeneousmixtures are more obviously mixtures and notcompounds. Suppose we wanted to separate theheterogeneous mixture we described earlier, ofsand in water: all we would need would be amesh screen through which the liquid could bestrained, leaving behind the sand.

Despite the apparent simplicity of filtration,it can become rather complicated, as we shall see,in the treatment of sewage to turn it into waterthat poses no threat to the environment. Further-more, it should be noted that sometimes theseprocesses—distillation and filtration—are usedin tandem. Suppose we had a mixture of sandand salt water taken from the beach, and wewanted to separate the mixture. The first stepwould involve the simpler process of filtration,separating the sand from the salt water. Thiswould be followed by the separation of purewater and salt through the distillation process.


Distillation

In distillation, the more volatile component ofthe mixture—that is, the part that is more easilyvaporized—is separated from the less volatileportion. With regard to the illustration usedabove, of separating water from salt, clearly thewater is the more volatile portion: its boilingpoint is much, much lower than that of salt, andthe heat required to vaporize the salt is so greatthat the water would be long since vaporized bythe time the salt was affected.

Once the volatile portion has been vapor-ized, or turned into distillate, it experiences con-densation—the cooling of a gas to form a liquid,after which the liquid or condensate is collected.Note that in all these stages of distillation, heat isthe agent of separation. This is especially impor-tant in the process of fractional distillation, dis-cussed below.

The process of removing water vapor fromsalt water, described earlier, is an example ofwhat is called “single-stage differential distilla-tion.” Sometimes, however—especially when a




high level of purity in the resulting condensate isdesired—it may be necessary to subject the con-densate to multiple processes of distillation toremove additional impurities. This process iscalled rectification.

PURIFIED WATER AND ETHA-

NOL. If distillation can be used to separatewater from salt, obviously it can also be used toseparate water from microbes and other impuri-ties through a more detailed process of rectifica-tion. Distilled water, purchased for drinking andother purposes, is just one of the more commonapplications of the distillation process. Anotherwell-known use of this process is the productionof ethyl alcohol, made famous (or perhaps infa-mous) by stories of bootleggers producing

homemade whiskey from “stills” or distilleries inthe woods.

Ethanol, the alcohol in alcoholic beverages,is produced by the fermentation of glucose, asugar found in various natural substances. Thechoice of substances is a function of the desiredproduct: grapes for wine; barley and hops forbeer; juniper berries for gin; potatoes for vodka,and so on. Glucose reacts with yeast, fermentingto yield ethanol, and this reaction is catalyzed byenzymes in the yeast. The reaction continuesuntil the alcohol content reaches a point equal toabout 13%.

After that point, the yeast enzymes can nolonger survive, and this is where the productionof beer or wine usually ends. Fortified wine or

A WATER FILTRATION PLANT IN CONTRA COSTA COUNTY, CALIFORNIA. (James A. Sugar/Corbis. Reproduced by permission.)

Distillationand Filtration



malt liquor—variations of wine and beer, respec-tively, that are more than 13% alcohol, or 26proof—are exceptions. These two products arethe result of mixtures between undistilled beerand wine and products of distillation having ahigher alcohol content.

Distillation constitutes the second stage inthe production of alcohol. Often, the fermenta-tion products are subjected to a multistage recti-fication process, which yields a mixture of up to95% ethanol and 5% water. This mixture is calledan azeotrope, meaning that it will not changecomposition when distilled further. Usually theproduction of whiskey or other liquor stops wellbelow the point of yielding an azeotrope, but in

some states, brands of grain alcohol at 190 proof(95%) are sold.

INDUSTRIAL DISTILLATION.

In contrast to ethanol, methanol or “wood alco-hol” is a highly toxic substance whose ingestioncan lead to blindness or death. It is widely used inindustry for applications in adhesives, plastics,and other products, and was once produced bythe distillation of wood. In this old productionmethod, wood was heated in the absence of air,such that it did not burn, but rather decomposedinto a number of chemicals, a fraction of whichwere methanol and other alcohols.

The distillation of wood alcohol was aban-doned for a number of reasons, not least ofwhich was the environmental concern: thou-

FEW INDUSTRIES EMPLOY DISTILLATION TO A GREATER DEGREE THAN DOES THE PETROLEUM INDUSTRY. SHOWN HERE

IS AN OIL REFINERY. (Mark L. Stephenson/Corbis. Reproduced by permission.)

Distillationand

Filtration




sands of trees had to be cut down for use in aninefficient process yielding only small portions ofthe desired product. Today, methanol is pro-duced from synthesis gas, itself a product of coalgasification; nonetheless, numerous industrialprocesses use distillation.

Distillation is employed, for instance, to sep-arate the hydrocarbons benzene and toluene, aswell as acetone from acetic acid. In nuclear powerplants that require quantities of the hydrogenisotope deuterium, the lighter protium isotope,or plain hydrogen, is separated from the heavierdeuterium by means of distillation. Few indus-tries, however, employ distillation to a greaterdegree than the petroleum industry.

Through a process known as fractional dis-tillation, oil companies separate hydrocarbonsfrom petroleum, allowing those of lower molec-ular mass to boil off and be collected first. Forinstance, at temperatures below 96.8°F (36°C),natural gas separates from petroleum. Other sub-stances, including petroleum ether and naphtha,separate before the petroleum reaches the156.2°F-165.2°F (69°C-74°C) range, at whichpoint gasoline separates.

Fractional distillation continues, with theseparation of other substances, all the way up to959°F (515°C), above which tar becomes the lastitem to be separated. Petroleum and petroleum-product companies account for a large portion ofthe more than 40,000 distillation units in opera-tion across the country. About 95% of all indus-trial separation processes involve distillation,which consumes large amounts of energy; forthat reason, ever more efficient forms of distilla-tion are continually being researched.

Filtration

In the simplest form of filtration, described ear-lier, a suspension (solid particles floating in a liq-uid) is allowed to pass through filter paper sup-ported in a glass funnel. The filter paper traps thesolid particles, allowing a clear solution (the fil-trate) to pass through the funnel and into areceiving container.

There are two basic purposes for filtration:either to capture the solid material suspended inthe fluid, or to clarify the fluid in which the solidis suspended. An example of the former occurs,for instance, when panning for gold: the water isallowed to pass through a sieve, leaving behindrocks that—the prospector hopes—contain gold.

An example of the latter occurs in sewage treat-ment. Here the solid left behind is feces—asundesirable as gold is desirable.

Filtration can further be classified as eithergaseous or liquid, depending on which of the flu-ids constitutes the filtrate. Furthermore, the forcethat moves the fluid through the filter—whethergravity, a vacuum, or pressure—is another defin-ing factor. The type of filter used can also serve todistinguish varieties of filtration.

LIQUID FILTRATION. In liquid fil-tration, such as that applied in sewage treatment,a liquid can be pulled through the filter by grav-itational force, as in the examples given. On theother hand, some sort of applied pressure, or apressure differential created by the existence of avacuum, can force the liquid through the filter.

Water filtration for purification purposes isoften performed by means of gravitation. Usual-ly, water is allowed to run down through a thicklayer of granular material such as sand, gravel, orcharcoal, which removes impurities. These layersmay be several feet thick, in which case the filteris known as a deep-bed filter. Water purificationplants may include fine particles of charcoal,known as activated carbon, in the deep-bed filterto absorb unpleasant-smelling gases.

For separating smaller volumes of solution,a positive-pressure system—one that uses exter-nal pressure to push the liquid through the fil-ter—may be used. Fluid may be introducedunder pressure at one end of a horizontal tank,then forced through a series of vertical platescovered with thick filtering cloths that collectsolids. In a vacuum filter, a vacuum (an area vir-tually devoid of matter, including air) is createdbeneath the solution to be filtered, and as a result,atmospheric pressure pushes the liquid throughthe filter.

When the liquid is virtually freed of impuri-ties, it may be passed through a second variety offilter, known as a clarifying filter. This type of fil-ter is constructed of extremely fine-mesh wiresor fibers designed to remove the smallest parti-cles suspended in the liquid. Clarifying filtersmay also use diatomaceous earth, a finely pow-dered material produced from the decay ofmarine organisms.

GAS FILTRATION. Gas filtration isused in a common appliance, the vacuum clean-er, which passes a stream of dust-filled air


Distillationand

Filtration

through a filtering bag inside the machine. Thebag traps solid particles, while allowing clean airto pass back out into the room. This is essential-ly the same principle applied in air filters andeven air conditioning and heating systems,which, in addition to regulating temperature,also remove dust, pollen, and other impuritiesfrom the air.

Various industries use gas filtration, not onlyto purify the products released into the atmos-phere, but also to filter the air in the workplace,as for instance in a factory room where fine air-borne particles are a hazard of production. Thegases from power plants that burn coal and oilare often purified by passing them through filter-ing systems that collect particles.


AZEOTROPE: A solution that has been

subjected through distillation and is still

not fully separated, but cannot be separat-

ed any further by means of distillation. An

example is ethanol, which cannot be dis-

tilled beyond the point at which it reaches

a 95% concentration with water.

CONDENSATE: The liquid product

that results from distillation.

CONDENSATION: The cooling of a

gas to form a liquid or condensate.

DISTILLATE: The vapor collected from

the volatile material or materials in distilla-

tion. This vapor is then subjected to con-

densation.

DISTILLATION: The use of heat to

separate the components of a liquid and/or

gas. Generally, distillation is used to sepa-

rate mixtures that are homogeneous.

FILTRATE: The liquid or gas separated

from a solid by means of filtration.

FILTRATION: The separation of solids

from a fluid (either gas or liquid) by allow-

ing the fluid to pass through a filter. Gen-

erally, filtration is used to separate mix-

tures that are heterogeneous.




ing different properties. An example is

sand in a container of water.





mixture.


composition, composed of molecules or

atoms of differing types. Compare with

pure substance.

PURE SUBSTANCE: A substance—

either an element or compound—that has

an unvarying composition. This means

that by changing the proportions of atoms,

the result would be an entirely different

substance. Compare with mixture.

SOLUTE: The substance or substances

that are dissolved in a solvent to form a

solution.


in which one or more substances is dis-

solved in an another substance—for exam-


SOLVENT: The substance that dissolves

a solute to form a solution.

SUSPENSION: A term that refers to a

mixture in which solid particles are sus-

pended in a fluid.

VOLATILE: Easily vaporized.

KEY TERMS



Sewage Treatment

Sewage treatment is a complex, multistageprocess whereby waste water is freed of harmfulcontents and rendered safe, so that it can bereturned to the environment. This is a criticalpart of modern life, because when people con-gregate in cities, they generate huge amounts ofwastes that contain disease-causing bacteria,viruses, and other microorganisms. Lack ofproper waste management in ancient andmedieval times led to devastating plagues incities such as Athens, Rome, and Constantinople.

Indeed, one of the factors contributing tothe end of the Athenian golden age of the fifthcentury B.C. was a plague, caused by lack of prop-er sewage-disposal methods, that felled many ofthe city’s greatest figures. The horrors created byimproper waste management reached theirapogee in 1347-51, when microorganisms car-ried by rats brought about the Black Death, inwhich Europe’s population was reduced by one-third.

AEROBIC AND ANAEROBIC

DECAY. Small amounts of sewage can be dealtwith through aerobic (oxygen-consuming)decay, in which microorganisms such as bacteriaand fungi process the toxins in human waste.With larger amounts of waste, however, oxygen isdepleted and the decay mechanism must beanaerobic, or carried out in the absence ofoxygen.

For people who are not connected to amunicipal sewage system, and therefore mustrely on a septic tank for waste disposal, wasteproducts pass through a tank in which they aresubjected to anaerobic decay by varieties ofmicroorganisms that do not require oxygen tosurvive. From there, the waste goes through thedrain field, a network of pipes that allow thewater to seep into a deep-bed filter. In the drainfield, the waste is subjected to aerobic decay byoxygen-dependant microorganisms before iteither filters through the drain pipes into theground, or is evaporated.

MUNICIPAL WASTE TREAT-

MENT. Municipal waste treatment is a muchmore involved process, and will only be

described in the most general terms here. Essen-tially, large-scale sewage treatment requires theseparation of larger particles first, followed bythe separation of smaller particles, as the processcontinues. Along the way, the sewage is chemical-ly treated as well. The separation of smaller solidparticles is aided by aluminum sulfate, whichcauses these particles to settle to the bottommore rapidly.

Through continual processing by filtration,water is eventually clarified of biosolids (that is,feces), but it is still far from being safe. At thatpoint, all the removed biosolids are dried andincinerated; or they may be subjected to addi-tional processes to create fertilizer. The water, oreffluent, is further clarified by filtration in adeep-bed filter, and additional air may be intro-duced to the water to facilitate aerobic decay—for instance, by using algae.

Through a long series of such processes, thewater is rendered safe to be released back into theenvironment. However, wastes containingextremely high levels of toxins may require addi-tional or different forms of treatment.

WHERE TO LEARN MORE

“Case Study: Petroleum Distillation” (Web site).<http://www.pafko.com/history/h_distill.html>(June 6, 2001).

“Classification of Matter” (Web site). <http://www.cards.anoka.k12.mn.us/projects/html>_98/smith/smith2.ht ml (June 6, 2001).

“Fractional Distillation” (Web site). <http://www.geoci-ties.com/chemforum/fdistillation.htm> (June 6,2001).

“Introduction to Distillation” (Web site). <http://lorien.ncl.ac.uk/ming/distil/distil0.htm> (June 6, 2001).

Kellert, Stephen B. and Matthew Black. The Encyclopediaof the Environment. New York: Franklin Watts, 1999.


Seuling, Barbara. Drip! Drop! How Water Gets to YourTap. Illustrated by Nancy Tobin. New York: HolidayHouse, 2000.

“Sewage Treatment” (Web site). <http://www.dcs.exeter.ac.uk/water/sewage.htm> (June 6, 2001).




361

SC IENCE OF EVERYDAY TH INGSr e a l - l i f e CHEMISTRY

ORGAN IC CHEM ISTRY

ORGAN IC CHEM ISTRY

ORGAN IC CHEM ISTRY

POLYMERS



ORGAN IC CHEM ISTRYOrganic Chemistry

CONCEPTThere was once a time when chemists thought“organic” referred only to things that were living,and that life was the result of a spiritual “lifeforce.” While there is nothing wrong with view-ing life as having a spiritual component, spiritualmatters are simply outside the realm of science,and to mix up the two is as silly as using mathe-matics to explain love (or vice versa). In fact, the“life force” has a name: carbon, the commondenominator in all living things. Not everythingthat has carbon is living, nor are all the areasstudied in organic chemistry—the branch ofchemistry devoted to the study of carbon and itscompounds—always concerned with livingthings. Organic chemistry addresses an array ofsubjects as vast as the number of possible com-pounds that can be made by strings of carbonatoms. We can thank organic chemistry for muchof what makes life easier in the modern age: fuelfor cars, for instance, or the plastics found inmany of the products used in an average day.

HOW IT WORKS

Introduction to Carbon

As the element essential to all of life, and hencethe basis for a vast field of study, carbon isaddressed in its own essay. The Carbon essay, inaddition to examining the chemical properties ofcarbon (discussed below), approaches a numberof subjects, such as the allotropes of carbon.These include three crystalline forms (graphite,diamond, and buckminsterfullerene), as well asamorphous carbon. In addition, two oxides ofcarbon—carbon dioxide and carbon monox-ide—are important, in the case of the former, to

the natural carbon cycle, and in the case of thelatter, to industry. Both also pose environmentaldangers.

The purpose of this summary of the carbonessay is to provide a hint of the complexitiesinvolved with this sixth element on the periodictable, the 14th most abundant element on Earth.In the human body, carbon is second only to oxy-gen in abundance, and accounts for 18% of thebody’s mass. Capable of combining in seeminglyendless ways, carbon, along with hydrogen, is atthe center of huge families of compounds. Theseare the hydrocarbons, present in deposits of fos-sil fuels: natural gas, petroleum, and coal.

A PROPENSITY FOR LIMIT -

LESS BONDING. Carbon has a valenceelectron configuration of 2s22p2. Likewise all themembers of Group 4 on the periodic table(Group 14 in the IUPAC version of the table)—sometimes known as the “carbon family”—haveconfigurations of ns2np2, where n is the numberof the period or row the element occupies on thetable. There are two elements noted for their abil-ity to form long strings of atoms and seeminglyendless varieties of molecules: one is carbon, andthe other is silicon, directly below it on the peri-odic table.

Just as carbon is at the center of a vast net-work of organic compounds, silicon holds thesame function in the inorganic realm. It is foundin virtually all types of rocks, except the calciumcarbonates—which, as their name implies, con-tain carbon. In terms of known elemental mass,silicon is second only to oxygen in abundance onEarth. Silicon atoms are about one and a halftimes as large as those of carbon; thus not evensilicon can compete with carbon’s ability to form


OrganicChemistry


an almost limitless array of molecules in variousshapes and sizes, and with various chemicalproperties.

Electronegativity

Carbon is further distinguished by its high valueof electronegativity, the relative ability of anatom to attract valence electrons. To mention afew basic aspects of chemical bonding, developedat considerably greater length in the ChemicalBonding essay, if two atoms have an electriccharge and thus are ions, they form strong ionicbonds. Ionic bonding occurs when a metal bondswith a nonmetal. The other principal type ofbond is a covalent bond, in which two unchargedatoms share eight valence electrons. If the elec-tronegativity values of the two elements involvedare equal, then they share the electrons equally;but if one element has a higher electronegativityvalue, the electrons will be more drawn to thatelement.

The electronegativity of carbon is certainlynot the highest on the periodic table. That dis-tinction belongs to fluorine, with an electronega-tivity value of 4.0, which makes it the most reac-tive of all elements. Fluorine is at the head ofGroup 7, the halogens, all of which are highly

reactive and most of which have high electroneg-ativity values. If one ignores the noble gases,which are virtually unreactive and occupy theextreme right-hand side of the periodic table,electronegativity values are highest in the upperright-hand side of the table—the location of flu-orine—and lowest in the lower left. In otherwords, the value increases with group or columnnumber (again, leaving out the noble gases inGroup 8), and decreases with period or rownumber.

With an electronegativity of 2.5, carbon tieswith sulfur and iodine (a halogen) for sixth place,behind only fluorine; oxygen (3.5); nitrogen andchlorine (3.0); and bromine (2.8). Thus its elec-tronegativity is high, without being too high.Fluorine is not likely to form the long chains forwhich is carbon is known, simply because itselectronegativity is so high, it overpowers otherelements with which it comes into contact. Inaddition, with four valence electrons, carbon isideally suited to find other elements (or othercarbon atoms) for forming covalent bondsaccording to the octet rule, whereby most ele-ments bond so that they have eight valence electrons.

“I JUST WANT TO SAY ONE WORD TO YOU...PLASTICS.” SIGNIFYING THAT PLASTICS WERE THE WAVE OF THE FUTURE,THESE WORDS WERE UTTERED TO DUSTIN HOFFMAN’S CHARACTER IN THE 1967 FILM THE GRADUATE. (The Kobal

Collection. Reproduced by permission.)


OrganicChemistry

Carbon’s Multiple Bonds

Carbon—with its four valence electrons—hap-pens to be tetravalent, or capable of bonding tofour other atoms at once. It is not necessarily thecase that an element has the ability to bond withas many other elements as it has valence elec-trons; in fact, this is rarely the case. Additionally,carbon is capable of forming not only a singlebond, with one pair of shared valence electrons,but a double bond (two pairs) or even a triplebond (three pairs.)

Another special property of carbon is itsability to bond in long chains that constitutestrings of carbons and other atoms. Further-more, though sometimes carbon forms a typicalmolecule (for example, carbon dioxide, or CO2, isjust one carbon atom with two oxygens), it is alsocapable of forming “molecules” that are reallynot molecules in the way that the word is typical-ly used in chemistry. Graphite, for instance, isjust a series of “sheets” of carbon atoms bondedtightly in a hexagonal, or six-sided, pattern, whilea diamond is simply a huge “molecule” com-posed of carbon atoms strung together by cova-lent bonds.

Organic Chemistry

Organic chemistry is the study of carbon, itscompounds, and their properties. The onlymajor carbon compounds considered inorganicare carbonates (for instance, calcium carbonate,alluded to above, which is one of the major formsof mineral on Earth) and oxides, such as carbondioxide and carbon monoxide. This leaves a hugearray of compounds to be studied, as we shall see.

The term “organic” in everyday languageconnotes “living,” but organic chemistry isinvolved with plenty of compounds not part ofliving organisms: petroleum, for instance, is anorganic compound that ultimately comes fromthe decayed bodies of organisms that once werealive. It should be stressed that organic com-pounds do not have to be produced by livingthings, or even by things that once were alive;they can be produced artificially in a laboratory.

The breakthrough event in organic chem-istry came in 1828, when German chemistFriedrich Wöhler (1800-1882) heated a sampleof ammonium cyanate (NH4OCN) and convert-ed it to urea (H2N-CO-NH2). Ammonium cyan-ite is an inorganic material, whereas urea, a wasteproduct in the urine of mammals, is an organicone. “Without benefit of a kidney, a bladder, or a


dog,” as Wöhler later said, he had managed to transform an inorganic substance into anorganic one.

Ammonium cyanate and urea are isomers:substances having the same formula, but possess-ing different chemical properties. Thus they haveexactly the same numbers and proportions ofatoms, yet these atoms are arranged in differentways. In urea, the carbon forms an organic chain,and in ammonium cyanate, it does not. Thus, toreduce the specifics of organic chemistry evenfurther, this discipline can be said to constitutethe study of carbon chains, and ways to rearrangethem to create new substances.


Organic Chemistry and Modern Life

At first glance, the term “organic chemistry”might sound like something removed from

IT IS VIRTUALLY IMPOSSIBLE FOR A PERSON IN MODERN

AMERICA TO SPEND AN ENTIRE DAY WITHOUT COMING

INTO CONTACT WITH AT LEAST ONE, AND MORE LIKELY

DOZENS, OF PLASTIC PRODUCTS, ALL MADE POSSIBLE

BY ORGANIC CHEMISTRY. HERE, A FASHION MODEL

PARADES A DRESS MADE OF PVC. (Reuters NewMedia

Inc./Corbis. Reproduced by permission.)


OrganicChemistry


everyday life, but this could not be further fromthe truth. The reality of the role played by organ-ic chemistry in modern existence is summed upin a famous advertising slogan used by E. I. duPont de Nemours and Company (usuallyreferred to as “du Pont”): “Better Things for Bet-ter Living Through Chemistry.”

Often rendered simply as “Better LivingThrough Chemistry,” the advertising campaignmade its debut in 1938, just as du Pont intro-duced a revolutionary product of organic chem-istry: nylon, the creation of a brilliant youngchemist named Wallace Carothers (1896-1937).Nylon, an example of a polymer (discussedbelow), started a revolution in plastics that wasstill unfolding three decades later, in 1967. Thatwas the year of the film The Graduate, whichincluded a famous interchange between the char-acter of Benjamin Braddock (Dustin Hoffman)and an adult named Mr. McGuire (WalterBrooke):

• Mr. McGuire: I just want to say one word toyou... just one word.

• Benjamin Braddock: Yes, sir.

• Mr. McGuire: Are you listening?

• Benjamin Braddock: Yes, sir, I am.

• Mr. McGuire: Plastics.

The meaning of this interchange was thatplastics were the wave of the future, and that anintelligent young man such as Ben should investhis energies in this promising new field. Instead,Ben puts his attention into other things, quiteremoved from “plastics,” and much of the plotrevolves around his revolt against what he per-ceives as the “plastic” (that is, artificial) characterof modern life.

In this way, The Graduate spoke for a wholegeneration that had become ambivalent concern-ing “better living through chemistry,” a phrasethat eventually was perceived as ironic in view ofconcerns about the environment and the manyartificial products that make up modern life.Responding to this ambivalence, du Pontdropped the slogan in the late 1970s; yet the real-ity is that people truly do enjoy “better livingthrough chemistry”—particularly organic chemistry.

APPLICATIONS OF ORGANIC

CHEMISTRY. What would the world be likewithout the fruits of organic chemistry? First, itwould be necessary to take away all the variousforms of rubber, vitamins, cloth, and paper madefrom organically based compounds. Aspirins andall types of other drugs; preservatives that keepfood from spoiling; perfumes and toiletries; dyes

ANOTHER BYPRODUCT OF ORGANIC CHEMISTRY: PETROLEUM JELLY. (Laura Dwight/Corbis. Reproduced by permission.)


OrganicChemistry

and flavorings—all these things would have to goas well.

Synthetic fibers such as nylon—used ineverything from toothbrushes to parachutes—would be out of the picture if it were not for theenormous progress made by organic chemistry.The same is true of plastics or polymers in gen-eral, which have literally hundreds upon hun-dreds of applications. Indeed, it is virtuallyimpossible for a person in twenty-first centuryAmerica to spend an entire day without cominginto contact with at least one, and more likelydozens, of plastic products. Car parts, toys, com-puter housings, Velcro fasteners, PVC (polyvinylchloride) plumbing pipes, and many more fix-tures of modern life are all made possible by plas-tics and polymers.

Then there is the vast array of petrochemi-cals that power modern civilization. Best-knownamong these is gasoline, but there is also coal, stillone of the most significant fuels used in electri-cal power plants, as well as natural gas and vari-ous other forms of oil used either directly orindirectly in providing heat, light, and electricpower to homes. But the influence of petrochem-icals extends far beyond their applications forfuel. For instance, the roofing materials and tarthat (quite literally) keep a roof over people’sheads, protecting them from sun and rain, are theproduct of petrochemicals—and ultimately, oforganic chemistry.

Hydrocarbons

Carbon, together with other elements, forms somany millions of organic compounds that evenintroductory textbooks on organic chemistryconsist of many hundreds of pages. Fortunately,it is possible to classify broad groupings oforganic compounds. The largest and most signif-icant is that class of organic compounds knownas hydrocarbons—chemical compounds whosemolecules are made up of nothing but carbonand hydrogen atoms.

Every molecule in a hydrocarbon is builtupon a “skeleton” composed of carbon atoms,either in closed rings or in long chains. Thechains may be straight or branched, but in eachcase—rings or chains, straight chains orbranched ones—the carbon bonds not used intying the carbon atoms together are taken up byhydrogen atoms.

Theoretically, there is no limit to the numberof possible hydrocarbons. Not only does carbonform itself into apparently limitless molecularshapes, but hydrogen is a particularly good part-ner. It has the smallest atom of any element onthe periodic table, and therefore it can bond toone of carbon’s valence electrons without gettingin the way of the other three.

There are two basic varieties of hydrocar-bon, distinguished by shape: aliphatic and aro-matic. The first of these forms straight orbranched chains, as well as rings, while the sec-ond forms only benzene rings, discussed below.Within the aliphatic hydrocarbons are three vari-eties: those that form single bonds (alkanes),double bonds (alkenes), and triple bonds(alkynes.)

ALKANES. The alkanes are also knownas saturated hydrocarbons, because all the bondsnot used to make the skeleton itself are filled totheir capacity (that is, saturated) with hydrogenatoms. The formula for any alkane is CnH2n+2,where n is the number of carbon atoms. In thecase of a linear, unbranched alkane, every carbonatom has two hydrogen atoms attached, but the two end carbon atoms each have an extrahydrogen.

What follows are the names and formulasfor the first eight normal, or unbranched, alka-nes. Note that the first four of these receivedcommon names before their structures wereknown; from C5 onward, however, they weregiven names with Greek roots indicating thenumber of carbon atoms (e.g., octane, a refer-ence to “eight.”)

• Methane (CH4)• Ethane (C2H6)• Propane (C3H8)• Butane (C4H10)• Pentane (C5H12)• Hexane (C6H14)• Heptane (C7H16)• Octane (C8H18)

The reader will undoubtedly notice a num-ber of familiar names on this list. The first four,being the lowest in molecular mass, are gases atroom temperature, while the heavier ones areoily liquids. Alkanes even heavier than those onthis list tend to be waxy solids, an example beingparaffin wax, for making candles. It should benoted that from butane on up, the alkanes havenumerous structural isomers, depending on



OrganicChemistry

whether they are straight or branched, and theseisomers have differing chemical properties.

Branched alkanes are named by indicatingthe branch attached to the principal chain.Branches, known as substituents, are named bytaking the name of an alkane and replacing thesuffix with yl—for example, methyl, ethyl, and soon. The general term for an alkane which func-tions as a substituent is alkyl.

Cycloalkanes are alkanes joined in a closedloop to form a ring-shaped molecule. They arenamed by using the names above, with cyclo- asa prefix. These start with propane, or rathercyclopropane, which has the minimum numberof carbon atoms to form a closed shape: threeatoms, forming a triangle.

ALKENES AND ALKYNES. Thenames of the alkenes, hydrocarbons that containone or more double bonds per molecule, are par-allel to those of the alkanes, but the family end-ing is -ene. Likewise they have a common formu-la: CnH2n. Both alkenes and alkynes, discussedbelow, are unsaturated—in other words, some ofthe carbon atoms in them are free to form otherbonds. Alkenes with more than one double bondare referred to as being polyunsaturated.

As with the alkenes, the names of alkynes(hydrocarbons containing one or more triplebonds per molecule) are parallel to those of thealkanes, only with the replacement of the suffix -yne in place of -ane. The formula for alkenes isCnH2n-2. Among the members of this group areacetylene, or C2H2, used for welding steel. Plasticpolystyrene is another important product fromthis division of the hydrocarbon family.

AROMATIC HYDROCARBONS.

Aromatic hydrocarbons, despite their name, donot necessarily have distinctive smells. In fact thename is a traditional one, and today these com-pounds are defined by the fact that they havebenzene rings in the middle. Benzene has a for-mula C6H6, and a benzene ring is usually repre-sented as a hexagon (the six carbon atoms andtheir attached hydrogen atoms) surrounding acircle, which represents all the bonding electronsas though they were everywhere in the moleculeat once.

In this group are products such as naphtha-lene, toluene, and dimethyl benzene. These lasttwo are used as solvents, as well as in the synthe-sis of drugs, dyes, and plastics. One of the morefamous (or infamous) products in this part of

the vast hydrocarbon network is trinitrotoluene,or TNT. Naphthalene is derived from coal tar,and used in the synthesis of other compounds.A crystalline solid with a powerful odor, it isfound in mothballs and various deodorant-disinfectants.

PETROCHEMICALS. As for petro-chemicals, these are simply derivatives of petrole-um, itself a mixture of alkanes with somealkenes, as well as aromatic hydrocarbons.Through a process known as fractional distilla-tion, the petrochemicals of the lowest molecularmass boil off first, and those having higher massseparate at higher temperatures.

Among the products derived from the frac-tional distillation of petroleum are the following,listed from the lowest temperature range (that is,the first material to be separated) to the highest:natural gas; petroleum ether, a solvent; naphtha,a solvent (used for example in paint thinner);gasoline; kerosene; fuel for heating and dieselfuel; lubricating oils; petroleum jelly; paraffinwax; and pitch, or tar. A host of other organicchemicals, including various drugs, plastics,paints, adhesives, fibers, detergents, syntheticrubber, and agricultural chemicals, owe theirexistence to petrochemicals.

Obviously, petroleum is not just for makinggasoline, though of course this is the first prod-uct people think of when they hear the word“petroleum.” Not all hydrocarbons in gasolineare desirable. Straight-chain or normal heptane,for instance, does not fire smoothly in an inter-nal-combustion engine, and therefore disruptsthe engine’s rhythm. For this reason, it is given arating of zero on a scale of desirability, whileoctane has a rating of 100. This is why gas sta-tions list octane ratings at the pump: the higherthe presence of octane, the better the gas is forone’s automobile.

Hydrocarbon Derivatives

With carbon and hydrogen as the backbone, thehydrocarbons are capable of forming a vast arrayof hydrocarbon derivatives by combining withother elements. These other elements arearranged in functional groups—an atom orgroup of atoms whose presence identifies a spe-cific family of compounds. Below we will brieflydiscuss some of the principal hydrocarbon deriv-



OrganicChemistry

atives, which are basically hydrocarbons with theaddition of other molecules or single atoms.

Alcohols are oxygen-hydrogen moleculeswedded to hydrocarbons. The two most impor-tant commercial types of alcohol are methanol,or wood alcohol; and ethanol, which is found inalcoholic beverages, such as beer, wine, andliquor. Though methanol is still known as “woodalcohol,” it is no longer obtained by heatingwood, but rather by the industrial hydrogena-tion of carbon monoxide. Used in adhesives,fibers, and plastics, it can also be applied as a fuel.Ethanol, too, can be burned in an internal-combustion engine, when combined with gaso-line to make gasohol. Another significant alcoholis cholesterol, found in most living organisms.Though biochemically important, cholesterolcan pose a risk to human health.

Aldehydes and ketones both involve a dou-ble-bonded carbon-oxygen molecule, known as acarbonyl group. In a ketone, the carbonyl groupbonds to two hydrocarbons, while in an alde-hyde, the carbonyl group is always at the end of ahydrocarbon chain. Therefore, instead of twohydrocarbons, there is always a hydrocarbon andat least one other hydrogen bonded to the carbonatom in the carbonyl. One prominent example ofa ketone is acetone, used in nail polish remover.Aldehydes often appear in nature—for instance,as vanillin, which gives vanilla beans their pleas-ing aroma. The ketones carvone and camphorimpart the characteristic flavors of spearmintleaves and caraway seeds.

CARBOXYLIC ACIDS AND ES-

TERS. Carboxylic acids all have in commonwhat is known as a carboxyl group, designated bythe symbol -COOH. This consists of a carbonatom with a double bond to an oxygen atom, anda single bond to another oxygen atom that is, inturn, wedded to a hydrogen. All carboxylic acidscan be generally symbolized by RCOOH, with Ras the standard designation of any hydrocarbon.Lactic acid, generated by the human body, is acarboxylic acid: when a person overexerts, themuscles generate lactic acid, resulting in a feelingof fatigue until the body converts the acid towater and carbon dioxide. Another example of acarboxylic acid is butyric acid, responsible in partfor the smells of rancid butter and human sweat.

When a carboxylic acid reacts with an alco-hol, it forms an ester. An ester has a structuresimilar to that described for a carboxylic acid,

with a few key differences. In addition to itsbonds (one double, one single) with the oxygenatoms, the carbon atom is also attached to ahydrocarbon, which comes from the carboxylicacid. Furthermore, the single-bonded oxygenatom is attached not to a hydrogen, but to a sec-ond hydrocarbon, this one from the alcohol. Onewell-known ester is acetylsalicylic acid—betterknown as aspirin. Esters, which are a key factor inthe aroma of various types of fruit, are oftennoted for their pleasant smell.

Polymers

Polymers are long, stringy molecules made ofsmaller molecules called monomers. They appearin nature, but thanks to Carothers—a tragic fig-ure, who committed suicide a year before Nylonmade its public debut—as well as other scientistsand inventors, synthetic polymers are a funda-mental part of daily life.

The structure of even the simplest polymer,polyethylene, is far too complicated to discuss inordinary language, but must be represented bychemical symbolism. Indeed, polymers are a sub-ject unto themselves, but it is worth noting herejust how many products used today involve poly-mers in some form or another.

Polyethylene, for instance, is the plastic usedin garbage bags, electrical insulation, bottles, anda host of other applications. A variation on poly-ethylene is Teflon, used not only in nonstickcookware, but also in a number of other devices,such as bearings for low-temperature use. Poly-mers of various kinds are found in siding forhouses, tire tread, toys, carpets and fabrics, and a variety of other products far too lengthy to enumerate.

WHERE TO LEARN MORE

Blashfield, Jean F. Carbon. Austin, TX: Raintree Steck-Vaughn, 1999.

“Carbon.” Xrefer (Web site). <http://www.xrefer.com/entry/639742> (May 30, 2001).

Chemistry Help Online for Students (Web site). <http://members.tripod.com/chemistryhelp/> May 30,2001).

Knapp, Brian J. Carbon Chemistry. Illustrated by DavidWoodroffe. Danbury, CT: Grolier Educational, 1998.

Loudon, G. Marc. Organic Chemistry. Menlo Park, CA:Benjamin/Cummings, 1988.



OrganicChemistry


ALKANES: Hydrocarbons that form

single bonds. Alkanes are also called satu-

rated hydrocarbons.

ALKENES: Hydrocarbons that form

double bonds.

ALKYL: A general term for an alkane

that functions as a substituent.

ALKYNES: Hydrocarbons that form

triple bonds.

ALLOTROPES: Different versions of

the same element, distinguished by molec-

ular structure.

AMORPHOUS: Having no definite

structure.

COVALENT BONDING: A type of

chemical bonding in which two atoms

share valence electrons.

CRYSTALLINE: A term describing a

type of solid in which the constituent parts

have a simple and definite geometric

arrangement repeated in all directions.

DOUBLE BOND: A form of bonding

in which two atoms share two pairs of

valence electrons. Carbon is also capable of

single bonds and triple bonds.

ELECTRONEGATIVITY: The relative

ability of an atom to attract valence elec-

trons.

FUNCTIONAL GROUPS: An atom or

group of atoms whose presence identifies a

specific family of compounds.

HYDROCARBON: Any chemical com-

pound whose molecules are made up of

nothing but carbon and hydrogen atoms.

HYDROCARBON DERIVATIVES: Fam-

ilies of compounds formed by the joining

of hydrocarbons with various functional

groups.

ISOMERS: Substances having the same

chemical formula, but that are different

chemically due to disparities in the

arrangement of atoms.

OCTET RULE: A term describing the

distribution of valence electrons that takes

place in chemical bonding for most ele-

ments, which end up with eight valence

electrons.

ORGANIC CHEMISTRY: The study of

carbon, its compounds, and their proper-

ties. (Some carbon-containing com-

pounds, most notably oxides and carbon-

ates, are not considered organic.)

SATURATED: A term describing a

hydrocarbon in which each carbon is

already bound to four other atoms. Alka-

nes are saturated hydrocarbons.

SINGLE BOND: A form of bonding in

which two atoms share one pair of valence

electrons. Carbon is also capable of double

bonds and triple bonds.

SUBSTITUENTS: Branches of alka-

nes, named by taking the name of an alka-

ne and replacing the suffix with yl—for

example, methyl, ethyl, and so on.

TETRAVALENT: Capable of bonding

to four other elements.

TRIPLE BOND: A form of bonding in

which two atoms share three pairs of

valence electrons. Carbon is also capable of

single bonds and double bonds.

UNSATURATED: A term describing a

hydrocarbon in which the carbons

involved in a multiple bond (a double

bond or triple bond) are free to bond with

other atoms. Alkenes and alkynes are both

unsaturated.


that occupy the highest principal energy

level in an atom. These are the electrons

involved in chemical bonding.

KEY TERMS


OrganicChemistry

“Organic Chemistry” (Web site). <http://edie.cprost.sfu.ca/~rhlogan/organic.html> (May 30, 2001).

“Organic Chemistry.” Frostburg State University Chem-istry Helper (Web site). <http://www.chemhelper.com/> (May 30, 2001).

Sparrow, Giles. Carbon. New York: Benchmark Books, 1999.





POLYMERSPolymers

CONCEPTFormed from hydrocarbons, hydrocarbon deriv-atives, or sometimes from silicon, polymers arethe basis not only for numerous natural materi-als, but also for most of the synthetic plastics thatone encounters every day. Polymers consist ofextremely large, chain-like molecules that are, inturn, made up of numerous smaller, repeatingunits called monomers. Chains of polymers canbe compared to paper clips linked together inlong strands, and sometimes cross-linked toform even more durable chains. Polymers can becomposed of more than one type of monomer,and they can be altered in other ways. Likewisethey are created by two different chemicalprocesses, and thus are divided into addition andcondensation polymers. Among the naturalpolymers are wool, hair, silk, rubber, and sand,while the many synthetic polymers includenylon, synthetic rubber, Teflon, Formica, Dacron,and so forth. It is very difficult to spend a daywithout encountering a natural polymer—evenif hair is removed from the list—but in the twenty-first century, it is probably even harder toavoid synthetic polymers, which have collectivelyrevolutionized human existence.

HOW IT WORKS

Polymers of Silicon and Carbon

Polymers can be defined as large, typically chain-like molecules composed of numerous smaller,repeating units known as monomers. There arenumerous varieties of monomers, and sincethese can be combined in different ways to formpolymers, there are even more of the latter.

The name “polymer” does not, in itself,define the materials that polymers contain. Ahandful of polymers, such as natural sand or synthetic silicone oils and rubbers, are builtaround silicon. However, the vast majority ofpolymers center around an element that occupiesa position just above silicon on the periodic table:carbon.

The similarities between these two are sogreat, in fact, that some chemists speak of Group4 (Group 14 in the IUPAC system) on the peri-odic table as the “carbon family.” Both carbonand silicon have the ability to form long chains ofatoms that include bonds with other elements.The heavier elements of this “family,” however(most notably lead), are made of atoms too big toform the vast array of chains and compounds forwhich silicon and carbon are noted.

Indeed, not even silicon—though it is at thecenter of an enormous range of inorganic com-pounds—can compete with carbon in its abilityto form arrangements of atoms in various shapesand sizes, and hence to participate in an almostlimitless array of compounds. The reason, inlarge part, is that carbon atoms are much smallerthan those of silicon, and thus can bond to oneanother and still leave room for other bonds.

Carbon is such an important element thatan entire essay in this book is devoted to it, whilea second essay discusses organic chemistry, thestudy of compounds containing carbon. In thepresent context, there will be occasional refer-ences to non-carbon (that is, silicon) polymers,but the majority of our attention will be devotedto hydrocarbon and hydrocarbon-derivativepolymers, which most of us know simply as“plastics.”


Polymers


Organic Chemistry

As explained in the essay on Organic Chemistry,chemists once defined the term “organic” asrelating only to living organisms; the materialsthat make them up; materials derived from them;and substances that come from formerly livingorganisms. This definition, which more or lessrepresents the everyday meaning of “organic,”includes a huge array of life forms and materials:humans, all other animals, insects, plants,microorganisms, and viruses; all substances thatmake up their structures (for example, blood,DNA, and proteins); all products that come fromthem (a list diverse enough to encompass every-thing from urine to honey); and all materialsderived from the bodies of organisms that wereonce alive (paper, for instance, or fossil fuels).

As broad as this definition is, it is not broadenough to represent all the substances addressedby organic chemistry—the study of carbon, itscompounds, and their properties. All living oronce-living things do contain carbon; however,organic chemistry is also concerned with carbon-containing materials—for instance, the syntheticplastics we will discuss in this essay—that havenever been part of a living organism.

It should be noted that while organic chem-istry involves only materials that contain carbon,

carbon itself is found in other compounds notconsidered organic: oxides such as carbon diox-ide and monoxide, as well as carbonates, mostnotably calcium carbonate or limestone. In otherwords, as broad as the meaning of “organic” is, itstill does not encompass all substances contain-ing carbon.

Hydrocarbons

As for hydrocarbons, these are chemical com-pounds whose molecules are made up of nothingbut carbon and hydrogen atoms. Every moleculein a hydrocarbon is built upon a “skeleton” ofcarbon atoms, either in closed rings or in longchains, which are sometimes straight and some-times branched.

Theoretically, there is no limit to the numberof possible hydrocarbons: not only does carbonform itself into seemingly limitless molecularshapes, but hydrogen is a particularly good part-ner. It is the smallest atom of any element on theperiodic table, and therefore it can bond to oneof carbon’s four valence electrons without gettingin the way of the other three.

There are many, many varieties of hydrocar-bon, classified generally as aliphatic hydrocar-bons (alkanes, alkenes, and alkynes) and aromat-ic hydrocarbons, the latter being those that con-

COTTON IS AN EXAMPLE OF A NATURAL POLYMER. (Carl Corey/Corbis. Reproduced by permission.)


Polymers


tain a benzene ring. By means of a basic alter-ation in the shape or structure of a hydrocarbon,it is possible to create new varieties. Thus, asnoted above, the number of possible hydrocar-bons is essentially unlimited.

Certain hydrocarbons are particularly use-ful, one example being petroleum, a term thatrefers to a wide array of hydrocarbons. Amongthese is an alkane that goes by the name of octane(C8H18), a preferred ingredient in gasoline.Hydrocarbons can be combined with variousfunctional groups (an atom or group of atomswhose presence identifies a specific family ofcompounds) to form hydrocarbon derivativessuch as alcohols and esters.


Types of Polymers and Polymerization

Many polymers exist in nature. Among these aresilk, cotton, starch, sand, and asbestos, as well asthe incredibly complex polymers known as RNA(ribonucleic acid) and DNA (deoxyribonucleicacid), which hold genetic codes. The polymersdiscussed in this essay, however, are primarily ofthe synthetic kind. Artificial polymers includesuch plastics (defined below) as polyethylene,styrofoam, and Saran wrap; fibers such as nylon,Dacron (polyester), and rayon; and other materi-als such as Formica, Teflon, and PVC pipe.

MODERN APPLIANCES CONTAIN NUMEROUS EXAMPLES OF SYNTHETIC POLYMERS, FROM THE FLOORING TO THE COUN-TERTOPS TO VIRTUALLY ALL APPLIANCES. (Scott Roper/Corbis. Reproduced by permission.)


PolymersAs noted earlier, most polymers are formedfrom monomers either of hydrocarbon or hydro-carbon derivatives. The most basic syntheticmonomer is ethylene (C2H4), a name whose -eneending identifies it as an alkene, a hydrocarbonformed by double bonds between carbon atoms.Another alkene hydrocarbon monomer is buta-diene, whose formula is C4H6. This is an exampleof the fact that the formula of a compound doesnot tell the whole story: on paper, the differencebetween these two appears to be merely a matterof two extra atoms each of carbon and hydro-gen. In fact, butadiene’s structure is much morecomplex.

Still more complex is styrene, which includesa benzene ring. Several other monomers involveother elements: chloride, in vinyl chloride; nitro-gen, in acrylonitrile; and fluorine, in tetrafluo-roethylene. It is not necessary, in the present con-text, to keep track of all of these substances,which in any case represent just some of themore prominent among a wide variety of syn-thetic monomers. A good high-school or collegechemistry textbook (either general chemistry ororganic chemistry) should provide structuralrepresentations of these common monomers.Such representations will show, for instance, thevast differences between purely hydrocarbonmonomers such as ethylene, propylene, styrene,and butadiene.

When combined into polymers, themonomers above form the basis for a variety ofuseful and familiar products. Once the carbondouble bonds in tetrafluoroethylene (C2F4) arebroken, they form the polymer known as Teflon,used in the coatings of cooking utensils, as well asin electrical insulation and bearings. Vinyl chlo-ride breaks its double bonds to form polyvinylchloride, better known as PVC, a material usedfor everything from plumbing pipe to toys toSaran wrap. Styrene, after breaking its doublebonds, forms polystyrene, used in containers andthermal insulation.

POLYMERIZATION. Note that sever-al times in the preceding paragraph, there was areference to the breaking of carbon doublebonds. This is often part of one variety of poly-merization, the process whereby monomers jointo form polymers. If monomers of a single typejoin, the resulting polymer is called a homopoly-mer, but if the polymer consists of more than onetype of monomer, it is known as a copolymer.This joining may take place by one of two


processes. The first of these, addition polymer-ization, is fairly simple: monomers add them-selves to one another, usually breaking doublebonds in the process. This results in the creationof a polymer and no other products.

Much more complex is the process known ascondensation polymerization, in which a smallmolecule called a dimer is formed as monomersjoin. The specifics are too complicated to discussin any detail, but a few things can be said hereabout condensation polymerization. Themonomers in condensation polymerization mustbe bifunctional, meaning that they have a func-tional group at each end. When characteristicstructures at the ends of the monomers react toone another by forming a bond, they create adimer, which splits off from the polymer. Theproducts of condensation polymerization arethus not only the polymer itself, but also a dimer,which may be water, hydrochloric acid (HCl), orsome other substance.

A Plastic World

A DAY IN THE LIFE. Though “plas-tic” has a number of meanings in everyday life,

A SYNTHETIC POLYMER KNOWN AS KEVLAR IS USED IN

THE CONSTRUCTION OF BULLETPROOF VESTS FOR LAW-ENFORCEMENT OFFICERS. (Anna Clopet/Corbis. Reproduced by

permission.)


Polymers and in society at large (as we shall see), the scien-tific definition is much more specific. Plastics arematerials, usually organic, that can be caused toflow under certain conditions of heat and pres-sure, and thus to assume a desired shape whenthe pressure and temperature conditions arewithdrawn. Most plastics are made of polymers.

Every day, a person comes into contact withdozens, if not hundreds, of plastics and poly-mers. Consider a day in the life of a hypotheticalteenage girl. She gets up in the morning, brushesher teeth with a toothbrush made of nylon, thenopens a shower door—which is likely to be plas-tic rather than glass—and steps into a moldedplastic shower or bathtub. When she gets out ofthe shower, she dries off with a towel containinga polymer such as rayon, perhaps while standingon tile that contains plastics, or polymers.

She puts on makeup (containing polymers)that comes in plastic containers, and later blow-dries her hair with a handheld hair dryer made ofinsulated plastic. Her clothes, too, are likely tocontain synthetic materials made of polymers.When she goes to the kitchen for breakfast, shewill almost certainly walk on flooring with a plas-tic coating. The countertops may be of formica, acondensation polymer, while it is likely that vir-tually every appliance in the room will containplastic. If she opens the refrigerator to get out amilk container, it too will be made of plastic, orof paper with a thin plastic coating. Much of thepackaging on the food she eats, as well as sand-wich bags and containers for storing food, is alsomade of plastic.

And so it goes throughout the day. Thephone she uses to call a friend, the computer shesits at to check her e-mail, and the stereo in herroom all contain electrical components housedin plastic. If she goes to the gym, she may workout in Gore-tex, a fabric containing a very thinlayer of plastic with billions of tiny pores, so thatit lets through water vapor (that is, perspiration)without allowing the passage of liquid water. Onthe way to the health club, she will ride in a carthat contains numerous plastic molds in thesteering wheel and dashboard. If she plays a com-pact disc—itself a thin wafer of plastic coatedwith metal—she will pull it out of a plastic jewelcase. Finally, at night, chances are she will sleep insheets, and with a pillow, containing syntheticpolymers.

A SILENT REVOLUTION. Thescenario described above—a world surroundedby polymers, plastics, and synthetic materials—represents a very recent phenomenon. “Beforethe 1930s,” wrote John Steele Gordon in an arti-cle about plastics for American Heritage, “almosteverything people saw or handled was made ofmaterials that had been around since ancienttimes: wood, stone, metal, and animal and plantfibers.” All of that changed in the era just beforeWorld War II, thanks in large part to a brilliantyoung American chemist named WallaceCarothers (1896-1937).

By developing nylon for E. I. du Pont deNemours and Company (known simply as“DuPont” or “du Pont”), Carothers and his col-leagues virtually laid the foundation for modernpolymer chemistry—a field that employs morechemists than any other. These men created whatGordon called a “materials revolution” by intro-ducing the world to polymers and plastics, whichare typically made of polymers.

Yet as Gordon went on to note, “It has beena curiously silent revolution.... When we think ofthe scientific triumphs of [the twentieth centu-ry], we think of nuclear physics, medicine, spaceexploration, and the computer. But all thesedevelopments would have been much impeded,in some cases impossible, without... plastics. Andyet ‘plastic’ remains, as often as not, a term ofopprobrium.”

AMBIVALENCE TOWARD PLAS-

TICS. Gordon was alluding to a cultural atti-tude discussed in the essay on Organic Chem-istry: the association of plastics, a physical mate-rial developed by chemical processes, with thecondition—spiritual, moral, and intellectual—ofbeing “plastic” or inauthentic. This was symbol-ized in a famous piece of dialogue about plasticsfrom the 1967 movie The Graduate, in which anonplussed Ben Braddock (Dustin Hoffman) lis-tens as one of his parents’ friends advises him toinvest his future in plastics. As Gordon noted,“however intergenerationally challenged thathalf-drunk friend of Dustin Hoffman’s parentsmay have been... he was right about the impor-tance of the materials revolution in the twentiethcentury.”

One aspect of society’s ambivalence overplastics relates to very genuine concerns aboutthe environment. Most synthetic polymers aremade from petroleum, a nonrenewable resource;



Polymersbut this is not the greatest environmental dangerthat plastics present. Most plastics are notbiodegradable: though made of organic materi-als, they do not contain materials that willdecompose and eventually return to the ground.Nor is there anything in plastics to attractmicroorganisms, which, by assisting in thedecomposition of organic materials, help to facil-itate the balance of decay and regeneration nec-essary for life on Earth.

Efforts are underway among organicchemists in the research laboratories of corpora-tions and other institutions to developbiodegradable plastics that will speed up thedecomposition of materials in the polymers—aprocess that normally takes decades. Until suchreplacement polymers are developed, however,the most environmentally friendly solution tothe problem of plastics is recycling. Today onlyabout 1% of plastics are recycled, while the restgoes into waste dumps, where they account for30% of the volume of trash.

Long before environmental concerns cameto the forefront, however, people had begunalmost to fear plastics as a depersonalizing aspectof modern life. It seemed that in a given day, aperson touched fewer and fewer things that camedirectly from the natural environment: the“wood, stone, metal, and animal and plantfibers” to which Gordon alluded. Plastics seemedto have made human life emptier; yet the truth ofthe matter—including the fact that plastics addmore than they take away from the landscape ofour world—is much more complex.

The Plastics Revolution

Though the introduction of plastics is typicallyassociated with the twentieth century, in fact the“materials revolution” surrounding plasticsbegan in 1865. That was the year when Englishchemist Alexander Parkes (1813-1890) producedthe first plastic material, celluloid. Parkes couldhave become a rich man from his invention, buthe was not a successful marketer. Instead, theman who enjoyed the first commercial success inplastics was—not surprisingly—an American,inventor John Wesley Hyatt (1837-1920).

Responding to a contest in which a billiard-ball manufacturer offered $10,000 to anyone whocould create a substitute for ivory, which wasextremely costly, Hyatt turned to Parkes’s cellu-loid. Actually, Parkes had given his creation—

developed from cellulose, a substance found inthe cell walls of plants—a much less appealingname, “Parkesine.” Hyatt, who used celluloid tomake smooth, hard, round billiard balls (therebywinning the contest) took out a patent for theprocess involved in making the material he haddubbed “Celluloid,” with a capital C.

Though the Celluloid made by Hyatt’sprocess was flammable (as was Parkesine), itproved highly successful as a product when heintroduced it in 1869. He marketed it successful-ly for use in items such as combs and baby rattles,and Celluloid sales received a powerful boostafter photography pioneer George Eastman(1854-1932) chose the material for use in thedevelopment of film. Eventually, Celluloid wouldbe applied in motion-picture film, and eventoday, the adjective “celluloid” is sometimes usedin relation to the movies. Actually, Celluloid(which can be explosive in large quantities) wasphased out in favor of “safety film,” or celluloseacetate, beginning in 1924.

Two important developments in the creationof synthetic polymers occurred at the turn of thecentury. One was the development of Galalith, anivory-like substance made from formaldehydeand milk, by German chemist Adolf Spitteler. Aneven more important innovation happened in1907, when Belgian-American chemist LeoBaekeland (1863-1944) introduced Bakelite. Thelatter, created in a reaction between phenol andformaldehyde, was a hard, black plastic thatproved an excellent insulator. It soon foundapplication in making telephones and householdappliances, and by the 1920s, chemists had fig-ured out how to add pigments to Bakelite, thusintroducing the public to colored plastics.

SYNTHETIC RUBBER. Through-out these developments, chemists had only avague understanding of polymers, but by the1930s, they had come to accept the model ofpolymers as large, flexible, chain-like molecules.One of the most promising figures in the emerg-ing field of polymer chemistry was Carothers,who in 1926 left a teaching post at Harvard Uni-versity to accept a position as director of thepolymer research laboratory at DuPont.

Among the first problems Carothers tackledwas the development of synthetic rubber. Natur-al rubber had been known for many centurieswhen English chemist Joseph Priestley (1733-1804) gave it its name because he used it to rub



Polymers out pencil marks. In 1839, American inventorCharles Goodyear (1800-1860) accidentally dis-covered a method for making rubber moredurable, after he spilled a mixture of rubber andsulfur onto a hot stove. Rather than melting, therubber bonded with the sulfur to form a muchstronger but still elastic product, and Goodyearsoon patented this process under the name vulcanization.

Natural rubber, nonetheless, had manyundesirable properties, and hence DuPont putCarothers to the task of developing a substitute.The result was neoprene, which he created byadding a chlorine atom to an acetylene deriva-tive. Neoprene was stronger, more durable, andless likely to become brittle in cold weather thannatural rubber. It would later prove an enormousboost to the Allied war effort, after the Japaneseseized the rubber plantations of Southeast Asia in 1941.

NYLON. Had neoprene, which Carothersdeveloped in 1931, been the extent of his achieve-ments, he would still be remembered by sciencehistorians. However, his greatest creation still layahead of him. Studying the properties of silk, hebecame convinced that he could develop a moredurable compound that could replicate the prop-erties of silk at a much lower cost.

Carothers was not alone in his efforts, asGordon showed in his account of events at theDuPont laboratories:

One day, an assistant, Julian Hill, noticedthat when he stuck a glass stirring rod into agooey mass at the bottom of a beaker theresearchers had been investigating, he could drawout threads from it, the polymers forming spon-taneously as he pulled. When Carothers wasabsent one day, Hill and his colleagues decided tosee how far they could go with pulling threadsout of goo by having one man hold the beakerwhile another ran down the hall with the glassrod. A very long, silk-like thread was produced.

Realizing what they had on their hands,DuPont devoted $27 million to the researchefforts of Carothers and his associates at the lab,and in 1937, Carothers presented his boss withthe results, saying “Here is your synthetic textilefabric.” DuPont introduced the material, nylon,to the American public the following year withone of the most famous advertising campaigns ofall time: “Better Things for Better LivingThrough Chemistry.”

The product got an additional boostthrough exposure at the 1939 World’s Fair.When DuPont put 4,000 pairs of nylon stock-ings on the market, they sold in a matter ofhours. A few months later, four million pairssold in New York City in a single day. Womenstood in line to buy stockings of nylon, a muchbetter (and less expensive) material for thatpurpose than silk—but they did not have longto enjoy it. During World War II, all nylon wentinto making war materials such as parachutes,and nylon did not become commercially avail-able again until 1946.

As Gordon noted, Carothers would surelyhave won the Nobel Prize in chemistry for hiswork—“but Nobel prizes go only to living recip-ients....” Carothers had married in 1936, and byearly 1937, his wife Helen was pregnant. (Pre-sumably, he was unaware of the fact that he wasabout to become a father.) Though highly enthu-siastic about his work, Carothers was always shyand withdrawn, and in Gordon’s words, “he hadfew outlets other than work.” He was, however, atalented singer, as was his closest sibling, Isobel, aradio celebrity. Her death in January 1937 senthim into a bout of depression, and on April 29,he killed himself with a dose of cyanide. Sevenmonths later, on November 27, Helen gave birthto a daughter, Jane.

HOW PLASTICS HAVE EN-

HANCED LIFE. Despite his tragic end,Carothers had brought much good to the worldby sparking enormous interest in polymerresearch and plastics. Over the years that fol-lowed, polymer chemists developed numerousproducts that had applications in a wide varietyof areas. Some, such as polyester—a copolymerof terephthalic acid and ethylene—seemed to fitthe idea of “plastics” as ugly, inauthentic, andeven dehumanizing. During the 1970s, clothes ofpolyester became fashionable, but by the early1980s, there was a public backlash against syn-thetics, and in favor of natural materials.

Yet even as the public rejected synthetic fab-rics for everyday wear, Gore-tex and other syn-thetics became popular for outdoor and workoutclothing. At the same time, the polyester thatmany regarded as grotesque when used in cloth-ing was applied in making safer beverage bottles.The American Plastics Council dramatized thisin a 1990s commercial that showed a few secondsin the life of a mother. Her child takes a soft-



Polymers


drink bottle out of the refrigerator and drops it,

and the mother cringes at what she thinks she is

about to see next: glass shattering around her

child. But she is remembering the way things

were when she was a child, when soft drinks still

came in glass bottles: instead, the plastic bottle

bounces harmlessly.

Of course, such dramatizations may seem abit self-serving to critics of plastic, but the factremains that plastics enhance—and in somecases even preserve—life. Kevlar, for instance,enhances life when it is used in making canoesfor recreation; when used to make a bulletproofvest, it can save the life of a law-enforcement offi-cer. Mylar, a form of polyester, enhances life

ADDITION POLYMERIZATION: A

form of polymerization in which

monomers having at least one double bond

or triple bond simply add to one another,

forming a polymer and no other products.

Compare to condensation polymerization.

ALKENES: Hydrocarbons that contain

double bonds.

COPOLYMER: A polymer composed of

more than one type of monomer.

DIMER: A molecule formed by the

joining of two monomers.

DOUBLE BOND: A form of bonding

in which two atoms share two pairs of

valence electrons. Carbon is noted for its

ability to form double bonds, as for

instance in many hydrocarbons.

FUNCTIONAL GROUPS: An atom or

group of atoms whose presence identifies a

specific family of compounds. When

combined with hydrocarbons, various

functional groups form hydrocarbon

derivatives.

HOMOPOLYMER: A polymer that

consists of only one type of monomer.

HYDROCARBON: Any chemical com-

pound whose molecules are made up of

nothing but carbon and hydrogen atoms.

HYDROCARBON DERIVATIVES:

Families of compounds formed by the

joining of hydrocarbons with various func-

tional groups.

MONOMERS: Small, individual sub-

units, often built of hydrocarbons, that join

together to form polymers.

ORGANIC: A term referring to any

compound that contains carbon, except

for oxides such as carbon dioxide, or car-

bonates such as calcium carbonate (i.e.,

limestone).

ORGANIC CHEMISTRY: The study

of carbon, its compounds, and their

properties.

PLASTICS: Materials, usually organic,

that can be caused to flow under certain

conditions of heat and pressure, and thus

to assume a desired shape when the pres-

sure and temperature conditions are with-

drawn. Plastics are usually made up of

polymers.

POLYMERIZATION: The process

whereby monomers join to form polymers.

POLYMERS: Large, typically chain-like

molecules composed of numerous smaller,

repeating units known as monomers.


that occupy the highest principal energy

level in an atom. These are the electrons

involved in chemical bonding.

KEY TERMS


Polymers when used to make a durable child’s balloon—but this highly nonreactive material also saveslives when it is applied to make replacementhuman blood vessels, or even replacement skinfor burn victims.

Recycling

As mentioned above, plastics—for all their bene-fits—do pose a genuine environmental threat,due to the fact that the polymers break downmuch more slowly than materials from livingorganisms. Hence the need not only to developbiodegradable plastics, but also to work on moreeffective means of recycling.

One of the challenges in the recycling arenais the fact that plastics come in a variety ofgrades. Different catalysts are used to make poly-mers that possess different properties, with vary-ing sizes of molecules, and in chains that may belinear, branched, or cross-linked. Long chains of10,000 or more monomers can be packed closelyto form a hard, tough plastic known as high-density polyethylene or HDPE, used for bottlescontaining milk, soft drinks, liquid soap, andother products. On the other hand, shorter,branched chains of about 500 ethylenemonomers each produce a much less dense plas-tic, low-density polyethylene or LDPE. This isused for plastic food or garment bags, spray bot-tles, and so forth. There are other grades of plas-tic as well.

In some forms of recycling, plastics of allvarieties are melted down together to yield acheap, low-grade product known as “plastic lum-ber,” used in materials such as landscaping tim-bers, or in making park benches. In order toachieve higher-grade recycled plastics, the mate-rials need to be separated, and to facilitate this,recycling codes have been developed. Many plas-tic materials sold today are stamped with a recy-cling code number between 1 and 6, identifyingspecific varieties of plastic. These can be melted

or ground according to type at recycling centers,and reprocessed to make more plastics of thesame grade.

To meet the environmental challenges posedby plastics, polymer chemists continue toresearch new methods of recycling, and of usingrecycled plastic. One impediment to recycling,however, is the fact that most state and local gov-ernments do not make it convenient, for instanceby arranging trash pickup for items that havebeen separated into plastic, paper, and glassproducts. Though ideally private recycling cen-ters would be preferable to government-operatedrecycling, few private companies have the finan-cial resources to make recycling of plastics andother materials practical.

WHERE TO LEARN MORE

Bortz, Alfred B. Superstuff!: Materials That Have ChangedOur Lives. New York: Franklin Watts, 1990.


Galas, Judith C. Plastics: Molding the Past, Shaping theFuture. San Diego, CA: Lucent Books, 1995.

Gordon, John Steele. “Plastics, Chance, and the PreparedMind.” American Heritage, July-August 1998, p. 18.

Mebane, Robert C. and Thomas R. Rybolt. Plastics andPolymers. Illustrated by Anni Matsick. New York:Twenty-First Century Books, 1995.

Plastics.com (Web site). <http://www.plastics.com> (June5, 2001).

“Plastics 101.” Plastics Resource (Web site).<http://www2.plasticsresource.com/plastics_101/>(June 5, 2001).

“Polymers.” University of Illinois, Urbana/Champaign,Materials Science and Technology Department (Website). <http://matse1.mse.uiuc.edu/~tw/polymers/polymers.html> (June 5, 2001).

“Polymers and Liquid Crystals.” Case Western ReserveUniversity (Web site). <http://abalone.cwru.edu/>(June 5, 2001).




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