and liquid water, such as that in rain droplets, behaves physically like it is covered with
a thin membrane. Ducks appreciate this characteristic of water because it enables them
to float on a water surface. A duck will sink in water to which a detergent has been
added to lower the surface tension, which the birds find to be very distressing. Water is
transparent to visible light and to the longer-wavelength fraction of ultraviolet radiation.
This enables photosynthesis to occur in algae suspended under the surface of water.
Water, which freezes at 0 ̊C, has its maximum density as a liquid at 4 ̊C. This causes
bodies of water to become stratified with colder, denser layers on the bottom.
Arguably the most important physical property of water is its behavior with heat.
Liquid water has a heat capacity of 4.184 joules per gram per ̊C (J/g ̊C), which means
that 4.184 joules of heat energy are required to raise the temperature of 1 gram of
liquid water by 1 ̊C. This very high heat capacity stabilizes the temperatures of regions
near bodies of water. The high heat capacity of water is due to the fact that the water
molecules are strongly tied together by hydrogen bonds. A lot of heat energy is required
to break these bonds and enable the water molecules to move more rapidly at higher
temperatures.
Water has a very high heat of fusion of 334 joules per gram (J/g). This means that
a very large amount of heat is required to break apart water molecules held in fixed
positions by hydrogen bonding in ice in order to convert the water to a liquid state. As ice
is melted, so long as both solid ice and liquid water are present, the temperature remains
at the melting temperature, a constant 0 ̊C. Heat added to the system is used to break
molecules apart in the solid ice, rather than raising the temperature.
The heat of vaporization of water is 2,259 J/g. This means that 2,259 joules of
heat energy are required to vaporize a gram of liquid water. This is the highest heat of
vaporization of any common liquid. It has an enormous influence on bodies of water
and on the weather. By absorbing so much heat in changing from liquid to vapor, water
stabilizes atmospheric temperatures. However, the latent heat contained in water vapor
is released when the vapor condenses, which is what happens during rainfall. This heat
release warms masses of air causing them to rise, and it is the driving force behind
thunderstorms and hurricanes. Latent heat in the form of water vapor evaporated from
oceans near the equator is carried away from the equator in masses of air and released
when the water vapor condenses to form rain.
7.3. Water Distribution and Supply
Even more so than energy, the availability of affordable water will determine the
development of civilization in future decades. Much of the world suffers from a chronic
shortage of water. An idea of problems with water distribution may be had by examining
Figure 7.2 showing distribution of precipitation in the 48 contiguous states of the U.S.
The map shows distinct water shortages in Southern California, Arizona, Nevada, Utah,
and Colorado, areas of the country experiencing exceptionally high population growth
that also happen to be water deficient.
Throughout the first 3/4 of the 1900s, water use in the U.S. climbed steadily, but use
became level around 1980 and in the last two decades of the century actually decreased
Chap. 7. Water, The Ultimate Green Solvent 163