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the vapour pressure of the solvent, x be1 its mole fraction, p0be its vapour pressure i in the pure state. Then according to Raoult’s law Fig. 1.5 If a solution obeys p1µ x 1 Raoult's law for all and p1= x 1p 0 (1.20) concentrations, its 1 The proportionality constant is equal vapour pressure to the vapour pressure of pure solvent, p0. would vary linearly 1 from zero to the A plot between the vapour pressure and vapour pressure of the mole fraction of the solvent is linear the pure solvent. (Fig. 1.5). 1.5 Ideal and Non- Liquid-liquid solutions can be classified into ideal and non-ideal solutions on the basis of Raoult’s law. ideal Solutions 1.5.1 Ideal The solutions which obey Raoult’s law over the entire range of Solutions concentration are known as ideal solutions. The ideal solutions have two other important properties. The enthalpy of mixing of the pure components to form the solution is zero and the volume of mixing is also zero, i.e., D mix = 0, D mix = 0 (1.21) It means that no heat is absorbed or evolved when the components are mixed. Also, the volume of solution would be equal to the sum of volumes of the two components. At molecular level, ideal behaviour of the solutions can be explained by considering two components A and B. In pure components, the intermolecular attractive interactions will be of types A-A and B-B, whereas in the binary solutions in addition to these two interactions, A-B type of interactions will also be present. If the intermolecular attractive forces between the A-A and B-B are nearly equal to those between A-B, this leads to the formation of ideal solution. A perfectly ideal solution is rare but some solutions are nearly ideal in behaviour. Solution of n-hexane and n-heptane, bromoethane and chloroethane, benzene and toluene, etc. fall into this category. 1.5.2 Non-ideal When a solution does not obey Raoult’s law over the entire range of Solutions concentration, then it is called non-ideal solution. The vapour pressure of such a solution is either higher or lower than that predicted by Raoult’s law (equation 1.16). If it is higher, the solution exhibits positive deviation and if it is lower, it exhibits negative deviation from Raoult’s law. The plots of vapour pressure as a function of mole fractions for such solutions are shown in Fig. 1.6. The cause for these deviations lie in the nature of interactions at the molecular level. In case of positive deviation from Raoult’s law, A-B interactions are weaker than those between A-A or B-B, i.e., in this case the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules. This means that in such solutions, molecules of A (or B) will find it easier to escape than in pure state. This will increase the vapour 13 Solutions Reprint 2026-27