十万个为什么·科学启蒙30篇（2）
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Why Do Ice Cubes Float in Water?
为什么冰块会浮在水面上？
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Most solids sink in their own liquid form, but ice is a rare exception because it becomes less dense when frozen.
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Water molecules form hexagonal crystals as they freeze, creating empty spaces that increase volume without adding mass.
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This expansion makes solid ice about 9% less dense than liquid water, so it naturally rises to the surface.
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If ice sank instead, lakes and oceans would freeze from the bottom up, making aquatic life nearly impossible in winter.
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The floating layer of ice also acts like an insulating blanket, slowing further heat loss from the water below.
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Scientists call this property 'anomalous expansion of water', and it’s essential for Earth’s climate and ecosystems.
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You can observe this by freezing a plastic bottle full of water — it bulges or even cracks as ice forms.
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Unlike most substances, water reaches its maximum density at 4°C, not at its freezing point.