Vapor Pressure: Solvent versus Solution
Date: November 2007
Why is the vapor pressure of a solution is lower than
vapor pressure of a pure solvent? What is happening on the molecular
Imagine the entropy of the vapor from the pure solvent and that of the
solution. Since the solute does not have an appreciable vapor pressure,
then we can say that only the solvent is in vapor form above both the
pure solvent and the solution. In this case we can say that the entropy
of the two systems are practically the same.
Now imagine the entropy of the pure solvent and that of the solution.
Since there are more microstates (possible arrangements) in the solution
than in the pure solvent, we say that the solution is more disorganized,
has a higher entropy.
So now think of the change in entropy in going from a pure solvent to
the vapor phase, and that of the solution going to the vapor phase -
remembering that the vapor entropy in both systems are the same, and
that the pure solvent has a lower entropy than that of the solution.
This means that the change in entropy from the pure solvent to its vapor
is higher than that of the solution to its vapor.
If the change in entropy is higher and the final entropy state is the
same, then the process with a higher entropy change is more favored,
tends to be more spontaneous. Thus, we can expect that the pure solvent
will spontaneously form more vapor (at a particular temperature) when
compared to the solution.
Greg (Roberto Gregorius)
Your inquiry can be addressed at several levels of complexity. The simplest,
and probably adequate, is that the vapor pressure of a solvent depends upon
how many solvent molecules are present at the surface of the liquid. If the
number of molecules at the surface is reduced by the presence of a solute,
then the partial pressure of the solvent will be reduced in proportion to
the number of solute molecules that replace the solvent molecules. This
simple approach "explains" Raoult's Law that: P = Po*X
where P is the partial pressure of solvent at mol fraction, X. and Po is
the vapor pressure of the pure solvent.
Building off of that oversimplified picture you can see how various
interactions at the molecular level can increase, or decrease, the
relative number of mols of solute and/or solvent that could be present
in any particular set of conditions.
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Update: June 2012