Why can water evaporate below 100°C? Or, more in general, what is the mechanism behind the evaporation process in liquids?
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The molecules on the surface will have a random amount of kinetic energy $E$ with a probability proportional to $\exp(-E/kT)$, i.e. the Boltzmann distribution. It follows that occasionally (albeit rarely) a molecule will have a large enough kinetic energy $E$ to break away from the surface and evaporate.
That's the kinetic explanation. From a thermodynamic point of view, evaporation increases the entropy of the universe. And so the second law of thermodynamics drives evaporation until the air is sufficiently humid that equilibrium is achieved and the free energy is minimised.
lemon
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