If external pressure is lower than a liquid’s saturated vapor pressure, what happens?

• A. The liquid will stop boiling
• B. The liquid will begin to boil vigorously and continue as long as pressure remains low.
• C. The boiling point increases
• D. The liquid freezes

Answer: (B)

Boiling happens when a liquid’s vapor pressure becomes higher than the pressure surrounding it. The saturated vapor pressure is the amount of vapor pressure the liquid would have in equilibrium at that temperature. If the external pressure is lower than this saturated vapor pressure, bubbles of vapor form throughout the liquid and it boils vigorously. The boiling will continue as long as the outside pressure stays below the liquid’s vapor pressure. Lowering the external pressure lowers the boiling point, so the liquid boils at a lower temperature rather than increasing the boiling point. Freezing isn’t related to this situation.