When the number of molecules leaving the water exceeds the number of molecules returning, the water begins to evaporate freely. At this point, the water reaches its boiling point, or saturation temperature, because it is saturated with thermal energy. Further increases in temperature, while keeping the pressure constant, do not cause a double temperature change, but only result in the formation of steam.

Pure water does not exist in nature due to its high solubility and contains various impurities. Water impurities are classified into three general categories: dissolved solids, suspended and colloidal solids, and gases.