Phases and phase changes

Question

A drop of water on a kitchen counter evaporates in a matter of minutes. However, only a relatively small fraction of the molecules in the drop move rapidly enough to escape through the drop's surface. Why, then, does the entire drop evaporate rather than just a small fraction of it?

Answer 1

It is true that only a relatively few water molecules have enough enough kinetic energy to escape in any given microsecond of time.  It is also true that when these fast ones do escape, they take with them some energy which will tend to cool the drop (evaporative cooling).   However, this energy is very rapidly replenished by heat flowing in from the kitchen counter, so the drop temperature does not change much.   Thus in the next microsecond of time additional molecules escape and over time the drop is reduced until its dimensions approach the molecular scale.  After that things get more complicated.