An aqueous solution containing urea was found to have boiling point more than the normal boiling point of water (373.13 K). When the same solution was cooled it was found that its freezing point is less than the normal freezing point of water (273.13 K). Explain these observations.

Explanation

Boiling point : Boiling point of a liquid is the temperature at which its vapour pressure is equal to the atmospheric pressure. Water boils at 373.13 K because at this temperature, the vapour pressure of water is equal to the atmospheric pressure (1.013 bar). However, the vapour prcesurc of a liquid decreases in the presence of non-volatile solute like urea. In order that the vapour pressure of the solution reaches the atmospheric pressure, we shall have to raise the temperature beyond the normal boiling temperature of pure water. Thus, boiling temperature of a solution is always higher than that of pure solvent.
Freezing point : Freezing point of a substance is the temperature at which the solid phase is in dynamic equilibrium with the liquid phase. We may define the freezing point of a substance as the temperature at which the vapour pressure of the substance in liquid phase is equal to its vapour pressure in the solid phase. When a non-volatile solid is added to the solvent, its vapour pressure decreases and now it would become equal to the that of the solid phase at a lower temperature. Thus the freezing point of a solution of urea is lower than that of pure water.