Illustrate elevation in boiling point with the help of vapour pressure temperature curve of a solution. Show that elevation in boiling point is a colligative property.

Explanation

Boiling point of a liquid is the temperature at which the pressure on the liquid becomes equal to the atmospheric pressure. Addition of a non-volatile solute decreases the vapour pressure of the solution. Now solution will have to be heated to a higher temperature so that its vapour pressure becomes equal to the atmospheric pressure.
Elevation in boiling point `DeltaT_(b)=T_(b)-T_(b)^(@)`. The curve AB represents the pure solvent. The curve for solution is represented by CD. The vapour pressure curve CD for solution lies below the curve AB of pure solvent. P presents the atmospheric pressure. Let `T_(b)^(@)` be the boiling point of pure solvent and `T_(b)` be the boiling point of the solution.
Rise in temperature, `DeltaT_(b)` is directly proportional to the decrease in vapour pressure. For dilute solutions, decrease in vapour pressure is proportional to the molality (m) of the solution. Hence, rise in temperature is directly proportional to the molality of the solution, i.e., `DeltaT_(b)propmorDeltaT_(b)=K_(b)*m`, where `K_(b)` is constant called molal elevation constant. Since, elevation in boiling point depends upon the molality of the solution. Hence, it is a colligative property.