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
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