a
During the depression of freezing point in a solution, liquid solvent and solid solvent are in equilibrium. During freezing of a solution, only the solvent freezes out and the equilibrium exists between solid and liquid forms of the solvent. Vapour pressure of the solid and liquid forms must be the same at freezing point, because otherwise, the system would not be at equilibrium, the lowering of the vapour pressure leads to the lowering of temperature at which the vapour pressures of the liquid and frozen forms of the solution will be equal.