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Which of the following solutions shows a positive deviation from Raoult's law?

1. Ethanol and acetone
2. Ethanol and methanol
3. HNO3 and H2O
4. Bromoethane and chloroethane

Correct answer: Ethanol and acetone

Solution

Positive deviation from Raoult's law occurs when A-B interactions are weaker than A-A and B-B interactions, causing higher vapour pressure than ideal. Ethanol-acetone: ethanol forms H-bonds with itself; when mixed with acetone, the H-bonds of ethanol are partially broken (acetone cannot donate H-bonds), weakening intermolecular attractions. This leads to higher vapour pressure — positive deviation. Ethanol-methanol: both form similar H-bonds; mixture behaves nearly ideally or with very small deviation. HNO3-H2O: forms stronger interactions (HNO3 partially dissociates, and H-bonds are stronger), giving negative deviation. Bromoethane-chloroethane: both are similar non-polar/weakly polar molecules; nearly ideal mixture.

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