9

Why can water evaporate below 100°C? Or, more in general, what is the mechanism behind the evaporation process in liquids?

rob
  • 89,569
Rob
  • 151
  • 1
    If you have a bucket of water molecules, at any temperature above freezing, there is always a chance that some of the molecules will have enough kinetic energy to evaporate into the atmosphere. The higher the temperature, the more chance this will occur. Even below freezing, there is still a slight probability of evaporation. –  Jul 03 '16 at 11:20
  • See also: https://en.wikipedia.org/wiki/Vapor_pressure – Nayuki Jul 03 '16 at 18:26
  • Salt disolves in water at room temperature. Would you say that that salt is in liquid state? However salt melts (turns liquid) at 801 °C . The same happens with evaporation. Water alone boils (turns gaseous) at 100 °C. But it can disolve in a gas (like air) at room temperature. – leonbloy Jul 03 '16 at 19:36
  • @leonbloy - Water can "dissolve" (rather rapidly, in fact) in a total vacuum. The mechanism for dissolving salt in water is somewhat different from the mechanism for evaporating water into air. – Hot Licks Jul 03 '16 at 22:04

1 Answers1

9

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
  • 13,220
  • Is the kinetic energy for leaving the liquid higher when you have a surface tension? – Rob Jul 03 '16 at 11:56
  • 1
    @Rob Yes, the higher the surface tension, the stronger the atomic bonds, and so the more kinetic energy is required for evaporation. So surfaces with large surface tensions will evaporate at a much lower rate. – lemon Jul 03 '16 at 12:34
  • does that mean that everything evaporate? what about solids? – njzk2 Jul 03 '16 at 20:04
  • 1
    @njzk2 Yes, even solids! But, according to my back of the envelope calculation, the evaporation rate of, say, a metal, will be something like $10^{40}$ times lower than that of water. – lemon Jul 03 '16 at 20:15