Depression of Freezing Point

Jul 25, 2022, 16:45 IST

About Depression of Freezing Point

Freezing point is the temperature at which solid and liquid states of a substance have the same vapour pressure. 

At the freezing point of pure solvent the rates at which its molecules stick together to form a solid and leave it to return to a liquid are equal. When a solute is present fewer solvent molecules in solution are in contact with surface of the solid because solute particles get in their way. Therefore, solvent molecules adhere to the surface more slowly. However the rate at which molecules leave the solid which is pure solvent is unchanged. Therefore even at the melting point of pure solvent there is net flow of molecules away from the solid and solid melts. Only if temperature is lowered will that flow be stopped and the equilibrium restored.

It is observed that the freezing point of the solution (Tf) containing non volatile solute is always less than the freezing point of the pure solvent  

Experiments have shown that for dilute solutions the depression in freezing point (ΔTf) is directly proportional to the molal concentration of the solute in a solution.

Molal freezing point depression constant of the solvent or cryoscopic constant, is defined as the depression  in freezing point which may theoretically be produced by dissolving 1 mole of any solute in 1000g of the solvent.

[Note: Similar to Kb the value of Kf depends on solvent nature& is given by Kf = RTf2/1000 Lf where R is gas constant , Tf is freezing point of pure solvent and Lf is the latent heat of fusion per gram of solvent]

Get detail expression of Depression of Freezing Point