Colligative properties of dilute solutions :

   In the preceding section, we described physical properties of pure liquids. In a binary solution behaving ideally, it was considered that the vapour pressure of the solution is the sum of the crystal partial pressure of the components constituting the solution. But, when a non-volatile solute, (e.g., sugar) is added to a volatile solvent, it is found that the vapour pressure of the solvent is lowered. In other words, the addition of a non-volatile solute to a volatile solvent alters or modifies the properties of the solution. In such cases the properties of the solution depends on the number of solute particles ( atoms, molecules or ions) and not on the nature of the solute particles. Now, we use the term colligative to  describe those properties of a solution which may be determined by taking simple arithmetic averages of the properties of solute and solvent.

        The colligative properties of solutions are those properties which depend upon the number of solute particles (e.g., the mole-fraction of the solute in the solution) and not the nature of the solute.

    As we shall see, the nature of the solute may modify the properties of the solutions, if the solute is volatile, dissociates or associates in its vapour phase or in solution. The colligative properties of solutions are : (1) Relative lowering of vapour pressure, (2) Elevation in boiling point, (3) Depression in freezing point, (4) Osmotic pressure, etc.