When water freezes its molecules lock into a roomy honeycomb full of gaps, so ice takes up more space than the water it came from — and lighter things float.
What's actually happening
Freezing normally packs molecules tighter. Cool almost any liquid and its molecules slow down, huddle closer, and lock into a dense solid that sinks in its own melt. Solid wax sinks in liquid wax; solid iron sinks in molten iron. Water is the famous exception, and the reason is the shape of its molecule.
A water molecule is a bent little magnet — oxygen slightly negative, hydrogens slightly positive — and each one wants to hold hands with exactly four neighbours, at fixed angles. In liquid water the molecules are too jittery to maintain that arrangement and tumble closer together than the handshake geometry "wants". But as the liquid cools toward freezing, the jostling weakens, the hydrogen bonds win, and the molecules snap into an open hexagonal lattice full of empty space. Ice is water forced into a roomier formation: about 9% less dense than the liquid it came from. Less dense floats.
The consequences are enormous. Because ice floats, a winter lake freezes from the top down, and the floating lid insulates the water below — fish spend winter in liquid water at 4 °C under the ice. If ice sank, lakes would freeze solid from the bottom up, and each summer's thaw might not undo it. Aquatic life as we know it depends on water's one backwards habit.
- 1Fill a small plastic bottle to the brim with water, cap it loosely, and mark the level.
- 2Freeze it overnight. The ice stands clearly above your mark — same molecules, more volume.
- 3Now float an ice cube in a full glass of water and ask: will it overflow when the cube melts? It won't. The cube's submerged 90% displaces exactly the water the whole cube will become.