For a non-ideal solution with a negative deviation:

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

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.