Saturation temperature and saturation pressure are terms in gas-liquid equilibrium. If it is not filled with liquid in a sealed container, part of liquid molecules will enter the upper space, which is called "evaporation ". As the number of vapour molecules in space increases, the pressure of vapour generated by it also increases. At a certain time, the number of vapour molecules in space will not increase any more. At this time, the number of molecules leaving the liquid and the number of molecules returning the liquid from Space reach a dynamic balance, which is also called "saturated state ". At this time, the pressure generated by vapour is called "saturated pressure ". For the same substance, the saturation pressure is related to the temperature. The higher the temperature is, the greater the energy the molecule has, the easier it is to get rid of the liquid and gasification, and the corresponding saturation pressure is also higher. A certain temperature corresponds to a certain saturation pressure, and the two are not independent. Therefore, in the saturated state, the temperature corresponding to the saturation pressure is also called "saturation temperature ". Generally, the relationship between saturation temperature and saturation pressure of various substances can be found in the manual.
The gasification process of common water in the air can be divided into evaporation and boiling. Evaporation is conducted on the surface of water, and boiling is the process of gasification in the liquid at the same time. Under certain pressure, when the temperature of the liquid rises to the temperature of boiling, it is called "boiling point ts ".
For pure matter, there is no essential difference between evaporation and boiling, and boiling point is also called "evaporation temperature ". For example, when heating the liquid sealed in the constant pressure vessel in Figure 6, the temperature t of the liquid at the beginning is lower than the boiling point ts, and all of them are in the liquid state, which is called supercooled liquid (Figure 6a ); when the temperature of the liquid is heated up to the boiling point (Figure 6b), the liquid will begin to gasification, which is called saturated liquid; in the gasification stage, the quantity of vapour increases continuously, the temperature remains the same as the boiling point (figure 6c), until all the liquid vapates into vapour (figure 6d), which is called saturated vapour. The gas and liquid in the container have the same temperature in the gasification stage. The relationship between boiling point and pressure is the same as that between saturation temperature and saturation pressure. Therefore, the boiling point is the saturation temperature under the same pressure. The two have the same meaning, but different statements. The state of gas-liquid coexistence is called saturated. Heating the saturated vapor continuously, the temperature of the vapor rises, exceeding the saturated temperature, which is called superheated vapor (figure 6e ).
Condensation process is the inverse process of evaporation. For pure substances, condensation temperature is also called liquefaction temperature, which is equal to the evaporation temperature under the same pressure. Saturation temperature can unify the two.
