Interesting question. It certainly is not easy to find solutes to raise
the vapor pressure of water. I think it can't be done, and here's my
I hate to introduce a new term, but doing so makes this argument a
lot easier. There's something called the chemical potential, which is a
measure of how easy it is for molecules of some type to leave some
material. There's a spontaneous net flow of molecules from regions of
high chemical potential to regions of low chemical potential, until the
chemical potentials become equal. That's just like the spontaneous flow
of heat from regions of high temperature to ones of low temperature,
until the temperature becomes uniform. Let's call the chemical
potential of water mu.
Now think of four regions. One is the solution. One is some vapor
in equilibrium with the solution. Another is pure water, and the last
is the vapor it equilibrates with. In equilibrium mu(above solution) =
mu(solution) and mu(water)= mu(above water).
Let's imagine we were wrong about how solutes affect vapor
pressure, and the vapor pressure above the solution were more than the
vapor presure above the water. Higher pressure gas has higher mu. That
would mean the mu(above solution) > mu(above water). So then
mu(solution) > mu(water). That would mean that water would
spontaneously flow from the solution to any region of pure water. So
the solution would spontaneously separate into a pure water region and
a denser region. So it wasn't in equilibrium to begin with. So our
assumption that you could have, in equilibrium, higher vapor pressure
above the solution is self-contradictory.
The equilibrium vapor pressure above the solution is always lower.
(republished on 07/24/06)