DidYou Know?Do your muscles ache after a long workout? Unless you have injured yourself,the ache is probably due to lactic acid. Intense exercise causes the energy-releasing process of cell metabolism to surpass the supply of oxygen to yourmuscle cells. New chemical reactions begin, releasing the energy you need.These reactions also produce lactic acid. Unfortunately, lactic acid reducesthe ability of muscle tissue to contract - and causes you pain.

You have seen how to classify compounds as molecular or ionic. There areother useful ways to classify compounds. You can group them based on theirchemical and physical properties.

Acids form a very important and common group of chemicals with distinctiveand useful properties. The tangy flavour of some fruits and other foods anddrinks is due' to one physical property of acids: their sour taste. Acids are used toremove rust and to refine and process metals. Acids can perform these functionsbecause of their useful and sometimes dangerous chemical properties.

Bases are bitter-tasting compounds with a slippery feel. The bitternessof tonic water is due to a base called quinine. Like acids, bases have useful,distinctive properties. Many cleaning products are bases.So are medical drugs such as lidocaine, a local anaes- I

thetic used by dentists. Because they are corrosive, 4however, bases must always be handled with care. '

Common household acids and bases are shownin Figure 2.7. !'

)

""-

~ All these consumerproducts contain acids and bases.Which of them have you used?Did you check the packages forsafety warnings? How did youchange your behaviour afterreading the warnings?

Defining AcidsHow do chemists explain the properties of acidsand bases? In 1884, the Swedish chemist SvanteArrhenius (1859-1927) proposed a theory of acidsand bases based on their behaviour in water. Purewater, a molecular compound, does not conduct "-

electricity. Both acidic and basic solutions, however,are electrolytes, so they must contain freely moving ions.One common feature of acids is that they all contain hydrogen.Therefore, Arrhenius defined an acid as a substance that reactsand releases hydrogen ions, H+ (aq), in a water solution.

One molecule of hydrogen chloride gas, HCI(g), forms the ionsH+ (aq) and CI-(aq) in water. This change is shown below. Accordingto Arrhenius's definition, hydrogen chloride dissolved in water, HC~aq),is acidic.

HCI(g) ~ H+ (aq) + CI-(aq)

Table 2.9 gives examples of some important acids and their uses.

Chapter 2 Names, Formulas, and Properties. MHR 63

Some anthropods use theproperties of acids to theiradvantage. For example, milli-pedes and scorpions producecorrosive acids to deter predators.Ants secrete drops of an acid tomark their pathways.

Table 2.9 Some Important Acids

?Pure water contains equal,minute quantities of hydrogenions and hydroxide ions. In anypure sample of water, an extremelytiny proportion of the H20(~)molecules are ionized. That is,they are broken into a hydrogenion (H+ (aq)) and a hydroxide ion(OH-(aq)). As a result, the amountsof the two ions in any givensample of pure water are equal.

Defining BasesArrhenius defined a base as a substance that dissolves in water and releaseshydroxide ions, OH-(aq), into the solution. Sodium hydroxide, NaOH(s), forexample, is an ionic compound that dissolves readily. The number of hydroxideions in the solution increases as NaOH(s) crystals break up into ions in solution.Arrhenius's theory, therefore, correcdy predicts that the solution, NaOH(aq),will be basic. One formula unit of solid sodium hydroxide, NaOH(s), dissolvesin water to form OH-(aq) and Na + (aq), as shown below. According to Arrhenius's

definition, sodium hydroxide dissolved in water, NaOH(aq), is basic.

NaOH(s) ~ Na+(aq) + OH-(aq)

Table 2.10 gives more important information about several useful bases.

Table 2.10 Some Important Bases

Indicators and pHMany common acids and bases form colourless solutions. These look just likewater but may be much more hazardous. One safe way to tell whether a solutionis acidic or basic is to use an acid-base indicator. An acid-base indicator is achemical that changes colour in response to the concentration of hydrogen orhydroxide ions in a solution. Litmus, shown in Figure 2.8 on the next page, isred in acids and blue in bases. Phenolphthalein is another common indicator.It is colourless in acids but turns pink in bases.

64 MHR . Unit 1 Energy and Matter in Chemical Change

To express the acidic or basic nature of a solution, chemists use the pH scale.This scale is shown in Figure 2.9. The pH of a solution is a number thatexpresses the quantity of hydrogen ions, H+ (aq), in each litre of a solution. ThepH scale usually ranges from 0 to 14. Pure water has a pH of 7. Solutions witha pH of 7 are said to be neutral. They are neither acidic nor basic. Solutionswith a pH less than 7 are acidic. When pH is above 7, a solution is basic.

~ Litmus is a compoundthat is extracted from lichens, plant-like members of the Kingdom Fungi.Litmus paper is made by dippingpaper in litmus solution.

The pH scale allows chemists to express a wide range of measurements usinga small and easily understood range of numbers. This is possible becausepH numbers are exponents of measurements expressed as powers of 10.For example, a bottle of acid with a pH of 1 contains 1 000 000 (106) timesmore H+ (aQ) in each litre than a solution with a pH of 6. The Richter scale forexpressing earthquake intensity and the decibel scale of sound intensity areexponential scales similar to pH. You can find out more about exponentialscales from your mathematics teacher or a senior mathematics textbook.

ant bites sea

blooddraincleanerstomach

aciddetergentsnormal

rainwater

Ineutral

~ The pH scale. The symbol "pH" stands for "powerof the hydrogen ion concentration." pH values have no units.

Chapter 2 Names, Formulas, and Properties. MHR 65

water

I

As you can see in Figure 2.10, there are two common ways to fInd the pHof a solution. You can observe the colour of pH paper that has been dippedinto the solution. Indicators in the paper turn different colours in solutionsof various pH values. Alternatively, you can use a pH meter or pH computerprobe. Both of these instruments measure an electrical property of the solutionthat depends on its pH. Although they are more precise than pH paper, pHmeters are expensive. They are also delicate, and require frequent calibration(adjustment to give accurate results) using solutions of known pH.

The table below summarizes some of the observable properties of acids andbases. In the activity that follows, you will use and compare several differentmethods to classify solutions as acidic or basic.

Sodium hydroxide is also knownas lye. The word "lye" comes fromthe term "alkaline," which issometimes used to mean a basicsolution. A basic substance iscalled an alkali, from an Arabicword meaning "ashes of saltwort"(a plant). In pioneer times, waterthat had been poured throughplant ashes was mixed with animalfat to produce soap. What doesthat suggest about the chemicalsin the soap?

Hydrangea flowers function like natural acid-baseindicators for the soil in which they grow. In acidic soil,the flowers are blue. In basic soil, the flowers are pink.

You can use pH paper or a pHmeter to measure how acidic or basic asolution is.

Table 2.11 Properties of Acids and Bases

66 MHR . Unit 1 E~ergy and Matter in Chemical Cha~ge

Find OutHome-Grown Indicators

There are many natural indicators. In this activity,you will use "clear" tea and purple grape juice totest household chemicals and decide whetherthey are acidic or basic.

2. Pour about 20 mL of grape juice into a clearglass or jar. Add about 80 mL of water to thejuice. Use four more containers, and dividethe grape juice indicator into five roughly

equal portions.

3. Add vinegar to one portion and household

ammonia to another. Record the colour of

your indicator in acidic and basic solutions.

CAUTION: Household ammonia is a

poisonous, caustic liquid and should

be handled carefully.

4. Place an AspirinTM tablet in a plastic bag,

and crush it using a rolling pin or hammer.

Dissolve the powder in a small quantity of

water. Add the AspirinTM solution to the third

container of grape juice indicator. Record

the colour change, if any.

Many common products, such as RolaidsTM, are acids or bases.

5. Repeat step 4 using a TumsTM or RolaidsTMtablet. Rinse all solutions down the sink.Clean and dry the five containers you used.

6. Pour roughly equal volumes of the cool teainto the five glasses or jars. Add lemon juiceto one portion, and record the colour of thetea in an acidic solution. Add baking sodato a second portion, and record the colourof the tea in a basic solution.

Safety Precautions

nCeDrm~. Ammonia solutions can irritate your skin

and lungs. Avoid breathing the vapour fromthe solution.

. If you get any chemicals on your skin, rinse

with copious amounts of cold water. Informyour teacher.

5 clear glasses or jarsrolling pin or hammer

20 mL householdammonia

20 mL lemon juice2 Aspirin TM tablets

2 TumsTM orRolaids TM (tablets)

7. Test the AspirinTM and TumsTM or RolaidsTMtablets as before, but use tea as an indicator.Dispose of solutions as directed by yourteacher. Clean and dry the five containersyou used.

Materials

kettle

teapottea bag4 plastic bagswater20 mL purple

grape juice20 mL vinegarbaking soda

What Did You Find Out? . f!1.1'!:.1 i~~'J,ii~: !: li~~ lli~li~ i~~DIJ)j

Procedure . ~[~jii[li.11!: "J~ '~l!~l i~~:~[(.)~l'iIJ

2. Classify AspirinTM and TumsTM or RolaidsTMtablets as acidic or basic.

68MHRe Matter in Chemical

1. Why were no tests performed on one portionof indicator solution?

1. Make a cup of tea, without milk or sugar.Let the tea cool while you test the grapejuice indicator in steps 2 to 5.

Naming AcidsCommon names for many acidic solutions come from the compounds foundin the solutions. Table 2.12 gives the formulas and names of some chemicalsin two "families" of acids. Try to find and describe patterns in the table inyour own words before reading the naming rules below. Look back to therules for naming polyatomic ions on page 51 for help in spotting patterns.

Table 2.12 Naming Inorganic Acids

pidYou Know?

It is also correct to name an acid as an aqueous solution. For example,hydrosulfuric acid, HZS(aq), is also called aqueous hydrogen sulfide. Sulfuricacid, HZSO4(aq), is also called aqueous hydrogen sulfate.

Study the guidelines below to test if you were able to identify the patternsin Table 2.12. You can use these guidelines to name other acids besides thoselisted in the table.

. Solutions of compounds named For example, aqueous hydrogen

"hydrogen _ide" are named chloride is also called hydrochloric"hydro_ic acid." acid, HCl(aq).

When acids are composed of hydrogen and polyatomic ions that containoxygen, the names of the acids are related to the names of the polyatomic ions.

. Solutions of compounds named For example, aqueous hydrogen

"hydrogen _ate" are named chlorate is also called chloric"_ic acid." acid, HCIO3(aq).

. Solutions of compounds named For example, aqueous hydrogen

"hydrogen per_ate" are named perchlorate is also called perchloricper_ic acid. acid, HCIO4(aq)'

. Solutions of compounds named For example, aqueous hydrogen

"hydrogen _ite" are named chlorite is also called chlorous"_ous acid." acid, HCIOZ(aq).

. Solutions of compounds named For example, aqueous hydrogen

hydrogen "hypo_ite" are hypochlorite is also callednamed "hypo_ous acid". hypochlorous acid HCIO(aq)

Table 2.13 summarizes the names of acids that contain hydrogen, chlorine,and oxygen.

Table 2.13 A Family of Oxygen-Containing Acids

Chapter 2 Names, Formulas, and Properties. MHR 69

Acids that are based on chainsof carbon atoms are called organicacids. They are named using adifferent system. Their formulasbegin with carbon (C) and endwith the hydrogen atom or atomsthat can dissociate. Ethanoicacid (CH3COOH(aq)), the mainingredient in vinegar, is an exampleof an organic acid. Ethanoic acidis also called acetic acid.

Safety and Acid-Base NeutralizationAcids have several distinctive chemical properties. For example, strong acidsreact noticeably with magnesium and several other metals. In the reaction,hydrogen gas is produced and the metal is consumed. Many acids also reactwith the chemicals that make up skin and other living tissue. These reactiveand corrosive properties make acid spills difficult and hazardous to handleand clean up.

Fortunately, when acids and bases react with each other, they neutralizeeach other. In other words, they lose their acidic and basic properties whenthey react chemically. Hydrochloric acid, HCl(aq), for example, reacts withsodium hydroxide, NaOH(aq), to produce water and sodium chloride, NaCl(aq)'If the correct amounts of acid and base are mixed, the two hazardous solutionsbecome nothing more than salt dissolved in water!

Some chemicals can act both asan acid (to neutralize a base) and asa base (to neutralize an acid). Sodiumhydrogen carbonate, NaHCO3(s) (bakingsoda), is an example. Baking soda alsodissolves readily and can be handledsafely. It is often part of the safetysupplies in chemical laboratories. Bakingsoda and other safety supplies used inlabs are shown in Figure 2.11.

~"I. Spill-absorbing "pillows," cat litter, andbaking soda are common chemical safety supplies.Baking soda neutralizes spilled acids or bases. Howwould you use the other items in the photo in caseof an acid spill?

70 MHR . Unit I Energy and Matter in Chemical Change

Neutralization reactions are used for large-scale cleanups, too. For example,sulfuric acid spills can occur during manufacture, transport, or use. If a large-scale spill such as a transport accident occurs, emergency teams are called into contain and clean up the spill. The team uses large quantities of sodiumhydrogencarbonate or calcium oxide to neutralize the acid. The resultingmaterial is then carried away for proper disposal. Finally the team washesdown the area with water.

pidYouKnow?

Section 2.3 SummaryIn this section you explored some of the properties of acids and bases. Youlearned about acids and bases that are solutions in water of various substances.But what about water itself? You learned that pure water has a pH of 7.0. Inthe final section of Chapter 2, you will explore the key properties of water,which are essential to life on Earth.

Check Your Understanding1. Write the names and formulas of two common acids and two common

bases. State at least one use for each acid and base.

2. Make a table that compares the following characteristics of acids and bases.

(a) taste (b) effect on litmus

(c) pH (d) Arrhenius definition

3. Identify two ways to measure pH. Then, describe one advantage and onedrawback of each method.

4. Phosphoric acid has the fonnula H3PO4(aq)o Name each of the followingrelated acids.

(a) H3PO3(aq) (b) H3POS(aq) (c) H3PO2(aq)

5. The formula for nitric acid is HNO3(aq). What is the correct fonnulafor each acid named below?

(a) nitrous acid (b) pernitric acid (c) hyponitrous acid

6. Which of the following solutions has the greater quantity of H+ (aq)for each litre of solution: orange juice (PH 4.0) or baking soda solution(pH S.O)? Explain your answer.

7. Litmus shows its acidic colour most clearly when pH is less than 4. Thebasic colour is not distinct until pH is greater than S. How does thisobservation limit the usefulness of litmus as an acid-base indicator?

8. Apply Cheese and yogurt are made by the action of certain bacteriaon milk. The pH of fresh milk is about 6.5, the pH of cheese is usuallyabout 5.5, and the pH of yogurt is about 4.5.

(a) Which of these common foods is most acidic?

(b) Does the taste of these foods support your answer to (a)? Explain

your response.

rh.ntpr? N.mp. Fnrmnl.. .nrl Prnnprtip. 8 MHR 71

Sulfuric acid is one of the mostimportant industrial chemicals.It is used to make detergents,pharmaceuticals, rayon fibres,dyes, and paper. It is alsoused in other processes,such as petroleum refiningand metal processing.