Why do lakes freeze from top to bottom?
The fact that ice is less dense than water has a profound ecological significance. Consider, for example, the temperature changes in the fresh water of a lake in a cold climate. As the temperature of water near the surface is lowered, its density increases. The colder water then sinks toward the bottom, while warmer water, which is less dense, rises to the top. This normal convection motion continues until the temperature throughout the water reaches 40C. Below this temperature, the density of water begins to decrease with decreasing temperature, so that it no longer sinks. On further cooling, the water begins to freeze at the surface. The ice layer is formed does not sink because it is less dense than liquid; it even acts as a thermal insulator for the water below it. Was ice heavier, it would sink to the bottom of the lake and eventually the water would freeze upward? Most living organisms in the body of water could not survive. Fortunately, this does not happen, and it is this unusual property of water that makes the sports of ice fishing possible.
Plot of density versus temperature for liquid water