Colligative Properties
The presence of solute particles can make the physical properties (such as boiling point and freezing
point) of the solution different from those of the pure solvent. Such effects are more easily studied
systematically in cases where the solution is relatively dilute and the solute is nonvolatile (negligible
presence in the gas phase; does not exert vapor pressure of its own). The more numerous these
solute particles are in solution, the more pronounced the changes on the physical properties.
Physical properties that depend on the number of dissolved particles in the solution but not on their
chemical identity or nature are known as colligative properties.
VAPOR-PRESSURE LOWERING (RAOULT’S LAW)
A CLOSER LOOK
Raoult’s law is the basis of the technique of distillation, used to separate substances with
different boiling points or volatilities.When solute B is added to pure solvent A, the vapor pressure of A above the solvent decreases. If the
vapor pressure of A above pure solvent A is designated by P°A and the vapor pressure of A above the
solution containing B is PA, the change in vapor pressure, defined as ∆P = P°A – PA, is
∆P = XBP°A
where XB is the mole fraction of the solute B in solvent A. For a two-component system (that is, no
other kind of solute present), XB = 1 – XA, and so by substituting this into the equation, together with
the definition ∆P = P°A – PA, one obtains Raoult’s law: