Example 1.7

Example . Fig. . : The vapour pressure curve for solution lies below the curve for pure water. The diagram shows that D T b denotes the elevation of boiling point of a solvent in solution. . bar or The lowering of vapour pressure of a solution causes a lowering of the freezing point compared to that of the pure solvent (Fig. . ). We know that at the freezing point of a substance, the solid phase is in dynamic equilibrium with the liquid phase. Thus, the freezing point of a substance may be defined as the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase. A solution will freeze when its vapour pressure equals the vapour pressure of the pure solid solvent as is clear from Fig. . . According to Raoult’s law, when a non-volatile solid is added to the solvent its vapour pressure decreases and now it would become equal to that of solid solvent at lower temperature. Thus, the freezing point of the solvent decreases. Let f T be the freezing point of pure solvent and f T be its freezing point when non-volatile solute is dissolved in it. The decrease in freezing point. f f f  T T T is known as depression in freezing point. Similar to elevation of boiling point, depression of freezing point ( D T f ) for dilute solution (ideal solution) is directly proportional to molality, m of the solution. Thus, D T f µ m or D T f = K f m ( . ) The proportionality constant, K f , which depends on the nature of the solvent is known as Freezing Point Depression Constant or Molal