Ideal and non-ideal solutions

An ideal solution obeys Raoult's law over the whole range of composition. It also has ΔmixH = 0 (no heat taken in or given out on mixing) and ΔmixV = 0 (the volumes simply add).

Ideal behaviour needs A–B attractions nearly equal to the A–A and B–B attractions. No solution is perfectly ideal, but n-hexane with n-heptane, bromoethane with chloroethane, and benzene with toluene come close.

A non-ideal solution does not obey Raoult's law over the whole range; its vapour pressure is either higher than predicted (positive deviation) or lower (negative deviation).

Positive deviation: A–B attractions are weaker than A–A and B–B, so molecules escape more easily. Ethanol with acetone (acetone breaks some of ethanol's hydrogen bonds) and carbon disulphide with acetone behave this way; mixing is endothermic and the volume increases (ΔmixH > 0, ΔmixV > 0).

Negative deviation: A–B attractions are stronger, so escape is harder. Phenol with aniline (hydrogen bond between the phenolic H and the lone pair on N) and chloroform with acetone (chloroform's H bonds to acetone's O) behave this way; mixing is exothermic and the volume shrinks (ΔmixH < 0, ΔmixV < 0).

Azeotropes are binary mixtures whose liquid and vapour have the same composition, so they boil at a constant temperature and cannot be separated by fractional distillation.

A large positive deviation gives a minimum boiling azeotrope: fractional distillation of fermented ethanol-water stops at about 95% ethanol by volume.

A large negative deviation gives a maximum boiling azeotrope: nitric acid and water, about 68% nitric acid and 32% water by mass, boiling at 393.5 K.