Elevation of boiling point

COLLIGATIVE PROPERTIES ( Elevation of boiling 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.

ELEVATION OF BOILING POINT

The boiling point of a liquid is the temperature at which the vapour pressure of the liquid becomes equal to the atmospheric pressure.

As we know on adding a non volatile solute to the solvent, the vapour pressure of the solution lowers than the pure solvent. Therefore, the solution has to be heated to the higher temperature for reaching the vapour pressure equal to atmospheric pressure.
For example the water boils a 373.15 K (100 degree celcius) because at this temperature the vapour pressure of water is 1.013 bar (1 atm). But the vapour pressure of sucrose is less than 1.013 bar at 373.15 K. In order to maintain the vapour pressure of solution equal to 1.013 bar for boiling the temperature should be raised above the boiling point of pure solvent (water). Thus, the boiling point of solution is always higher than that of the boiling point of pure solvent.

In the above figure it is very clear that T_b is the boiling point of solution which is higher than T_b⁰, the boiling point of pure solvent.

The increase in boiling point ∆T_b =  T_b - T_b⁰, is known as elevation of boiling point.

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

∆T_b α m 

∆T_b = K_b m 

m is the molality of the solution and the proportionality constant, K_b is called Boiling point elevation constant or Molal elevation constant (Ebullioscopic Constant). The unit of K_b 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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