Why Does Ice Float? (And Why Life Depends on It)

Drop a cube of solid wax into melted wax and it sinks. Drop ice into water and it floats. Water is the odd one out — and that oddity keeps lakes alive.

An open crystal

A water molecule is bent, with slightly positive hydrogen atoms and a slightly negative oxygen. Neighbouring molecules attract through hydrogen bonds. In liquid water these bonds constantly break and re-form, letting molecules pack fairly closely. When water freezes, each molecule locks into hydrogen bonds with four neighbours in a hexagonal lattice — a structure with lots of empty space.

Less dense

That open structure makes ice about 9% less dense than liquid water, so it floats with roughly 90% below the surface — the origin of “the tip of the iceberg”.

Why it matters

• Lakes and rivers freeze from the top down. The ice layer insulates the water below, so fish survive winter in the liquid water underneath.
• If ice sank, bodies of water would freeze from the bottom up, and many would freeze solid.

Densest at 4°C

Liquid water is densest at about 4°C. In winter, water at 4°C sinks to the bottom of a lake, while colder water and ice stay on top.

The downside

Water expands about 9% when it freezes — enough to burst pipes and crack rocks, which is how frost slowly breaks mountains apart.