What do you understand by depression of freezing point? Derive the expression between depression of freezing point and molar mass of solute.

• Freezing point of a substance is the temperature at which it’s liquid and solid phases have same vapour pressure. Depression in freezing point may be defined as the decrease of freezing point of a solvent on the addition of a non-volatile solute.

• E.g. Ethylene glycol is used as an antifreeze in car radiators which depresses the freezing point of water below 273K.

• Relation between depression of freezing point and molar mass of solute,

If w₂ gram of a solute having molar mass M₂ is present in w₁ gram of the solvent and produces a depression in freezing point T_f of the solvent, then the molality of the solute is given by,

m = w₂/M₂ 1000/w₁

Thus, the relation can be given by

T_f = K_fm.