The reason for this is that some molecules have enough kinetic energy to overcome the attraction of neighboring molecules and gravity to escape into the gas phase. . If the gas space is closed, the vapor pressure will increase to a definite value for a given temperature and remain there. This is called the equilibrium vapor pressure. . A dynamic equilibrium is achieved. Molecules are leaving and returning to the liquid.
Vapor pressure is inversely proportional to attractive forces.
How easily a molecule leaves a substance depends on the strength of the attraction to other molecules, atoms, or ions. As the temperature increases the vapor pressure increases. Vapor pressure is independent of the amount of substance present as long as some of each phase is present after equilibrium is reached. The normal boiling point is the temperature at which the vapor pressure of the liquid is equal to 1 atmosphere - 760 torr. As atmospheric pressure changes the boiling point changes.