Why does ice float on water?

Almost every substance is denser as a solid than as a liquid, because cooling slows molecules and lets them pack more tightly. Water is a notable exception, and the reason is the geometry of hydrogen bonding.

A water molecule is bent, with the oxygen carrying a partial negative charge and the hydrogens partial positive. This lets molecules form hydrogen bonds with each other. In liquid water these bonds constantly form and break, and molecules can slide past one another and pack fairly closely.

When water freezes, the molecules lock into a crystalline lattice that maximises hydrogen bonding. That arrangement is hexagonal and contains substantial empty space; each molecule is held at a fixed distance from its neighbours rather than being free to move closer. The lattice is therefore less dense than the liquid, by roughly nine percent, which is why ice floats and why water expands as it freezes.

Water is unusual in another related way: it reaches maximum density at about four degrees Celsius rather than at its freezing point. Cooling below that makes it less dense again as the lattice-like structuring begins.

The consequences are far-reaching. Because ice floats and forms an insulating layer at the surface, lakes and rivers freeze from the top down rather than solidifying throughout, allowing aquatic life to survive winter beneath. If ice sank, bodies of water would freeze from the bottom and many would never fully thaw.

The expansion also explains familiar practical effects: burst pipes in cold weather, cracked bottles left in a freezer, and the weathering of rock as water freezes in crevices.