The actual identity of the solute molecules is unimportant
to the lowering of vapor
pressure. Since theirs is a "spoiling"
role in decreasing the frequency with which solvent molecules
approach the surface, only their numbers matter. If a substance
such as a salt dissociates into two particles or ions in solution,
then it is doubly effective. One mole of NaCl lowers the vapor
pressure of water by twice as much as a mole of glucose, because
it yields twice the number of particles in an aqueous solution.
Example. At 35°
C the vapor pressure of water is 42.2 mm Hg. What is the vapor
pressure of an aqueous solution of glucose that has one glucose
molecule for every 100 water molecules?
Example. The elemental
abundance table in Chapter 8 shows that ocean water can be
considered as a solution with 330 NaCl "molecules" for every
33,000 water molecules. The vapor pressure of pure water on
a hot summer day (35° C) is
42.2 mm Hg. What is the vapor pressure of water in the middle
of the ocean at that temperature?
Lowering of the equilibrium vapor pressure of a liquid by
ions or molecules of a solute is known as a colligative
property (meaning "collective" or "joint") because
the size of the effect depends only on the total
number of solute molecules or ions, and not on their
There are three other common colligative properties of solutions:
boiling point elevation, freezing
point lowering, and osmotic
pressure. In all four cases, adding solute molecules
or ions decreases the escaping tendency of solvent molecules
from the liquid. Therefore some adjustment in temperature
or pressure must be made to restore equilibrium between the
liquid and the other phase