How does vapor pressure relate to flashing of a liquid?

• A. If vapor pressure significantly exceeds ambient pressure, flashing occurs
• B. Vapor pressure has no effect
• C. Higher vapor pressure reduces flashing
• D. Only temperature matters

Answer: (A)

Vapor pressure is the pressure at which a liquid is in equilibrium with its vapor at a given temperature. Flashing occurs when the surrounding pressure is reduced below the liquid’s vapor pressure, so the liquid can no longer stay in equilibrium and part of it rapidly vaporizes to reach the new balance. If the vapor pressure at that temperature significantly exceeds the ambient pressure, a substantial portion of the liquid will flash into vapor, driven by that pressure difference.

This is why the option stating that flashing happens when vapor pressure greatly exceeds ambient pressure is the best: vapor pressure directly governs the tendency to vaporize, and a larger gap to the ambient pressure means more flashing. It’s not correct to say vapor pressure has no effect, or that higher vapor pressure reduces flashing, or that only temperature matters—ambient pressure matters too, because a higher surrounding pressure can suppress flashing even at the same temperature.