Ice fishermen sit on top of frozen lakes in the winter and catch fish in the liquid water below through holes cut in the ice sheet. The fact that the lake doesn't freeze solid is a good thing for those fish who live there and for the hungry fishermen. In this problem we will investigate how long it takes for this sheet of ice to form and why the lake doesn't freeze solid during the winter. The water on the surface of the lake only starts to freeze when the air temperature is below the freezing temperature of water.

Why does the ice only form on the surface and not throughout the entire volume of the lake?

This is because below 4°c, water unlike other materials becomes less dense when it's temperature is further lowered.

Explanation:

Due to the unusual nature of water; at about 4°c, the behavior of the density of water in relation to its temperature reverses. This means that water becomes less dense as it becomes colder below 4°c. The colder parts therefore floats to the top of the water body while the warmer part sinks allowing the top to freeze and the remaining body below to remain in its liquid state.

The freezing of the top of the lake alone protects the remaining depth of water from freezing by acting as an insulator and preventing further heat loss from the water to the ambient space. If this had not been the case, and water froze all through, marine lives will freeze to death and it will be more difficult to melt the ice come the next summer.

This behavior is due to the hydrogen bonding of the water molecules.