What is meant by positive and negative deviations from Raoult's law and how is the sign of `Delta_(mix)H` related to positive and negative deviations from Raoult's law ?

Explanation

Solutions having vapour pressures more than that expected from Raoult's law are said to exhibit positive deviation. In these solutions solvent - solute interactions are weaker and `Delta _("sol")` H is positive because stronger A-A or B-B interactions are replaced by weaker A-B interactions. Breaking of the stronger interactions requires more energy & less energy is released on formation of weaker interactions. So overall `Delta_("sol")` H is positive . Similarly `Delta_("sol")`V is positive ie. the volume of solution is some what more than sum of volumes of solvent and solute. So there is expansion in volume on solution formation. Similarly in case of solutions exhibiting negative deviations, A-B interactions are stronger than A-A & B-B. So weaker interactions are replaced by stronger interactions so, there is release of energy i.e. `Delta_("sol")H` is negative.