Ideal And Non-ideal Solutions

Non-ideal solutions deviate from Raoult's law when intermolecular forces between unlike molecules differ from those between like molecules. A positive deviation occurs when A–B interactions are weaker than A–A and B–B interactions, meaning molecules escape more easily into the vapour phase, raising the vapour pressure above the Raoult's law prediction. Ethanol and water: in pure form, both substances have extensive hydrogen bonding. Upon mixing, the A–B (ethanol–water) hydrogen bonds are weaker than the original H-bonds in the pure liquids, reducing cohesive forces and causing positive deviation. Acetone–chloroform (Option A) shows negative deviation because a new H-bond forms between C=O of acetone and C–H of chloroform, increasing interactions. Benzene–toluene (Option B) are nearly ideal because their structures are similar, not a deviation example. Acetone–water (Option D) actually shows slight positive deviation, but the stated reason about strengthening interactions is incorrect — positive deviation arises from weakening, not strengthening. This concept is directly covered in NCERT Chapter 2 under non-ideal solutions. Plausibility check: a mixture that shows positive deviation has a maximum in the vapour pressure-composition curve, often forming a minimum-boiling azeotrope.