Question

What is meant by + ve and - ve deviations from Raoult's law and how is the sign of △H solution related to + ve and - ve deviations from Raoult's law ?

Answer 1

Solutions showing positive deviations, the partial vapour pressure of each component of solution is greater than the vapour Pressure as expected according to Raoult's law. 

Solutions showing negative deviations, the partial vapour pressure of each component of solution is less than vapour pressure as expected according to Raoult's law.

For positive deviation △_mix H = + ve

For negative deviation △_mix H = - ve