Question

What do you understand by depression of freezing point?

Answer 1

The decrease in the freezing point of a solvent caused by the addition of a non-volatile solute to it is termed as depression of freezing point.

For a pure solvent, solutions I and II, the graph in Fig. shows how the vapour pressure changes with temperature; solution II is more concentrated than solution I. The liquid solvent’s and the frozen solvent’s vapour pressures equalise at point A, establishing their equilibrium. Consequently, temperature T_f, the freezing point of a pure solvent, is represented by point A.

The pure solid solvent is often the solid that separates out of a solution when it freezes. A solution with a lower vapour pressure can only be in equilibrium with the frozen solvent if the frozen solvent’s vapour pressure is likewise lower. The temperature will be lower for this to occur. Because of this, the solution will freeze at a lower temperature than pure solvent.

It is evident from Fig. that solution I freezes at temperature T₁, or point B, when its vapour pressure equals that of the frozen solvent. The freezing point of a pure solvent, T_f, is clearly higher than temperature T₁. The depression of the freezing point for solution I is the difference `Delta T_(f_1)` between the two freezing points, T_f and T₁. Because Solution II is more concentrated, its vapour pressure is even lower. At point C, or temperature T₂, which is still lower, its vapour pressure equals that of the frozen solvent. The depression of the freezing point for solution II is the difference `Delta T_(f_2)` between the freezing point of pure solvent (T_f) and the freezing point of solution II (T₂). Thus,

Depression of freezing point for solution I = T_f − T₁

= `Delta T_(f_1)`

Depression of freezing point for solution II = T_f − T₂

= `Delta T_(f_2)`