The boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure. For Example, vapour pressure of water is at 373K . Therefore, water boils because its vapour pressure at this temperature becomes equal to one atmospheric pressure which is 1.013bar. The vapour pressure of an aqueous solution of sucrose is less than at and therefore the solution will not boil at 373K .
In order to make the solution boil, its temperature must be increased so that its vapour pressure becomes equal to . Thus, boiling point of a solution is always higher than the boiling point of the pure solvent in which the solvent is prepared.
The elevation in boiling point on the addition of a non-volatile solute to a solvent can be easily illustrated graphically as shown above. It is clear from the vapour pressure that the pure solvent becomes equal to atmospheric pressure at ( corresponding to temperature ) while the vapour pressure of the solution becomes equal to atmospheric pressure at ( corresponding to the temperature ).