Negative Deviation
Behaviour in which a solution exerts a lower vapour pressure than Raoult's law predicts.
Formula / equation
p < x p°
Unit
Pa
In depth
Negative deviation occurs when unlike molecules attract each other more strongly than like molecules do, often through new hydrogen bonding, so escape into the vapour becomes harder. Mixing is exothermic and accompanied by contraction, and a maximum-boiling azeotrope may result.
Examples in the real world
Chloroform with acetone, which form a hydrogen bond and warm on mixing; nitric acid with water.