Why ice floats: fresh water is densest at 4 °C

Ice floats because fresh water is densest at about 4 °C and expands as it freezes, so solid ice is lighter than the liquid beneath it. Cool most liquids and they shrink steadily until they solidify; water shrinks only until about 4 °C, then starts to expand again as it approaches 0 °C.

When it freezes, the molecules lock into an open, hexagonal lattice held together by hydrogen bonds, which takes up about 9% more room than the liquid. That is why roughly 7/8 of an iceberg sits below the waterline, according to the U.S. Coast Guard’s Navigation Center. There is nothing magic about 4 °C itself. As University of Illinois physicists explain, two effects compete: cooling molecules settle into open, low-energy arrangements that take up more space, while ordinary cooling packs them closer together. At about 4 °C the two cancel.

That balance is how a lake survives winter. In autumn, cooling surface water sinks and the lake overturns until all of it reaches 4 °C. Colder water then stays on top, freezes, and the ice thickens only from underneath, slowly, because ice conducts heat poorly. On most lakes it grows to a metre or two at most, and a deep lake keeps a pool of 4 °C water below, where fish and other organisms wait out the winter.

The ocean is different, because the 4 °C rule is a freshwater rule. Salt lowers the temperature of maximum density faster than it lowers the freezing point, and at a salinity of 24.7 the two meet at about −1.4 °C.

Nearly all ocean water is saltier than that, so seawater keeps getting denser all the way to its freezing point of about −2 °C. Cold surface water keeps sinking, which is why most of the deep ocean sits below 4 °C.