十万个为什么·科学阅读30篇（1）

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为什么盐能融化道路结冰却不能融化湖面厚冰？

Salt lowers water’s freezing point, so ice melts when salt dissolves into surface moisture.
On roads, thin ice layers mix easily with salt, forming brine that spreads and accelerates melting.
Frozen lakes have thick, dense ice with little surface liquid—so salt cannot dissolve effectively.
Without liquid water to carry ions, salt crystals just sit on top without triggering melting.
Wind, sunlight, and traffic help road salt work faster, but lakes lack those factors.
Even large amounts of salt won’t melt deep lake ice because heat transfer is too slow.
Scientists measure ‘eutectic temperature’—the lowest point where saltwater stays liquid—at -21°C.
Below that, salt stops working entirely, regardless of quantity or mixing.
In nature, only shallow ponds or icy puddles respond well to small salt doses.
That’s why winter safety teams never use salt on frozen rivers or lakes.