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Solutions 177
12-3 Colligative Properties
Some of the properties of solutions are dependent upon the chemical and physical
nature of the individual solute. However, there are solution properties that
depend only on the number of solute particles and not their identity. These
properties are colligative properties and they include:
• Vapor pressure lowering
• Freezing point depression
• Boiling point elevation
• Osmotic pressure
Vapor Pressure Lowering
If a liquid is placed into a sealed container, molecules will evaporate from the surface
of the liquid and will eventually establish a gas phase over the liquid that is
in equilibrium with the liquid phase. This is the vapor pressure of the liquid. This
vapor pressure is temperature dependent, the higher the temperature the higher
the vapor pressure. If a solution is prepared, then the solvent contribution to the
vapor pressure of the solution depends upon the vapor pressure of the pure solvent,
P° solvent, and the mole fraction of the solvent. We can find the contribution
of solvent to the vapor pressure of the solution by the following relationship:
P solvent X solvent P° solvent
A similar calculation gives the solute contribution.
P solute X solute P° solute
There may be more than one solute present. If there is more than one solute,
we find the contribution of each solute in the same way. If the solute is nonvolatile,
P° solute 0.
The vapor pressure of a solution is the sum of the contributions of all solutes
and the solvent.
P solution P solvent P solute or P solution X solvent P° solvent X solute P° solute
This relationship is Raoult’s law.
Not all solutions obey Raoult’s law. Any solution that follows Raoult’s law is an
ideal solution. However, many solutions are not ideal solutions. A solution may
have a vapor pressure higher than predicted by Raoult’s law. A solution may
have a vapor pressure lower than predicted by Raoult’s law. Solutions with a