Water is at its heaviest at 4°C — which is why ice floats
Almost every substance gets denser as it cools and freezes. Water cheats, and that quirk keeps ponds — and the fish in them — alive through winter.
Cool a glass of water and it gets denser, right up until about 4 °C, where it reaches its maximum density of roughly 1,000 kg per cubic metre. Keep cooling, and water does something almost no other liquid does: it starts to expand again, becoming lighter as it approaches freezing.
When it finally freezes, the molecules lock into an open, hexagonal lattice held by hydrogen bonds. That cage takes up more room than liquid water, so ice is about 9% less dense — and floats.
If water behaved normally, ice would sink, and lakes would freeze solid from the bottom up.
Instead, the coldest water rises, freezes on top, and forms an insulating lid. Beneath it, liquid water survives the winter, letting fish and other life ride out the cold. A single molecular oddity — water’s density peaking above its freezing point — is one of the reasons aquatic ecosystems exist at all.
Sources & references
2 referencesWell-established. Corroborated by 2 independent sources.
