In boiling water when we can see, at atmospheric pressure, temperature to 100 ℃, water began to boil. But water not all at once into steam, but with the absorb heat, steam increasing. In the stage of coexistence of steam and liquid, it is called "saturated state ". The state under steam called "saturated steam", water called "saturated water ". In the whole vaporization stage, steam and water have the same temperature, so called "saturated temperature ".
Distillation column at the top of the situation is similar, qi dan and liquid nitrogen is coexistence of saturated state, have the same saturation temperature. But, the same temperature of saturated liquid and saturated steam belong to different state. Saturated steam heat cold cemented saturated liquid, temperature remains constant, these heat called "condensation latent heat"; saturated liquid absorption heat exasperating into saturated steam, temperature also maintain saturated temperature unchanged, these heat called "evaporation latent heat ". For the same substance, under the same pressure, the two are equal in numerical value.
