十万个为什么·科学启蒙30篇（3）
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Why Does Ice Float on Water?
冰为什么能浮在水面上
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Ice floats because it is about 9% less dense than liquid water—a rare property among substances.
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When water cools below 4°C, its molecules begin forming open hexagonal crystals held by hydrogen bonds.
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These crystals leave empty spaces between molecules, making solid ice take up more volume than the same mass of water.
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That lower density means ice weighs less per liter than water, so it rises instead of sinking.
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If ice sank, lakes and oceans would freeze from the bottom up, killing most aquatic life each winter.
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This unique behavior also causes pipes to burst when water freezes inside them—expanding outward with great force.
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Water reaches its maximum density at 4°C, which is why deeper lake water stays near that temperature year-round.
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Antarctic ice shelves float because they’re connected to land-based glaciers but extend over seawater.
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Scientists study ice density to understand climate patterns, since melting sea ice doesn’t raise sea level—but land ice does.
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No other common substance expands when freezing; most solids sink in their own melted form.