ANS:A - increases

The decrease in mutual solubility with an increase in temperature can be explained by the principles of thermodynamics, specifically the concept of entropy.

1. Entropy Increase: As temperature rises, the molecules in the liquid phase gain kinetic energy and move more rapidly. This increased molecular motion disrupts the intermolecular forces holding the molecules of the liquids together. As a result, the solvent molecules become less capable of surrounding and dissolving the solute molecules effectively.
2. Entropy Effect on Solubility: The decrease in solubility with increasing temperature is driven by changes in the entropy of the system. When a solute dissolves in a solvent, there is usually a decrease in the entropy of the system due to the ordering of solvent molecules around the solute molecules. However, as temperature increases, the gain in entropy from increased molecular motion outweighs the decrease in entropy from solvation, leading to a net decrease in solubility.
3. Phase Separation: At higher temperatures, the decrease in solubility can lead to phase separation, where the solute and solvent form distinct layers or phases within the mixture. This phase separation is often observable in systems where the solubility decreases significantly with temperature.