$word_{count}$: 220

Non-ideal solutions deviate from Raoult's law when A-B interactions differ from A-A and B-B. Positive deviation: A-B weaker than average of A-A, B-B. Consequences: $P_{total}$ > Raoult prediction, delta_H_{mix} > 0 (endothermic), delta_V_{mix} > 0 (expansion). Examples: ethanol + water (H-bonds disrupted), acetone + CS2, benzene + methanol. Forms minimum boiling azeotrope — mixture boils at lower temperature than either pure component. Ethanol-water azeotrope: 95.5% ethanol at 78.1 C. Negative deviation: A-B stronger than A-A, B-B. Consequences: $P_{total}$ < Raoult prediction, delta_H_{mix} < 0 (exothermic), delta_V_{mix} < 0 (contraction). Examples: acetone + chloroform (H-bond between C=O and H-CCl3), HCl + water, HNO3 + water. Forms maximum boiling azeotrope — boils above both pure components. HCl-water: 20.2% HCl at 108.6 C. Identification from data: compare observed $P_{total}$ with calculated ideal value. If higher — positive; if lower — negative. From structure: disrupted strong interactions = positive; new strong interactions formed = negative. Azeotropes cannot be separated by simple distillation because liquid and vapour have identical composition at the azeotropic point. Methods to break: pressure-swing distillation, entrainer addition, molecular sieves.