A mixture of two or more substances in which there is an interaction between the molecules or atoms of the individual components. Raoult’s law applies to ideal mixtures in which the forces between the particles in the mixture are the same as those in the pure liquids. Mixtures that exhibit a positive deviation from Raoult’s law have vapour pressures greater than that of an ideal mixture (see Fig. 31). This is caused by the intermolecular forces between the molecules being less than for the pure liquids. Heat is absorbed when the liquids mix. That is, the enthalpy change of mixing is endothermic. An example is ethanol and water, which has a maximum vapour pressure for a mixture containing 95.6 per cent of ethanol by mass. Mixtures that exhibit a negative deviation from Raoult’s law have vapour pressures that are less than those of an ideal mixture. This is due to stronger intermolecular forces in the mixture than for the pure liquids. Heat is therefore evolved on mixing. An example is the mixing of nitric acid and water, which react to form hydroxonium ions and nitrate ions.

Fig. 31 Non-ideal mixture (positive deviation)