十万个为什么·科学阅读30篇（2）
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Why Does Ice Float on Water While Most Solids Sink in Their Liquids?
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？
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Water expands when it freezes, making ice about 9% less dense than liquid water.
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This unusual behavior comes from hydrogen bonds forming a hexagonal lattice with open spaces.
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Most substances contract on cooling, so their solids sink — but water reaches maximum density at 4°C.
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As surface water cools below 4°C, it becomes less dense and rises, eventually freezing on top.
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This layer of ice insulates lakes and rivers, allowing aquatic life to survive winter below.
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Without this property, freshwater ecosystems would freeze solid from the bottom up.
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The density anomaly also affects ocean circulation and global climate regulation.
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Scientists study ice structure to improve materials for cold-weather engineering and cryopreservation.
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Even tiny impurities or pressure can shift freezing points and crystal shapes slightly.
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This single molecular quirk supports life on Earth in ways few other elements can match.