Thermodynamics Of Mixing And Excess Properties

For an ideal solution, two defining conditions hold: (1) there is no enthalpy change upon mixing, ΔH_mix = 0, because the intermolecular interactions between unlike molecules (A–B) are identical to those between like molecules (A–A and B–B); (2) there is no volume change upon mixing, ΔV_mix = 0. However, mixing two distinct substances always increases the randomness or disorder of the system — molecules of each type are now spread throughout the mixture. Therefore, ΔS_mix > 0 (positive entropy change). Since ΔG_mix = ΔH_mix − TΔS_mix = 0 − T(positive) < 0, ideal solutions form spontaneously at all temperatures. Option B (ΔH > 0) describes a non-ideal solution with positive deviation from Raoult's law — mixing is endothermic for systems like ethanol-water. Option C (all zero) would imply no mixing ever occurred, contradicting physical reality. Option D (ΔH < 0) describes an exothermic mixing process typical of solutions with negative deviation (e.g., acetone-chloroform). The thermodynamic treatment of ideal solutions is from NCERT Chemical Thermodynamics and Solutions chapters and appears in JEE Advanced in integrated form. Plausibility check: ΔG < 0 confirms spontaneous mixing, consistent with the everyday observation that miscible liquids mix freely without external driving forces.