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

Concept:

• At equilibrium, the rate of evaporation equals the rate of condensation.

• According to Le Chatelier's Principle, if a system at equilibrium is disturbed, the system will shift in a direction that counteracts the disturbance.

• Decreasing the pressure favors the phase that occupies more volume, which is the gaseous (vapour) phase.

• Evaporation is an endothermic process, meaning it absorbs heat from the surroundings.
Step 1: Determine the shift in equilibrium
When pressure is suddenly decreased, the system tries to increase the pressure back.
It does this by producing more gas molecules.
Therefore, the equilibrium shifts in favor of vaporization: \[ \text{Liquid} \rightarrow \text{Vapour} \]

Step 2: Evaluate the energy change
Vaporization requires energy to overcome the intermolecular forces in the liquid.
This energy (Latent Heat of Vaporization) is taken from the internal energy of the liquid itself or the immediate surroundings.
\[ \Delta H_{\text{vap}} \gt 0 \, (\text{Endothermic}) \]

Step 3: Analyze the temperature effect
Because the process is endothermic and happens "suddenly" (often adiabatic or near-adiabatic), the liquid loses its thermal energy to power the evaporation.
The loss of kinetic energy of the remaining molecules results in a drop in temperature.
Therefore, cooling occurs.