What is the vapor pressure of a liquid at its normal boiling temperature?
The vapour pressure is
And this is the principle behind vacuum distillation. If we reduce the ambient pressure, the vapour pressure of the liquid may become equal to the ambient pressure (that developed by the vacuum pump!) at LOWER temperatures, and thus we may distil a liquid that is involatile at 1 atmosphere. Are you with me?
This is a definition for a 2nd year university student. Such sophistication would not be required of an A-level student.