Ideal and Non-ideal Solutions

Once we know how each liquid contributes its vapour pressure through Raoult's law, we can use that law as a test for how a real liquid-liquid mixture behaves. Some mixtures follow Raoult's law faithfully at every composition; many others drift away from it. On this basis, liquid-liquid solutions are sorted into two categories:

• Ideal solutions — obey Raoult's law across the entire composition range.
• Non-ideal solutions — depart from Raoult's law, showing a vapour pressure either higher or lower than the law predicts. …