Henry's law is a gas law that states that the amount of dissolved gas is proportional to its partial pressure in the gas phase.

Positive deviations :

In these type of deviations,the partial vapour pressure of each component A and B of solution is higher than the vapour pressure calculated from Raoult's law,

For eg : Water and ethanol,chloroform and water.

In the case of positive deviation A-B interactions are weaker than those between A-A or B-B.This means that in such solutions molecules of A (or B) will find it easier to escape than in pure state.This will increase the vapour pressure and results in the positive deviation from Raoult's lawΔHmin=+ve

Negative deviations :

In this type of deviations,the partial vapour pressure of solutions is less than that of ideal solution of same composition.Boiling point of such a solution is relatively higher than the boiling points of both the components.This type of behavior is known as negative deviation.

Here the intermolecular attractive forces between A-A and B-B are weaker than those between A-B and leads to decrease in vapour pressure.

In case of solutions showing negative deviations,a slight decrease in volume and evolution of heat takes place on mixing

ΔVmix=−ve