If we leave a liquid in a container, over time we will see that its volume decreases, even if it is not visible to the naked eye. A particularity that we can observe in liquids at rest is that the particles that compose them have considerable mobility, with this they manage to overcome the forces of attraction and thus pass to the state gaseous. This process takes place at any temperature below the boiling temperature of the liquid and is called evaporation.
We can define the evaporation as a process in which particles leave the liquid's surface and enter a gaseous state.
An interesting fact that we must emphasize is that the same particles that left the liquid penetrate it, returning to the initial state – in this case, the liquid state, as shown in the figure above. We have to remember that these two mentioned processes happen at the same time, and always the balance is reached, making the two phases coexist. Thus, we can say that the greater the temperature of the liquid, the greater the movement of its particles, and with this, a greater number of them will leave the liquid.
THE Steam pressure it is nothing more than the pressure that the particles that have left the liquid exert on the surface of the liquid, or rather, the container. It is important to emphasize that the process can occur in reverse, that is, when the liquid temperature decreases, part of the gas molecules can return to a liquid state.
We can define the equilibrium vapor pressure as being the one in which the gaseous and liquid phases coexist in equilibrium. Evaporation occurs under any temperature condition, but the the amount of gas formed depends on the temperature. Steam pressure depends on temperature.
By Domitiano Marques
Graduated in Physics
Brazil School Team
Source: Brazil School - https://brasilescola.uol.com.br/fisica/pressao-vapor-evaporacao.htm