The Unique Properties of Water
Water is a unique substance. To understand how lakes
behave, it is useful to understand water’s physical and chemical
properties. The molecular structure of water and the way in which water
molecules associate with each other dictate these properties:
1. Water is an excellent solvent; many gases, minerals, and organic
compounds dissolve readily in it.
2. Water is a liquid at natural environmental temperatures and
pressures. Although this property seems rather common and obvious, it is
quite important. If water behaved at ordinary temperatures and pressures
like other inorganic compounds that are chemically similar to it, water
would only be present as a vapor—and lakes would not exist.
3. The temperature/density relationship of water is also unique. Most
liquids become more dense (heavier)as they cool. Water also rapidly
becomes more dense as its temperature drops, but only to a certain
point. Water reaches its maximum density at 39.2°F (3.94°C), then it
decreases slightly in density until it reaches 32°F (0°C), the
freezing point. At this point, ice forms and its density decreases
sharply. Ice, therefore, is much lighter than liquid water and thus
forms at the surface of lakes rather than at the lake bottom.
A second important consequence of the temperature/density relationship
of water is the thermal stratification of lakes. Energy is required to
mix fluids of differing densities, and the amount of energy necessary is
related to the difference in density. In the case of lakes, this energy
is provided primarily by wind. Therefore, the changes in water density
that accompany rapidly decreasing water temperatures in the metalimnion
during summer stratification are of great importance. The metalimnetic
density gradient provides a strong and effective barrier to lake mixing.
4. Water also has an unusually high “specific heat." Specific
heat is the amount of energy required to change the temperature of 1
gram of water by 1°C. Water also has a high "latent heat of
fusion,” which is the energy required to melt 1 gram of ice at 0°C.
These properties make lakes slow to thaw and warm in the spring and slow
to cool and freeze in the fall, thus providing exceptionally stable
thermal environments for aquatic organisms.
Because water gains and loses heat slowly, the presence of large lakes
can exert a significant influence on local and regional climate. A good
example is the Great Lakes, which have a dramatic effect on both air
temperature and precipitation in the states and provinces surrounding
them.
