十万个为什么·科学启蒙30篇（3）

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冰为什么能浮在水面上

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