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Q & A: evaporative cooling

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Most recent answer: 10/22/2007
Q:
When water evaporates when it is boiling does the temperature rise or drop, why?
- Peter Hastorf (age 36)
Taipei, Taiwan
A:
Evaporation itself cools the water. Thatís why you sweat on a hot day-itís a way to cool down. So if water was boiling and nothing else was sending heat in or out of it, it would cool down and stop boiling. If it keeps boiling, then something (maybe a stove) must be supplying the heat that went into the evaporation (called the latent heat of vaporization).

Why does evaporation cool the water? Water molecules stick together- another way of saying they lower their energy when theyíre in contact. The main reason for that is that positively charged parts of one molecule can sit near negatively charged parts of another. So for the molecules to pull apart (vaporize) something has to supply energy. That energy  comes from all the little thermal jiggles of the molecules, leaving them a little cooler.

Mike W.

(published on 10/22/2007)

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