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TABLE 11.1
Possible pollution problems in the U.S. water supply
Pollutant Source Risk
Lead Lead pipes in Nerve damage,
older homes; miscarriage, birth
solder in copper defects, high
pipes; brass blood pressure,
fixtures hearing problems
Chlorinated Industrial Cancer
solvents pollution
Trihalomethanes Chlorine Liver damage,
disinfectant kidney damage,
reacting with possible cancer
other pollutants
PCBs Industrial waste, Liver damage,
older transformers possible cancer
Bacteria and Septic tanks, Gastrointestinal
viruses outhouses, problems, serious
overflowing disease
sewer lines

unique solvent abilities, why solid water is less dense than liquid
water, its high specific heat, its high latent heat of vaporization,
and perhaps why no two snowflakes seem to be alike.

STRUCTURE OF WATER MOLECULES
In chapter 9, you learned that atoms combine in two ways.
Atoms from opposite sides of the periodic table form ionic
bonds after transferring one or more electrons. Atoms from
the right side of the periodic table form covalent bonds by
sharing one or more pairs of electrons. This distinction is
clear-cut in many compounds but not in water. The way atoms
share electrons in a water molecule is not exactly covalent, but
FIGURE 11.1 A freshwater stream has many potential uses.
it is not ionic either. As you learned in chapter 9, the bond that
is not exactly covalent or ionic is called a polar covalent bond.
In a water molecule, an oxygen atom shares a pair of elec-
trons with each of two hydrogen atoms with polar covalent
As described in chapter 4, water is also unusual because it bonds. Oxygen has six outer electrons and needs two more to
has a high specific heat. The same amount of sunlight falling satisfy the octet rule, achieving the noble gas structure of eight.
on equal masses of soil and water will warm the soil 5°C for Each hydrogen atom needs one more electron to fill its outer
each 1°C increase in water temperature. Thus, it will take five orbital with two. Therefore, one oxygen atom bonds with two
times more sunlight to increase the temperature of the water as hydrogen atoms, forming H 2 O. Both oxygen and hydrogen are
much as the soil temperature change. This enables large bodies more stable with the outer orbital configuration of the noble
of water to moderate the atmospheric temperature, making it gases (neon and helium in this case).
more even. Electrons are shared in a water molecule but not equally. Oxy-
A high latent heat of vaporization is yet another unusual gen, with its eight positive protons, has a greater attraction for the
property of water. This property enables people to dissipate large shared electrons than do either of the hydrogens with a single pro-
amounts of heat by evaporating a small amount of water. Since ton. Therefore, the shared electrons spend more time around the
people carry this evaporative cooling system with them, they oxygen part of the molecule than they do around the hydrogen
can survive some very warm desert temperatures, for example. part. This results in the oxygen end of the molecule being more
Finally, other properties of water are not crucial for life negative than the hydrogen end. When electrons in a covalent
but are interesting nonetheless. For example, why do all snow- bond are not equally shared, the molecule is said to be polar. A
flakes have six sides? Is it true that no two snowflakes are alike? polar molecule has a dipole (di = two; pole = side or end), mean-
The unique structure of the water molecule will explain water’s ing it has a positive end and a negative end.

11-3 CHAPTER 11 Water and Solutions 277