Question

When a liquid and its vapour are at equilibrium and the pressure is suddenly decreased:

• A. Vapour pressure of solution increases
• B. Heating occurs
• C. Cooling occurs
• D. Equilibrium remains unaffected

Hint:

Sudden pressure decrease ? more vaporization ? endothermic process ? temperature decreases (cooling).

Answer 1

The Correct Option is C

Step 1: System at equilibrium.
At liquid-vapour equilibrium, the rate of evaporation equals the rate of condensation, and the vapour pressure equals the external pressure.
Step 2: Effect of sudden pressure decrease.
When external pressure is suddenly reduced, the system responds by shifting the equilibrium toward the vapour phase (more molecules evaporate) to re-establish equilibrium.
Step 3: Energy consequence.
Evaporation is an endothermic process; it requires the latent heat of vaporisation. This energy is drawn from the liquid itself, reducing its internal energy.
Step 4: Conclusion.
The liquid loses energy to drive evaporation and its temperature drops: cooling occurs.
\[ \boxed{\text{Cooling occurs}} \]