| Feature | Ideal Solution | Positive Deviation | Negative Deviation |
|---|---|---|---|
| Raoult's Law | Perfectly obeyed | P_obs > P_Raoult | P_obs < P_Raoult |
| A-B vs A-A/B-B forces | A-B ≈ A-A ≈ B-B | A-B weaker than A-A, B-B | A-B stronger than A-A, B-B |
| _mix | 0 | > 0 (endothermic) | < 0 (exothermic) |
| _mix | 0 | > 0 (expansion) | < 0 (contraction) |
| _mix | > 0 | > 0 | > 0 |
| Azeotrope type | None | Minimum boiling (P_max at azeotrope) | Maximum boiling (P_min at azeotrope) |
| Examples | Benzene + toluene; n-hexane + n-heptane | Ethanol + water; Acetone + ; Acetone + benzene | + acetone; HCl + water; + water |
| Temperature effect on P | P increases linearly with x | P curve bows upward above ideal line | P curve bows downward below ideal line |
| Molar mass determination | Accurate (i=1 assumed) | Overestimates (apparent M < true M) | Can affect accuracy |
| NEET Trick | "No interaction change on mixing" | **"P"ositive = Ethanol+water = PEN" | "N"egative = +acetone |
Key Memory Rule: Positive deviation → WEAKER A-B bonds → molecules ESCAPE MORE easily → HIGHER vapour pressure → LOWER boiling point → MINIMUM boiling azeotrope.
Negative deviation → STRONGER A-B bonds → molecules ESCAPE LESS easily → LOWER vapour pressure → HIGHER boiling point → MAXIMUM boiling azeotrope.