P • A • R • T
•
1
THE NATURE OF WATER
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CHAPTER 1
THE WATER MOLECULE
Three-quarters of the surface of the earth is covered with water. While this is an
impressive statistic, it is pale beside the spectacular photographs that have come
to us from outer space. They reveal a beautiful blue planet bathed in water, partly
hidden by a veil of vapor.
Life came into being in this water. As living things became more complex and
specialized, they left the sea for the land, taking water with them as the major part
of their bodies. On the Planet Earth, water is life.
A philosopher observed that the proper study of mankind is man; the water
chemist paraphrases this: "The proper study of water is the water molecule." The
formula for water—H2O—by itself tells us only its composition and molecular
weight. It does nothing to explain the remarkable properties that result from its
unique molecular arrangement (see Figure 1.1). Two hydrogen atoms are located
105° apart, adjacent to the oxygen atom, so that the molecule is asymmetrical,
positively charged on the hydrogen side and negatively charged on the oxygen
side. For this reason, water is said to be dipolar. This causes the molecules to
agglomerate, the hydrogen of one molecule attracting the oxygen of a neighboring
molecule. The linking of molecules resulting from this attractive force is called
hydrogen bonding,
One of the consequences of hydrogen bonding is that molecules OfH 2 O cannot
leave the surface of a body of water as readily as they could without this intermolecular attraction. The energy required to rupture the hydrogen bond and liberate a molecule of H2O to form vapor is much greater than for other common
chemical compounds. Because of this fact, the water vapor—steam—has a high
energy content and is an effective medium for transferring energy in industrial
plant operations, buildings, and homes.
Water also releases more heat upon freezing than do other compounds. Furthermore, for each incremental change in temperature, water absorbs or releases
more heat—i.e., has great heat capacity—than many substances, so it is an effective heat transfer medium.
The freezing of water is unusual compared to other liquids. Hydrogen bonding
produces a crystal arrangement that causes ice to expand beyond its original liquid volume so that its density is less than that of the liquid and the ice floats. If
this were not the case, lakes would freeze from the bottom up, and life as we know
it could not exist.
Table 1.1 compares the boiling point and other heat properties of water with
similar molecules, such as hydrogen sulfide, and with dissimilar compounds that
are liquid at room temperature.
Because of the unusual structure of the water molecule, it is present in the
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Hydrogen
Molecule
Water Molecule
Oxygen
Molecule
Hydrogen
Molecule
Water Molecule
FIG. 1.1 The joining of diatomic hydrogen and oxygen molecules to
produce water molecules of a polar nature.
TABLE 1.1 Thermal Properties of Water and Similar Compounds
Substance
H2O
H2S
Methanol
Ethanol
Benzene
Specific
heat
Freezing point,
0
C
Boiling point,
0
C
Latent heat of
evaporation,
cal/g
1.00
O
-83
-98
-117
6
100
-62
65
79
80
540
132
263
204
94
0.57
0.54
0.39
FIG. 1.2 A steel needle, with a density about 7
times that of water, can be made to float because
of water's high surface tension.
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natural environment in all three states of matter, solid as ice, liquid as water, and
gas as vapor. It is the only chemical compound having this unusual character.
In addition to its unusual heat properties, water has physical properties quite
different from other liquids. Its high surface tension is easily demonstrated by the
experiment 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 a
capillary tube (Figure 1.3). This capillarity is partly responsible for the system of
circulation developed by living plants through their roots and tissue systems.
Meniscus
FIG. 1.3 A meniscus forms (left) when hydrogen atoms reach upward to
wet oxide surfaces at the water line in a glass tube. The drawing at the right
shows how hydrogen bonding of water to a thin glass tube causes the water
in the tube to rise above the level of the surrounding water. Some liquids
other 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 a
crystal orient themselves to neutralize the attractive forces between the ions in
the crystal structure. The liberated ions are then hydrated by these water molecules as shown in Figure 1.4, preventing them from recombining and recrystalizing. This solvency and hydration effect is shown quantitatively by water's relatively high dielectric constant.
(Water Molecule)
FIG. 1.4 The orientation of water molecules tends to keep ions from recombining and thus precipitating from solution. This accounts for water's capabilities as a solvent.
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Water ionizes so very slightly, producing only 10 7 moles of hydrogen and 10~7
moles of hydroxyl ions per liter, that it is an insulator—it cannot conduct electrical current. As salts or other ionizing materials dissolve in water, electrical conductivity develops. The conductivity of naturally occurring waters provides a
measure of their dissolved mineral content (Figure 1.5).
Specific Conductance,
Los Angeles
Omaha
Des Moines
Tucson
Chicago.
Davenport
Atlanta
Greenville, S.C.
Total Dissolved Solids, mg/L
FIG. 1.5 Dissolved solids content of water can be estimated from its specific conductance. For most public
water supplies, the conversion factor is 1.55^s conductance per milligrams per liter of total dissolved solids.
For other kinds of water, e.g., wastewater and boiler
water, the conversion factor must be established for
each situation.
Viscosity,
Centipoises
Surface Tension
dynes/cm
FIG. 1.6 Surface tension and viscosity both
decrease as water is heated.
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Another important phenomenon occurring in water solutions related to dissolved matter (solutes), rather than to water (solvent), is osmotic pressure. If two
aqueous solutions are separated by a membrane, water will pass from the more
dilute into the more concentrated one. This important process controls the performance of all living cells. It explains the effectiveness of food preservation by
salting; the salt creates a strong solution, disrupting the cells of organisms that
might cause food spoilage, as the water inside their bodies leaves in an attempt
to dilute the external salt solution. In specially designed membrane cells, the
osmotic flow of water across the membrane can be reversed by applying a sufficiently high pressure to the more concentrated solution. This process of "reverse
osmosis" 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 friction—the friction of one layer of molecules moving
across another. As water temperature rises, this internal friction decreases.
Because of the temperature effect, dissolved salts and gases can diffuse more rapidly through warmer water, chemical treatment is hastened, and the physical processes of sedimentation and degasification proceed faster. The effect of temperature 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., and 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, New
York, 1974.
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