The vapor pressure of a substance is the pressure at which the gaseous state of the substance is in thermodynamic equilibrium with the liquid state. Vapor pressure is measured in terms of the unit of pressure that is Pascal (Pa).
A substance with a higher vapor pressure is a more volatile liquid. This is due to the fact that a larger amount of the liquid has to be converted to the gaseous state before a thermodynamic equilibrium is reached between the two states. The vapor pressure of liquids with a lower boiling point is found to be higher than liquids with a higher boiling point. This is corroborated by the fact that liquids with a lower boiling point are more volatile.