What are Colligative Properties?
A we have discussed, solutions have different properties than either
the solutes or the
solvent used to make the solution. Those properties can be divided
into two main groups--colligative and non-colligative properties.
Colligative properties
depend only on the number of
dissolved particles in solution and not on their identity. Non-colligative
properties depend on the
identity of the dissolved species and the solvent.
To explain the difference between the two sets of solution properties, we
will compare the properties
of a 1.0 M aqueous sugar solution to a 0.5 M solution of
table salt (NaCl) in water.
Despite the concentration of sodium chloride being half of the sucrose
concentration, both solutions
have precisely the same number of dissolved particles because each sodium
chloride unit creates two
particles upon dissolution--a sodium ion, Na+, and a chloride
ion, Cl-.
Therefore, any difference in the properties of those two solutions is due
to a non-colligative property.
Both solutions have the same freezing point, boiling point, vapor pressure,
and osmotic pressure
because those colligative properties of a solution only depend on the
number of dissolved particles.
The taste of the two solutions, however, is markedly different. The sugar
solution is sweet and the
salt solution tastes salty. Therefore, the taste of the solution is not a
colligative property. Another
non-colligative property is the color of a solution. A 0.5 M
solution of CuSO4
is bright blue in contrast to the colorless salt and sugar solutions.
Other non-colligative properties
include viscosity, surface tension, and solubility.
Raoult's Law and Vapor Pressure Lowering
When a nonvolatile solute is added to a liquid
to form a solution, the vapor pressure above that solution decreases. To
understand why that might
occur, let's analyze the vaporization process of the pure solvent then do
the same for a solution.
Liquid molecules at the surface of a liquid can escape to the gas phase
when they have a sufficient
amount of energy to break free of the liquid's intermolecular forces. That
vaporization process is
reversible. Gaseous molecules coming into contact with the surface of a
liquid can be trapped by
intermolecular forces in the liquid. Eventually the rate of escape will
equal the rate of capture to
establish a constant, equilibrium vapor pressure above the pure liquid.
If we add a nonvolatile solute to that liquid, the amount of surface area
available for the escaping
solvent molecules is reduced because some of that area is occupied by
solute particles. Therefore, the
solvent molecules will have a lower probability to escape the solution than
the pure solvent. That fact
is reflected in the lower vapor pressure for a solution relative to the
pure solvent. That statement is
only true if the solvent is nonvolatile. If the solute has its own
vapor pressure, then the
vapor pressure of the solution may be greater than the vapor pressure of
the solvent.
Note that we did not need to identify the nature of the solvent or the
solute (except for its lack of
volatility) to derive that the vapor pressure should be lower for a
solution relative to the pure solvent.
That is what makes vapor pressure lowering a colligative property--it only
depends on the number of
dissolved solute particles.
summarizes our discussion so far. On the surface
of the pure solvent
(shown on the left) there are more solvent molecules at the surface than in
the right-hand solution
flask. Therefore, it is more likely that solvent molecules escape into the
gas phase on the left than on
the right. Therefore, the solution should have a lower vapor pressure than
the pure solvent.
