Ideal and Non-Ideal Solutions

Ideal solutions obey Raoult's Law and during their formation there is no change in heat and volume. Non-ideal solutions are those solutions which do not obey Raoult’s law and whose formation is accompanied by changes of heat and volume.

Most of the real solutions are non-ideal. They show considerable deviation from the ideal behaviour. Deviations are of two types - positive and negative.

Positive Deviation

Positive deviations are shown by liquid pairs for which the A-B molecular interactions are weaker than the A-A and or B-B molecular interactions. The total vapour pressure for such solutions is greater than predicted by Raoult's law. 

Examples of non-ideal solutions showing positive deviation from the ideal behavior are mixtures of liquids such as water-propanol, ethanol-chloroform, acetone- carbon disulfide, and ethanol-cyclohexane.

Negative Deviation

Negative deviations are shown by liquid pairs for which the A-B molecular interactions-are stronger than A-A or B-B molecular interactions. The total vapour pressure for such solutions is less than that predicted by Raoult’s law.

Examples of such liquid pairs are chloroform acetone, water-sulphuric acid, phenol-aniline, and water-HCl.