Question

Explain how the anomalous expansion of water helps in preserving aquatic life in very cold countries.

Answer 1

1. When the air temperature drops, the surface layer of water is chilled.
2. As the temperature of the surface layer drops towards $4^{o}C$, this layer becomes denser than the warmer water below it.
3. The denser water sinks and pushes up the deeper and warmer water which in turn is chilled at the surface.
4. This process continues until the temperature of the entire lake reaches $4^{o}C$.
5. Further cooling of the surface water below $4^{o}C$ makes it less dense than the deeper layers; consequently the surface layer does not sink but stays on top.
6. Continued cooling of the top layer to $0^{o}C$ leads to the formation of ice that floats on the water because ice has a lower density than water at any temperature.
7. Below the ice, however, the water temperature remains above $0^{o}C$.
8. The sheet of ice acts as an insulator that reduces the loss of heat from the lake or pond.
9. Further heat transferred from the ground beneath the lake helps to keep the water under the ice sheet from freezing.
10. As a result lakes do not usually freeze solid, even during prolonged cold spells, so fish and other aquatic life can survive during the winter.