Effect of pressure on solubility:\nFor a liquid–liquid solution whose mixed volume is less than the sum of pure-component volumes (negative volume change on mixing), how does solubility respond to pressure?

Introduction / Context:
When two liquids mix with a volume contraction (the solution volume is less than the sum of pure volumes), the process has a negative excess volume. According to thermodynamic principles, pressure tends to favor states with smaller molar volumes. Understanding this helps predict how solubility and miscibility shift with applied pressure in liquid–liquid systems (not solid–liquid dissolution).

Given Data / Assumptions:

• Liquid–liquid solution with negative volume change on mixing.
• No chemical reaction; only physical mixing.
• Temperature held constant during the pressure change.

Concept / Approach:
Le Chatelier’s principle (generalized) indicates that increasing pressure favors the side of an equilibrium with lower volume. If mixing decreases volume, higher pressure drives the equilibrium toward more mixed state, enhancing mutual solubility. Conversely, if mixing increased volume, pressure would disfavor mixing.

Step-by-Step Solution:
Recognize: ΔVmix < 0 for the solution.Apply pressure: system minimizes Gibbs energy G = H − T S + ∫V dP.Lower V states (more mixed) reduce G more under higher P.Therefore, mutual solubility increases as pressure rises.

Verification / Alternative check:
Phase diagrams for partially miscible liquid pairs often show the miscibility gap shrinking under elevated pressure when ΔVmix is negative.

Why Other Options Are Wrong:
“Increases with decrease in pressure” contradicts the volume argument. “Unchanged” ignores measurable pressure effects. “Independent of temperature” is irrelevant; the question is about pressure. “Becomes zero at high pressure” is not a general rule.

Common Pitfalls:
Confusing gas solubility (strong pressure dependence) with liquid–liquid mixing; also mixing up the sign of ΔVmix.

Final Answer:
Increases with rise in pressure