Question

Explain with a suitable diagram and appropriate example, why some non-ideal solution shows positive deviation from RaouIt’s law.

Answer 1

Some non-ideal solutions show positive deviation from Raoult’s law. Consider a solution of two components A and B. If A-B interactions in the solution are weaker than the A – A and B – B interactions in the two liquids forming the solution, then the escaping tendency of molecules A and B from the solution become more than in pure liquids.

The total vapour pressure will be greater than the corresponding vapour pressure as expected on the basis of Raoult’s law. This type of behaviour of solution is called positive deviation from Raoult’s law. The boiling point of such solutions are lowered.

Mathematically,

P_A < P_A⁰ x_A 

P_B < P_B⁰ x_B 

The total vapour pressure is less than P_A + P_B

Examples of solutions showing positive deviations 

1. Ethyl alcohol and water 

2. Benzene and acetone 

3. Ethyl alcohol and cyclohcxanc 

4. Carbon tetrachloride and chloroform.