Well, it's a little easier going the other way. Melting ice is endothermic -- you can see this by putting a thermometer in a glass of warm water, adding an ice cube, and watching the temperature go down as the ice melts. The melting process needs heat to proceed and takes it from the warm water.
Going the other way: your refrigerator makes ice by cooling the water off. The heat gets transferred to the environment by air flowing past the coils in the coolant loop after the compressor, usually located in the back of your refrigerator. The heat dissipated in these coils is the sum of quite a few sources -- heat leakage from the outside into the refrigerator, and energy expended by the compressor.
You can also freeze pure water by putting it into a vacuum. Water will boil when the vapor pressure exceeds the ambient pressure, and in a vacuum, this is the case at all temperatures for which the water is liquid. Boiling is endothermic (it takes energy to boil water), and so the water cools off as it boils. Eventually, what's left will freeze.
You can also freeze water by putting dry ice or liquid nitrogen into it. The dry ice will vaporize and the liquid nitrogen will boil, both indicating that they are receiving thermal energy given up by the freezing water.
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January 7th, 2006, 13:46
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