Ideal and Nonideal Solutions

1. Ideal solutions : Solutions with the following properties are called ideal solutions.

i. Ideal solutions obey Raoult's law over the entire range of concentrations.

ii. No heat is evolved or absorbed when two components forming an ideal solution are mixed -- the enthalpy of mixing is zero, $\Delta_{mix}H = 0$.

iii. There is no volume change when two components forming an ideal solution are mixed: the volume of an ideal solution is equal to the sum of the volumes of the two components taken for mixing, $\Delta_{mix}V = 0$.

iv. In an ideal solution, the solvent-solute, solute-solute and solvent-solvent molecular interactions are comparable.

v. The vapour pressure of an ideal solution always lies between the vapour pressures of the pure components, as shown in Fig. 2.2.

It is important to understand that perfectly ideal solutions are uncommon; solutions such as benzene + toluene behave nearly ideally.

2. Nonideal solutions :

i. These solutions do not obey Raoult's law over the entire range of concentrations.

ii. The vapour pressures of these solutions can be higher or lower than those of the pure components.

iii. Deviation from the Raoult's law : These solutions show two types of deviation from the Raoult's law.

A. Positive deviations from Raoult's law - The solutions in which the solute-solvent intermolecular attractions are weaker than those between solute-solute molecules and solvent-solvent molecules exhibit positive deviations. The vapour pressures of such solutions are higher than those of the pure components, as shown in Fig. 2.3. The solutions of ethanol and acetone, and of carbon disulphide and acetone, show positive deviations from the Raoult's law.

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