Boiling occurs when the vapor pressure of the liquid equals what?

• A. The vapor pressure at the surface
• B. External pressure
• C. The temperature of the surrounding air
• D. Humidity of the environment

Answer: (B)

Boiling happens when the vapor pressure of the liquid becomes equal to the pressure exerted by its surroundings. At that moment, bubbles of vapor can form inside the liquid and rise to the surface, driving rapid liquid-to-gas transition. The temperature at which this occurs is the boiling point and it depends on the external pressure: at sea level, water boils at 100°C; higher external pressure raises the boiling point, while lower external pressure lowers it. The other factors listed aren’t what set boiling: vapor pressure at the surface is just the pressure the liquid already has at that temperature, not the threshold that initiates boiling; the surrounding air temperature doesn’t determine the boiling point; and humidity doesn’t affect this phase transition.