Raoult's law states that in an ideal binary solution of two volatile components, the partial vapour pressure of each component is proportional to its mole fraction in the solution.

A plot of total pressure to mole fraction of one of the components gives:

Graph for Ideal Raoult's Law

If A and B form a non-ideal solution with positive deviation, then the following graph is observed:

Graph for Positive deviation of Raoult's Law

At what composition of the two components is the deviation (difference in total pressure) maximum?

My guess would be that maximum deviation is observed when mole fraction of both the components are equal.

My reasoning is that the deviation is observed because of the fact that interactions between A-A and B-B particles are not equal to the A-B particle interactions. When the quantity of both the components are equal, there is a maximum amount of A-B interactions. Hence, the deviation must be maximum.

Is this reasoning correct?

  • $\begingroup$ Are you talking for the postive deviation then for postive deviation vapour pressure should be greater than the value of individual vapour pressure mathematically you can differentiate the value it would be in the range $\endgroup$ Aug 12, 2020 at 8:50

1 Answer 1


There's no definitive answer to your question. You might want to look up azeotropic mixtures.

Essentially, to make an azeotrope, you need a set of liquids with really high deviation from "ideal" behaviour, and at the composition where the deviation is maximum, the slope is zero. At that composition, there isn't much variation in vapor pressure as you vary the mole fractions, and so both components of the liquid boil equally, as stated here:

An azeotrope or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling point mixtures.

Now, if you look at a few examples of azeotropes, it becomes clear that the maximum deviation is not achieved at the same composition ratio:

Sulfuric acid (98.3%) / water, boils at 338 °C ||Nitric acid (68%) / water, boils at 120.2 °C at 1 atm|| Perchloric acid (71.6%) / water, boils at 203 °C|| Hydrofluoric acid (35.6%) / water, boils at 111.35 °C

References: Azeotropes, wikipedia


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