Concept:Boiling occurs when the saturated vapour pressure of a liquid equals the external atmospheric pressure.
Explanation:At any temperature, some molecules escape as vapour and create vapour pressure above the liquid.
As temperature rises, the saturated vapour pressure increases.
When the saturated vapour pressure becomes equal to the atmospheric pressure, bubbles of vapour form inside the liquid and rise to the surface.
This temperature is called the boiling point.
For water at sea level, this occurs at
100∘C.
Option B describes a dynamic equilibrium between evaporation and condensation, which is not the definition of boiling point.
Options C and D describe net evaporation or net condensation, not the boiling point.
Answer:A. saturated vapour pressure equals the atmospheric pressure.