When a liquid is in a closed container, some molecules pass
from the liquid to the vapor phase and others from the vapor to the liquid phase. When the number
molecules that are exchanged in each direction is the same, the system is in equilibrium.
Under these conditions the pressure of the molecules in the vapor phase is known as the saturation vapor pressure
which increases with temperature
This is a physical process known as evaporation. Intermolecular forces
that maintain bound molecules in the liquid are broken without breaking bonds within the molecule.