Q.Which one of the following binary liquid mixtures exhibits positive deviation from Raoults law?

Ethanol + Water shows positive deviation from Raoult's law because mixing disrupts ethanol's own hydrogen bonding, weakening net intermolecular attraction and raising vapour pressure above the ideal value.

Raoult's law predicts the vapour pressure of an ideal solution assuming A-A, B-B and A-B intermolecular forces are all comparable. Real mixtures deviate:

• Positive deviation: occurs when A-B interactions in the mixture are WEAKER than the average of A-A and B-B interactions in the pure liquids. Molecules escape into the vapour phase more easily than Raoult's law predicts, so the observed total vapour pressure is HIGHER than the ideal value, and mixing is usually endothermic (Delta-mix H > 0) with a volume increase. Classic examples: ethanol + water, acetone + ethanol, acetone + carbon disulphide, cyclohexane + ethanol.

• Negative deviation: occurs when A-B interactions are STRONGER than A-A/B-B (often due to hydrogen bonding or dipole-dipole attraction forming BETWEEN the two different components), lowering the observed vapour pressure below Raoult's-law prediction. Classic examples: acetone + chloroform (forms a strong intermolecular H-bond), HCl + water, HNO3 + water, chloroform + benzene.

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