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.

If two liquids A and B form minimum boiling azeotrope at some specific composition then

A–B interactions are weaker than those between A–A or B–B.

The correct answer is option 4. A–B interactions are weaker than those between A–A or B–B.

To understand the behavior of a minimum boiling azeotrope, let’s break down the key concepts:

An azeotrope is a mixture of two or more liquids that boils at a constant temperature and with a constant composition, even when the vapor is condensed and reboiled. The boiling point of an azeotrope is typically different from that of the pure components.

A minimum boiling azeotrope boils at a lower temperature than either of the pure components. This happens at a specific composition of the mixture.

Vapor Pressure and Intermolecular Interactions

1. Vapor Pressure Concept:

The vapor pressure of a liquid is the pressure exerted by the vapor of the liquid when it is in equilibrium with its liquid phase. According to Raoult’s Law, the vapor pressure of a solution is the sum of the partial pressures of each component in the solution, weighted by their mole fractions.

2. Intermolecular Interactions:

The vapor pressure of a solution is influenced by the strength of intermolecular interactions.

Strong A–A or B–B Interactions: If the intermolecular forces between like molecules (A–A or B–B) are strong, the vapor pressure will be lower because fewer molecules can escape into the vapor phase.

Weak A–B Interactions: When the interaction between different molecules (A–B) is weaker, the vapor pressure increases because the molecules are more easily able to escape into the vapor phase.

Minimum Boiling Azeotrope Explained

1. Weaker A–B Interactions:

For a minimum boiling azeotrope, the interaction between different types of molecules (A–B) is weaker than the interactions between the same type of molecules (A–A or B–B). As a result, the solution has a higher vapor pressure than predicted by Raoult’s Law, leading to a lower boiling point.

2. Implications:

Higher Vapor Pressure:Due to weaker A–B interactions, more molecules are able to escape into the vapor phase, resulting in a higher vapor pressure.

Lower Boiling Point: Because of the higher vapor pressure, the mixture boils at a lower temperature compared to the boiling points of the pure components.

Summary

In the case of a minimum boiling azeotrope: The interactions between the different molecules (A–B) are weaker than those between the same type of molecules (A–A or B–B). This leads to a higher vapor pressure for the mixture compared to the pure components, and hence, the mixture boils at a lower temperature than either of the pure liquids.

So, the correct explanation is: A–B interactions are weaker than those between A–A or B–B.