At any temperature, some of the molecules in a liquid will
be moving so fast when they
encounter the liquid-gas interface
that they keep right on going into
the gas phase as vapor.
Most of the molecules of liquid have too
little energy, and are pulled
back from the interface into the liquid by the attractions
of their neighbors. The overall free
energy per mole of liquid rises
as the temperature increases,
and can be thought of as an average escaping tendency of molecules
from the liquid.
At the same time, molecules in the vapor above the liquid
also have a range of speeds and energies, and the slower-moving
among them may be captured when
they strike the liquid surface.
The likelihood that this will happen increases
with the number of gas molecules hitting
the liquid surface per second, which in turn depends on the
concentration or partial
pressure of vapor molecules above the liquid.
The higher this partial pressure of vapor, the more frequently
the molecules will strike the surface of the liquid, and the
greater will be the tendency of vapor molecules to mo ve back
into the liquid.