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8 September, 17:23

Molecules in a liquid don't all have the same speed, why does a liquid cool down when it evaporates?

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  1. 8 September, 17:56
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    You're right. The molecules in a jar of liquid have a wide range

    of different speeds. The average of all of their kinetic energies

    is what we sense as the liquid's temperature.

    Within the jar, there's conduction and convection going on

    constantly. Tiny warm samples conduct heat to the tiny cool

    samples next to them, so that the temperature stays uniform

    throughout the jar.

    When some of the liquid evaporates, it's the fastest molecules that

    escaped from the surface and never returned to the jar. With them

    went their little bit of kinetic energy, so the total kinetic energy of the

    molecules that got left behind decreased slightly, and since it was

    the most energetic ones that departed, the average KE of those that

    remained also decreased. Once this new average gets uniformly

    distributed throughout the jar, it'll show up as a decrease in temperature.
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