Question

What is meant by positive and negative deviations from Raoult’s law and how is the sign of Δ_mixH related to positive and negative deviations from Raoult’s law?

Answer 1

In +ve deviation, A-B interactions are weaker than those between A-A or B-B. In such molecules, A or B will find it easier to escape than in pure state. This increases vapour .pressure. In case of -ve deviation, A-B interaction = A-A or B-B. This leads to decrease in vapour pressure.

In +ve deviation, Δ_mixH is + ve 

In -ve deviation, Δ_mixH is - ve