十万个为什么·科学启蒙30篇（1）

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为什么冰能浮在水面上？

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