The properties of solution which depends upon number of solute particles present in the solution irrespective of their nature is called colligative properties of solution.

The following conditions are satisfied for normal colligative properties of solution.

(i) Solution should be very dilute.

(ii) Solute should be non-volatile, does not dissociable or associable.

Example:–

(a) Lowering of vapour pressure.

(b) Elevation in the boiling point.

(c) Depression in the freezing point.

(d) Osmotic pressure.

Lowering of vapour pressure – When a non-volatile solute is added in a solvent. 

The vapour pressure is lowered due to the following reason.

(i) Surface area of solution decreases from solvent molecules. V. P. ∝ surface area of solvent

(ii) Density of solution increases and rate of evaporation decreases.

Rate of Evaporation ∝ √1/density

Therefore V.P. of solution decreases from pure solvent.

V.P. of pure solvent = P_o

V.P. of solution = P_s

Lowering of V. P. = P_o – P_s 

Relative lowering of V.P.  = P_o – P_s /P_o

According to Raoult’s law

P_o – P_s/P_o = mole fraction of solute .