An ideal solution is one in which the attraction between components of the solution is the same as the interaction between the molecules of each component. Heat is neither absorbed nor evolved during the formation of ideal solution and the volume of the solution is equal to the sum of the volumes of the component liquids. The vapour pressures of ideal solutions can be calculated by averaging the properties of the liquids. The solutions in which properties of dissolved liquids are different from those of the liquids in the pure state and which are formed by evolution or absorption of heat are called non-ideal solutions. Raoult’s law states that partial pressures of component (say liquid A) in solution is proportional to the mole fraction. If all the components in solutions behave like ideal gases, then the total pressure of the solution is equal to the sum of the partial pressure of the individual components.

\[P_{Total} = \chi _AP_A^o + \chi _BP_B^o\]

where \(P_A^o\) and \(P_B^o\) are the vapour pressures of pure solvents A and B, respectively, and \(\chi _A\) and \(\chi _B\) are mole fractions of the solvents A and B in solution. The composition of vapour of an ideal solution can be determined by the partial pressures of the components.

If \(Y_A\) and \(Y_B\) are the mole fractions of the components A and B in the vapour phase, the partial

vapour pressures of A and B can be calculated using Dalton’s law of partial pressures.

\[P_A = Y_AP_{Total}\]

\[P_B = Y_BP_{Total}\]

A non-ideal solution is that solution (i) which does not obey Raoult’s law (ii) for which \(\Delta V_{mix}\) is not zero and (iii) for which \(\Delta H_{mix}\) is not zero. In non-ideal solutions, the solute–solvent interactions are weaker or stronger than the solute–solute and solvent–solvent interactions. The non-ideal solutions in which solute–solvent interactions are weaker or stronger than the solute– solute or solvent–solvent show positive deviations from Raoult’s law.

Which of the following solution pairs can be separated into its pure components by fractional distillation?

Benzene–toluene

The answer is (1) Benzene–toluene.

Fractional distillation is a process of separating the components of a liquid mixture by boiling the mixture and then collecting the vapors that come off at different temperatures. The components of the mixture will vaporize at different temperatures depending on their boiling points. The vapors that vaporize at lower temperatures will be collected first, and the vapors that vaporize at higher temperatures will be collected later.

Benzene and toluene have different boiling points. Benzene has a boiling point of 80°C, and toluene has a boiling point of 110°C. This means that when benzene and toluene are mixed, the benzene will vaporize at a lower temperature than the toluene. The benzene vapors can then be collected and condensed, leaving behind the toluene.

Water and nitric acid have different boiling points. Water has a boiling point of 100°C, and nitric acid has a boiling point of 83°C. This means that when water and nitric acid are mixed, the water will vaporize at a lower temperature than the nitric acid. The water vapors can then be collected and condensed, leaving behind the nitric acid.

Water and hydrochloric acid have an azeotropic mixture, which means that the boiling point of the mixture is the same as the boiling points of the pure liquids. This means that it is not possible to separate water and hydrochloric acid by fractional distillation.

Water and ethanol have different boiling points. Water has a boiling point of 100°C, and ethanol has a boiling point of 78°C. This means that when water and ethanol are mixed, the ethanol will vaporize at a lower temperature than the water. The ethanol vapors can then be collected and condensed, leaving behind the water.

Therefore, the only solution pair that can be separated into its pure components by fractional distillation is benzene–toluene.