十万个为什么·科学阅读30篇（2）

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为什么冰能浮在水面上，而大多数固体却沉入自己的液体中？

Water expands when it freezes, making ice about 9% less dense than liquid water.
This unusual behavior comes from hydrogen bonds forming a hexagonal lattice with open spaces.
Most substances contract on cooling, so their solids sink — but water reaches maximum density at 4°C.
As surface water cools below 4°C, it becomes less dense and rises, eventually freezing on top.
This layer of ice insulates lakes and rivers, allowing aquatic life to survive winter below.
Without this property, freshwater ecosystems would freeze solid from the bottom up.
The density anomaly also affects ocean circulation and global climate regulation.
Scientists study ice structure to improve materials for cold-weather engineering and cryopreservation.
Even tiny impurities or pressure can shift freezing points and crystal shapes slightly.
This single molecular quirk supports life on Earth in ways few other elements can match.