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Why does salt lower the temperature of ice?
That's a nice question. If the ice starts out very cold (colder than -21°C), then sprinkling some salt on it won't melt any of the ice and, unless the salt is even colder, won't cool the ice. If the ice is warmer than -21°C and, of course, colder than 0°C, then the salt will melt some of the ice. Melting ice draws energy from somewhere, because the molecules in the liquid water don't have as low-energy contacts with each other as in the solid. The energy will mostly come from the ice, cooling it down.
(published on 10/22/2007)
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