The process in which water changes from a liquid to gas is called vaporization. The two main types of vaporization are evaporation and boiling. Evaporation is the process in which water molecules on the surface of a liquid gain enough energy to escape into the atmosphere. Boiling is the process in which water molecules throughout the liquid gain enough energy to escape into the atmosphere. It occurs when the vapor pressure of a liquid becomes equal to the external pressure applied to the surface of the liquid.