What is meant by positive deviations from Raoult's law and how is the sign of $∆$H_mix related to positive deviation from Raoult's law?

1. When the vapour pressure of a solution is higher than the predicted value by Raoult's law, it is called positive deviation.
2. In such cases intermolecular interactions between solute and solvent particles (A and B) are weaker than those between solute - solute (A-A) and solvent - solvent (B-B).
3. Hence, the molecules of (A or B) will escape more easily from the surface of solution than
in their pure state. There fore, the vapour pressure of the solution will be higher.
4. Characteristics of a solution showing positive deviation

$\text{ i) }{\mathrm{P}}_{\mathrm{A}}>{\mathrm{P}}_{\mathrm{A}}^0{\mathrm{X}}_{\mathrm{A}};{\mathrm{P}}_{\mathrm{B}}>{\mathrm{P}}_{\mathrm{B}}^0{\mathrm{X}}_{\mathrm{B}}$
$\text{ ii) }{\mathrm{\Delta H}}_{\mathrm{mix}}>0\text{;i.e., +ve }$
$\text{ iii) }{\mathrm{\Delta V}}_{\mathrm{mix}}>0\text{;i.e., + ve }$