216. Miscible liquid pairs often show negative or positive deviation from Raoult’s law. What is the reason for such deviations? Give one example of each type of liquid pairs.

Answer:
When a solution does not obey Raoult’s law over the entire range of concentration, then it is called non-ideal solution.

The vapour pressure of such solution either be higher or lower. i.e positive when higher
negtive when lower.

The cause for these deviations lie in the nature of interactions at the molecular level.

In case of positive deviation from Raoult’s law,
A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. 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 result in positive deviation. Mixtures of ethanol and acetone behave in this manner.

In case of negative deviations from Raoult’s law, the intermolecular attractive forces between A-A and B-B are weaker than those between
A-B and leads to decrease in vapour pressure resulting in negative deviations. An example of this type is a mixture of phenol and aniline.
In this case the intermolecular hydrogen bonding between phenolic proton and lone pair on nitrogen atom of aniline is stronger than the
respective intermolecular hydrogen bonding between similar molecules.