Concept:Boiling occurs when the vapour pressure of the liquid equals the external or atmospheric pressure.Explanation:A liquid boils when its vapour pressure becomes equal to the surrounding atmospheric pressure.At this point, bubbles of vapour can form throughout the liquid and rise to the surface.If the vapour pressure is less than atmospheric pressure, the liquid does not boil.If it is greater, the liquid is above its boiling point and boils rapidly.Answer:Equal to the atmospheric pressure.