Why does ice float?

Most substances contract as they solidify, so the solid form sinks in its own liquid. Water does the opposite. As it cools toward freezing, hydrogen bonds between molecules organize them into a hexagonal crystal lattice that holds them further apart than they sit in the jostling liquid, so ice is about 9 percent less dense than water at the same temperature. Water is actually densest at about 4 degrees Celsius, which is why a cooling lake sinks its densest water to the bottom and leaves the coldest water at the surface, where it freezes into a layer that floats.

The consequences are large. A floating ice layer insulates the water beneath it, so lakes and ponds freeze from the top down and rarely freeze solid, which is what allows fish, amphibians, and aquatic plants to survive winter. If ice sank, bodies of water would freeze from the bottom up, accumulate ice year over year, and remain largely frozen, which would have made the evolution of aquatic life as we know it improbable. Sea ice floating at the poles also reflects sunlight, which is part of why its loss accelerates warming.

The same expansion is why pipes burst in freezing weather, why rocks split when water in cracks freezes, why frozen soda cans rupture, and why ice cubes crack when dropped into a drink as the outer layer warms and contracts unevenly. A small number of other substances, including silicon, gallium, and bismuth, also expand on freezing, but among common liquids water is nearly alone, and that single anomaly shapes most of the planet's surface.