Sat July 09, 2011 By: Akshaya Parthasarathy

what is deviation from ideality ?state examples of solutions showing positive or negative deviation

Expert Reply
Sat July 09, 2011

Many pairs of liquids are present in which there is no uniformity of attractive forces i.e. the adhesive and cohesive forces of attraction are not uniform between the two liquids, so that they show deviation from the Raoult's law which is applied only to ideal solutions.

Negative deviation

When adhesive forces between molecules of A and B are greater than the cohesive force between A and A, or B and B, then the vapor pressure of the solution is less than the expected vapor pressure from Raoult's law. This is called a negative deviation from Raoult's law. These cohesive forces are lessened not only by dilution but also attraction between two molecules through formation of hydrogen bonds. This will further reduce the tendency of A and B to escape.

For example, chloroform and acetone show such an attraction by formation of a hydrogen bond.

Positive deviation

When the cohesive forces between like molecules are greater than the adhesive forces, the dissimilarities of polarity or internal pressure will lead both components to escape solution more easily. Therefore, the vapor pressure will be greater than the expected from the Raoult's law, showing positive deviation. If the deviation is large, then the vapor pressure curve will show a maximum at a particular composition, e.g. benzene and ethyl alcohol, carbon disulfide and acetone, chloroform and ethanol.


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