Why Ice Floats

Almost every liquid becomes denser as it cools, since its molecules slow down and pack more tightly together, which is why a solid version of most substances sinks in its own liquid. Water breaks this pattern in a way that has enormous consequences for life on Earth. As water cools toward freezing, it does initially get denser, just like other liquids, but only down to about four degrees Celsius. Below that point, something unusual happens: the water molecules begin arranging themselves into the hexagonal crystal structure of ice, and that structure actually takes up more space than the same molecules do as a liquid, because of the specific way each water molecule bonds to its neighbors. Each molecule forms weak bonds with four others in a rigid, open lattice, and that open, spread-out arrangement is less compact than the way molecules jostle around more freely in liquid water. Because the same mass of water takes up more volume as ice, ice ends up less dense than liquid water, and less dense materials float on top of denser ones. This is why lakes and ponds freeze from the top down rather than the bottom up: as the surface water cools and eventually freezes, the resulting ice layer floats and insulates the liquid water beneath it, allowing fish and other aquatic life to survive the winter in the unfrozen water below, rather than being trapped in solid ice from the bottom up.