Positive Deviation
Behaviour in which a solution exerts a higher vapour pressure than Raoult's law predicts.
Formula / equation
p > x p°
Unit
Pa
In depth
Positive deviation occurs when unlike molecules attract each other more weakly than like molecules do, so mixing breaks strong interactions and molecules escape more readily. Such mixing is endothermic and accompanied by a volume increase, and it can produce a minimum-boiling azeotrope.
Examples in the real world
Ethanol with cyclohexane; acetone with carbon disulfide, whose mixture cools on stirring.