Hydrosphere Contents

2. Properties Of Water

Water density

Frozen lake

Freshwater is unusual in that it has maximum density at 4°C and not at the freezing point. Thus, as water cools from 4°C to 0°C it expands. This is very important in freshwater lakes in temperate/continental regions because as lake water cools towards 4°C it gets heavier and sinks. However, below 4°C this no longer happens; instead the cold water actually floats over the denser 4°C water. Ultimately the cold surface water cools below 0°C, freezes and forms a layer of ice on the surface of the lake. This in turn insulates the lake from further freezing, so a frozen lake actually has liquid water at depth. If water acted like other liquids and continued to increase in density as it cooled, then lakes would freeze solid from the bottom up. As a result, this would mean that most organisms that live in lakes could not survive the winter.

At what temperature is freshwater at maximum density and what is the significance of this?