Question

What is positive and negative deviation from ideality?

Answer 1

In mixtures showing a positive deviation from Raoult's Law, the vapour pressure of the mixture is always higher than you would expect from an ideal mixture. If the vapor pressure of a mixture is lower than expected from Raoult's law, there is said to be a negative deviation.

Answer 2

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If the vapor pressure of a mixture is lower than expected from Raoult's law, there is said to be a negative deviation. Negative deviations from Raoult's law arise when the forces between the particles in the mixture are stronger than the mean of the forces between the particles in the pure liquids.