UPSC NDA II 2023 General Ability Solved Paper

Concept:Boiling happens when the vapour pressure of the liquid equals the surrounding atmospheric pressure.Explanation:Heating a liquid increases its vapour pressure ($P_{\text{vap}}$).Boiling begins when $P_{\text{vap}}$ becomes equal to the external atmospheric pressure ($P_{\text{atm}}$).At this temperature, bubbles form throughout the liquid and rise to the surface.Option A states: “When atmospheric pressure becomes equal to the vapour pressure.”Option D says: “When vapour pressure becomes equal to the air pressure.”Both describe the same equality, but the standard definition is exactly as in option A.If $P_{\text{atm}} < P_{\text{vap}}$ (option B), boiling can occur at a lower temperature, but it is not the defining condition.If $P_{\text{atm}} > P_{\text{vap}}$ (option C), the liquid does not boil.Therefore, the correct situation for boiling is when the two pressures are equal.Answer:Option A (When atmospheric pressure becomes equal to the vapour pressure).