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.

The azeotropic mixture of water (boiling point 100°C) and HCl (boiling point 85°C) boils at 108.5°C. When this mixture is distilled it is possible to obtain.

Neither HCl nor H₂O in pure states

The correct answer is option 4. Neither HCl nor H₂O in pure states

When an azeotropic mixture of water and HCl is distilled, the resulting distillate will depend on the composition of the mixture and the separation technique used. Let's analyze each option:

(1) Pure hydrogen chloride.

This option is not possible. Since the boiling points of water and HCl are different, an azeotropic mixture is formed to create a constant boiling point. The azeotropic mixture boils at a temperature higher than the boiling point of HCl alone. Therefore, distilling the azeotropic mixture will not yield pure hydrogen chloride.

(2) Pure water.

This option is also not possible. Similarly, the azeotropic mixture boils at a temperature higher than the boiling point of water alone. Therefore, distilling the azeotropic mixture will not yield pure water.

(3) Pure water as well as pure HCl.

This option is not possible. As mentioned earlier, an azeotropic mixture forms to achieve a constant boiling point. Distillation of the azeotropic mixture will not result in the separation of pure water and pure HCl.

(4) Neither HCl nor H₂O in pure states.

This option is correct. Distilling the azeotropic mixture of water and HCl will not produce either pure HCl or pure water. The azeotropic mixture has a boiling point higher than the boiling points of its individual components, making it difficult to separate them through distillation while maintaining their purity.

Therefore, the correct answer is: (4) Neither HCl nor H₂O in pure states.