十万个为什么·科学启蒙30篇（4）
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Why Hot Water Can Freeze Faster Than Cold (Mpemba Effect)
为什么热水结冰有时比冷水更快（姆潘巴现象入门）
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Under certain conditions, hot water freezes faster than cold—a counterintuitive behavior first noted by a Tanzanian student named Erasto Mpemba.
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Possible causes include faster evaporation reducing mass, less dissolved gas increasing convection, and early supercooling differences.
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Hot water loses more volume to steam before freezing, so less liquid remains to solidify overall.
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Convection currents in warmer water circulate heat more efficiently toward the surface, speeding up cooling initially.
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Cold water may form insulating frost layers sooner, slowing further heat loss—while hot water freezes more uniformly.
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The effect is unreliable and depends heavily on container shape, air humidity, and starting temperatures.
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It does NOT violate thermodynamics—it simply shows that total freezing time involves many competing physical factors.
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Reproducing it consistently remains a challenge, which is why scientists still study Mpemba’s observation with modern tools.