Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor  There are two types of vaporization: evaporation and boiling. Evaporation is a phase transition from the liquid phase to vapor (a state of substance below critical temperature and critical pressure) that occurs at temperatures below the boiling temperature at a given pressure. Evaporation usually occurs on the surface. Evaporation may occur when the partial pressure of vapor of a substance is less than the equilibrium vapour pressure.