When a solution does not obey Raoult's law over the entire range of concentration, then it is called non-ideal solution.

The vapour pressure of such a solution is either higher or lower than that predicted by Raoult's law. If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult's law.

Positive deviation :

In case of positive deviation from Raoult's law, $A$-$B$ interactions are weaker than those between $A$-$A$ or $B$-$B$, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of $A$ (or $B$) will find it easier to escape than in pure state. This will increase the vapour pressure and result in positive deviation.

Example: Mixtures of ethanol and acetone.

In pure ethanol, molecules are hydrogen bonded. On adding acetone, its molecules get in between the host molecules and break some of the hydrogen bonds between them. Due to weakening of interactions, the solution shows positive deviation from Raoult's law. Another example: $\mathrm{CS}_{2}$ and Acetone