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P A R T 1THE NATURE OFWATER

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CHAPTER 1THE WATER MOLECULE

Three-quarters of the surface of the earth is covered with water. While this is animpressive statistic, it is pale beside the spectacular photographs that have cometo us from outer space. They reveal a beautiful blue planet bathed in water, partlyhidden by a veil of vapor.Life came into being in this water. As living things became more complex andspecialized, they left the sea for the land, taking water with them as the major partof their bodies. On the Planet Earth, water is life.A philosopher observed that the proper study of mankind is man; the waterchemist paraphrases this: "The proper study of water is the water molecule." Theformula for waterH 2Oby itself tells us only its composition and molecularweight. It does nothing to explain the remarkable properties that result from itsun ique m olecular arrangem ent (see Figure 1.1). Two hydrogen atoms are located105 apart, adjacent to the oxygen atom, so that the molecule is asymmetrical,positively charged on the hydrogen side and negatively charged on the oxygenside. For this reason, water is said to be dipolar. This causes the molecules toagglomerate, the hydrogen of one molecule attracting the oxygen of a neighboringmolecule. The linking of molecules resulting from this attractive force is calledhydrogen bonding,

One of the consequences of hydrogen bonding is that molecules OfH 2O cannotleave the surface of a body of water as readily as they could without this inter-molecular attraction. The energy required to rupture the hydrogen bond and lib-erate a molecule of H 2O to form vapor is much greater than for other commonchemical compounds. Because of this fact, the water vaporsteamhas a highenergy content and is an effective medium fo r transferring energy in industrialplant operations, buildings, and homes.Water also releases more heat upon freezing than do other compounds. Fur-thermore, for each incremental change in temperature, water absorbs or releasesmore heat i.e., has great heat capacitythan many substances, so it is an effec-tive heat transfer med ium.The freezing of water is unusu al compared to other liquids. Hydrogen bondingproduces a crystal arrangement that causes ice to expand beyond its original liq-uid volume so that its density is less than that of the liquid and the ice floats. Ifthis were not the case, lakes would freeze from the bottom up, and life as we knowit could not exist.Table 1.1 compares the boiling point and other heat properties of water withsimilar molecules, such as hyd rogen sulfide, and with dissimilar compounds thatare liquid at room temperature.Because of the unusual structure of the water molecule, it is present in the

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FIG. 1.1 The joining of diatomic hydrogen and oxygen molecules toproduce w ater molecules of a polar nature.

TABLE 1.1 Thermal Properties of Water and Similar Compounds

FIG. 1.2 A steel needle, with a density about 7times that of water, can be made to float becauseof water's high surface tension.

SubstanceH 2OH 2SMethanolEthanolBenzene

Specificheat1.000.570.540.39

Freezing point,0CO

-83-98-1176

Boiling point,0C100-62657980

Latent heat ofevaporation,cal/g54013226320 494

HydrogenMoleculeWater Molecule

OxygenMolecule

HydrogenMoleculeWater Molecule

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natural environment in all three states of matter, solid as ice, liquid as water, andgas as vapor. It is the only chemical compound having this unusual character.In addition to its unusual heat properties, water has physical properties quitedifferent from other liquids. Its high surface tension is easily demonstrated by theexperiment of "floating" a needle on the surface of water in a glass (Figure 1.2).This high surface tension, due to hydrogen bonding, also causes water to rise in acapillary tube (Figure 1.3). This capillarity is partly responsible for the system ofcirculation developed by living plants through their roots and tissue systems.

F IG . 1.4 The orientation of water molecules tends to keep ions from recom-bining and thus precipitating from solution. This accounts fo r water's capa-bilities as a solvent.

(Water Molecule)

F IG . 1.3 A meniscus forms (left) when hydrogen atoms reach upward towet oxide surfaces at the water line in a glass tube. The drawing at the rightshows how hydrogen bonding of water to a thin glass tube causes the waterin the tube to rise above the level of the surrounding water. Some liquidsother than water do not wet a glass surface. They form an inverted meniscus.

Water is often called the universal solvent. Water molecules in contact with acrystal orient themselves to neutralize the attractive forces between the ions inthe crystal structure. The liberated ions are then hydrated by these water mole-cules as shown in Figure 1.4, preventing them from recombining and recrystal-izing. This solvency and hydration effect is shown quantitatively by water's rela-tively high dielectric constant.

Meniscus

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Water ionizes so very slightly, producing only 10 7 moles of hydrogen and 10 ~ 7moles of hydroxyl ions per liter, that it is an insulatorit cannot conduct electri-cal current. As salts or other ionizing materials dissolve in water, electrical con-ductivity develops. The conductivity of naturally occurring waters provides ameasure of their dissolved mineral content (Figure 1.5).

F IG . 1.6 Surface tension and viscosity bothdecrease as water is heated.

Tota l Dissolved Solids, mg/LFIG . 1.5 Dissolved solids content of water can be esti-mated from its specific conductance. For most publicwater supplies, the conversion factor is 1.55^s conduc-tance per milligrams per liter of total dissolved solids.For other kinds of water, e.g., wastewater and boilerwater, the conversion factor must be established foreach situation.

Viscosity,Centipoises Surface Tensiondynes/cm

Los Angeles

Omaha

Tucsones MoinesChicago. Davenpor tAtlantaGreenville, S.C.

SpfcCo

a

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Another important phenomenon occurring in water solutions related to dis-solved matter (solutes), rather than to water (solvent), is osmotic pressure. If twoaqueous solutions are separated by a membrane, water will pass from the moredilute into the more concentrated one. This important process controls the per-formance of all living cells. It explains the effectiveness of food preservation bysalting; the salt creates a strong solution, disrupting the cells of organisms thatmight cause food spoilage, as the water inside their bodies leaves in an attemptto dilute the external salt solution. In specially designed membrane cells, theosmotic flow of water across the membrane can be reversed by applying a suffi-ciently high pressure to the more concentrated solution. This process of "reverseosmosis" is a practical one for desalination of water.Finally, viscosity is another property of water affecting its treatment and use.It is a measure of internal frictionthe friction of one layer of molecules movingacross another. As water temperature rises, this internal friction decreases.Because of the temperature effect, dissolved salts and gases can diffuse more rap-idly through warmer water, chemical treatment is hastened, and the physical pro-cesses of sedimentation and degasification proceed faster. The effect of tempera-ture on viscosity is shown in Figure 1.62

SUGGESTED READING

Boys, C. V.: Soap Bubbles, Doubleday, New York, 1959.Buswell, A. M ., and Rodebush, W. H.: "Water," Sd. Am., April 1956, p. 76.Carson, Rachel: The Sea Around Us, Oxford University Press, New York, 1951.Day, John A., an d Davis, Kenneth S.: Water: The Mirror of Science, Doubleday, New York,1961.King, Thompson: Water, Miracle of Nature, Macmillan, New York, 1953.Leopold, Luna B., and Davis, Kenneth S.: Water, Life Science Library, Time-Life, NewYork, 1974.