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 identity.

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