Exceptional Thermal Characteristics of Water

(Frozen lake on Mather Island, Antarctica; Author: Hannes Grobe, Alfred Wegener Institute)

The expansion of water with temperature is approximately linear. However, on a fine scale it is not exactly linear. In the temperature range from 0°C to 4°C water decreases in volume with increasing temperature, its coefficient of volume expansion is negative. Water also expands when it freezes, while most materials contract when they freeze. This has important consequences for plant and animal life in lakes. A lake cools at the surface, the cooler and denser water from the surface flows to the bottom. But when the air temperature drops below 4°C, the surface water is less dense than the water below, the downward flow ceases. The ice is less dense than water, and it floats. The water at the bottom always remains at 4°C unless the entire lake is frozen.

Related link: Water Structure and Science, an excellent web site on physics and chemistry of water.