Question

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

• A. vapour pressure of the solution increases
• B. heating occurs
• C. cooling occurs
• D. equilibrium remains unaffected

Hint:

This is the same principle behind why we feel cold when sweat evaporates or when using a "cooling spray."
Decrease in pressure \( \rightarrow \) More evaporation \( \rightarrow \) Absorption of heat \( \rightarrow \) Cooling.

Answer 1

The Correct Option is C

Step 1: Recall the equilibrium being disturbed.
At equilibrium, molecules are leaving the liquid to become vapour at the same rate that vapour molecules are returning to the liquid.
Step 2: Apply Le Chatelier's principle to the pressure drop.
Lowering the pressure suddenly favours whichever side of the equilibrium takes up more volume, and that is always the gas side, so more liquid evaporates to push the system back toward its old pressure.
Step 3: Track where the energy for this extra evaporation comes from.
Turning liquid into vapour needs energy to break the intermolecular forces holding the liquid together, and since nothing is added from outside, that energy is drawn straight out of the remaining liquid itself.
Step 4: Conclude the temperature effect.
Because the liquid is losing its own thermal energy to fuel this evaporation, its temperature drops. \[ \boxed{\text{cooling occurs}} \]