Question

Why do certain solutions show negative deviation from Raoult’s law?

Answer 1

1. A solution is said to exhibit negative deviation from Raoult’s law when its vapour pressure is less than that determined using Raoult’s law.
2. Those liquid pairs where the A-B molecular interaction forces are greater than the A-A or B-B molecular interaction forces exhibit the negative deviations.
3. For solutions showing negative deviations, `Delta V_"mixing" < 0` and `Delta H_"mixing" < 0`, i.e., the formation of such a solution is exothermic and involves a decrease in the volume.