Vapor pressure

Under certain external conditions, the liquid molecules in the liquid will evaporate into gaseous molecules, and the gaseous molecules will also hit the liquid surface and return to the liquid state. This is a two-phase change in a one-component system that reaches equilibrium after a certain period of time. At equilibrium, the gaseous molecular content reaches a maximum value, and the pressure that these gaseous molecules can hit by the liquid, referred to as vapor pressure.

The vapor pressure reflects that there is a small amount of molecules in the solution that have a tendency to leave the matrix and enter the space. This tendency is also called escape tendency.

The vapor pressure is not equivalent to atmospheric pressure.

In the saturated state, the partial pressure of water vapor in the humid air is equal to the vapor pressure of the pure water at the air temperature.