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 "hydrogen bond". This is important for it means that all of the molecules of water in a lake or other
body of water are, to an extent, bonded to each other. It prevents the free, independent movement of
each molecule that would allow water, for example, to easily evaporate and exist as a gas at normal
earth temperatures. Indeed, to form a gas, significant energy must be absorbed to liberate the molecules
from the bonding forces.
As water absorbs energy, the motion of the molecules increases and molecular forces bonding
the molecules are increasingly strained. The average distance between molecules increases and the
density of the water decreases. As energy is lost water cools and the distance decreases while density
increases. Maximum density is achieved at approximately 4EC. Figure 1.2.3 illustrates this relationship.
As energy is lost and temperature decreases below maximum density, bonding relationships change as
water approaches the transition to ice. The structure of ice is rigid. As is easily demonstrated, an
aqueous ice bath at standard pressure will theoretically remain at 0EC even with inputs of energy - until
the ice has melted. This property is often utilized during instrument calibrations for temperature. The
energy exchanged during the conversion of water to ice (or the reverse) without a change in
temperature is the energy associated with the phase change and is termed the "latent heat of fusion".
For pure water at standard pressure, the latent heat of fusion is approximately 80 calories per gram.
(From the table of conversions, 1 cal. equals the amount of energy necessary to increase the
temperature of 1 gram of water by 1o C.) Upon freezing, the rigid crystalline structure of water
increases the distance between molecules and density is significantly decreased.
1.2-8
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