十万个为什么·科学启蒙30篇（1）
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Why Does Ice Float on Water?
为什么冰能浮在水面上？
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Most substances become denser when they solidify, but water behaves unusually—it expands by about 9% when freezing into ice.
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This happens because water molecules form open hexagonal lattices held together by hydrogen bonds as they cool below 4°C.
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The spaces between molecules increase, lowering ice’s density to about 0.92 g/cm³—less than liquid water’s 1.00 g/cm³.
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Because ice is less dense, it floats rather than sinks, insulating lakes and rivers from freezing solid in winter.
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If ice sank, freshwater ecosystems would freeze from the bottom up, killing fish and disrupting seasonal cycles.
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This property also causes pipes to burst in cold weather—water expands as it freezes inside narrow metal tubes.
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Scientists call water’s density peak at 4°C one of its ‘anomalous properties,’ shared by very few other liquids.
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Antifreeze works by disrupting hydrogen bonding, preventing water molecules from forming rigid ice crystals easily.
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You can observe this density difference by layering colored saltwater, freshwater, and ice cubes in a tall glass.
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Even deep ocean water stays liquid under high pressure because salt lowers the freezing point and suppresses lattice formation.