An azeotropic mixture of two liquids boils at a lower temperature than either of them why?

It shows a positive deviation from Raoult’s law

The correct answer is option 4. It shows a positive deviation from Raoult’s law.

An azeotropic mixture of two liquids that boils at a lower temperature than either of the pure components exhibits a positive deviation from Raoult's law.

Raoult's law states that the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction. For an ideal solution, the interactions between different molecules are similar to those between like molecules.

Positive Deviation from Raoult's Law: When a mixture shows a positive deviation from Raoult's law, the interactions between different molecules (A and B) are weaker than the interactions between like molecules (A-A and B-B). This weaker interaction leads to a higher tendency for the molecules to escape into the vapor phase, resulting in a higher total vapor pressure than predicted by Raoult's law.

An azeotropic mixture is a specific composition of two or more liquids that behaves as a single substance when boiled or condensed. It has a constant boiling point and retains the same composition in both liquid and vapor phases.

For a positive deviation azeotrope, the higher vapor pressure at a specific composition means that the boiling point of the mixture is lower than the boiling points of either of the pure components. This occurs because the vapor pressure of the mixture is higher, making it easier for the mixture to reach the atmospheric pressure and boil.

Example : Consider a mixture of ethanol and water, which forms an azeotrope with a lower boiling point than either pure ethanol or pure water. The interactions between ethanol and water molecules are weaker than the interactions within pure ethanol or pure water, leading to a higher vapor pressure for the mixture. As a result, the azeotropic mixture boils at a lower temperature.

Summary: An azeotropic mixture that boils at a lower temperature than either of its components shows a positive deviation from Raoult's law because the weaker interactions between different molecules in the mixture result in a higher vapor pressure and a lower boiling point.