40. Equilibrium between liquid and vapour

The quantity of water in the open container is continuously reduced by evaporation. But if the container is closed tightly, this does not happen. At a constant temperature, the system will be in a state of thermal equilibrium and will remain in it for as long as necessary. How does this work? After all, fast molecules will still fly out of the liquid and move to the area occupied by vapour.

Here's the thing. At the first moment, after we pour the liquid and close the container, the liquid will evaporate and the density of vapour over the liquid will increase. At the same time, however, the number of molecules returning to the liquid as a result of chaotic thermal movement will increase. This process of turning vapour into a liquid, the reverse of the evaporation process, is called condensation. The higher the density of vapour, the more molecules it returns to the liquid. The picture is different in an open container. Molecules that have left the liquid may not return to the liquid.

As a result, a state of equilibrium will eventually be established in a closed container. The number of molecules leaving the liquid surface will be equal to the number of vapour molecules returning to the liquid at the same time. The balance is dynamic in nature. Simultaneously with the evaporation process, condensation occurs and both processes on average compensate each other.

Vapour in equilibrium with its liquid is called saturated vapour. If the air from the liquid container is pumped out, there will be nothing but saturated vapour above the liquid surface.