positive and negative deviations

positive deviation

if the vapour pressure of solution is greater than the vapour pressure of either of pure components then the solution is said to have positive deviations.

for such solution, boiling point of solution is lower than boiling point of either of the components

explanation

if a graph is plotted between composition and vap pressure of a solution which shows positive deviation from raoult's law, the total vap pressure cover rises to the maximum

let us consider the mixture of A and B components at point C. At the point C, the mixture has the highest vap pressure and therefore the lowest boiling point. on distilling this type of solution, the first fraction will be a constant boiling point mixture i.e azeotropic mixture having fixed composition.

example of positive deviation

ethanol water mixture. the BP of th solution with 4.5 percent water and 95.5 percent ethanol is 78.1.

negative deviation

if the vapour pressure of solution is lower than the vapour pressure of either of the pure components, then the solution is said to have negative deviation.

for such solution, boiling point of solution is greater than boiling points of either of pure components.

explanation

consider two components A and B which form non ideal solution with negative deviation. consider a solution with composition corresponding to C. on distilling this solution, distillate without change in its composition is obtained. point C has lowest vap pressure because less volatile component at C is in excess and forces of attraction between components A and B are very strong

example

solution of HCl in water to form hydrochloric acid. it has BP 110 with 20.24 of. acid