[Physics] Does evaporation decrease entropy

entropyevaporationthermodynamics

I have a container of water in thermal equilibrium (there are no temperature fluctuations inside the container).

Some molecules will evaporate out of the container of water thus decreasing its temperature. This is clearly a spontaneous process. But doesn't this mean that the entropy of the system is decreasing? The molecules that evaporated were clearly in thermal equilibrium with the other water molecules before. But now they gained some energy and the other water molecules lost energy.

How can this be?

Best Answer

No, in fact you could even view the spontaneous evaporation as being driven by the fact that it increases entropy.

Basically what's happening is the liquid particles have random speeds (with distribution characterized by temperature), and they bump into each other. Every once in a while, two particles near the interface will collide in just such a way that one of the particles will escape the liquid and transition to the gas phase. The particle that remains in the liquid will loose energy in the process, thereby slightly lowering the temperature of the liquid. However, generally a gas has considerably more specific entropy (entropy per particle) than liquid at fixed temperature/pressure [1]. So, the overall entropy of the system increases.

[1] Compare for example the molar entropy for solid, liquid, and gas phases of water in these tables from Wikipedia.

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