Explain why boiling point of solution is greater than that of pure solvent ?

Explanation

(i) The vapour pressure of a solution (P) containing non volatile solute is lower than that of pure solvent `(P^(@)).`
(ii) Consider a closed system is which a pure solvent is in equibrium with its vapur. At equilibrium the molar Gibbs free energies of solvent in a liquid and gaseous phase are equal `(Delta G=0).`
(iii) When a solute is added to this solvent the dissolution takes place and its free energy (G) decreases due to increase in entropy.
(iv) In order to maintain the equilibrium, the free energy of the vapour phase must also decrease.
(v) At a given temperature, the only way to lower the free energy of the vapour is to reduce its pressure.
(vi) Thus the vapour pressure of the solution must decrease to maintain the equilibrium.