Question

Why do only the surface of lakes freeze?

Answer 1

Ice is less dense than water, so it doesn't sink to the bottom of the lake.

Explanation:

In order for water to freeze, it must reach a temperature of `0 ^"o" "C"`. This occurs at the surface of a lake, which is why it freezes. However, water, being a special molecule, expands when it freezes, rather than contracts, meaning that the ice on top of the lake is less dense than the water underneath it. This allows it to float on top of the water.

Warm water is denser, so it sinks to the bottom, This effectively separates lakes into cold water, at the top, and warm water at the bottom:

Since the water below the ice on top of the lake never reaches `0 ^"o" "C"`, it never freezes.