DEPRESSION OF FREEZING POINT

COLLIGATIVE PROPERTIES (DEPRESSION OF FREEZING POINT)

(Latin : co means together, ligare means to bind)

The properties of the solutions which depend only on the number of solute particles but not on the nature of the solute are called colligative properties.

There are four colligative properties :

1.      Relative lowering of vapour pressure

2.      Elevation of boiling point

3.      Depression of freezing point

4.      Osmotic pressure.  
     

Depression of Freezing Point

f

    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 phase in the solid phase.

    As we know that the vapour pressure of solution decreases as we mix solid solute to it, therefore it has different boiling point than their pure solvent.

  

   Therefore, it very clear from the above figure that to freeze the solution the temperature of the solution should lower than the pure solvent. And when the curve of solution will touch the curve of frozen solvent then only solution will freeze. This figure clearly representing that the freezing point of solution (Tf ) is greater than the freezing point of solvent (Tf⁰).

Thus, the depression (decrease) in freezing point  ∆ Tf  = Tf⁰ -  Tf .

Experiment have shown that the depression of freezing point is directly proportional to the molal conentration of a solution. Thus

       ∆Tf  α  m 

 ∆Tf  =  Kf m 

m is the molality of the solution and the proportionality constant, Kf is called Freezing point deprssion constant or Molal Depression constant (Cryoscopic Constant). The unit of Kf is K kg mol^-1 . If w₂ and w₁ are the masses of solvent and solute respectively and M₂ is the molar mass of solute then the expression :

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