What is Vapour Pressure?

When the pressure above a liquid is reduced at constant temperature, it evaporates and the resulting gas is called a vapour. This vapour, like other gases, has a pressure called vapour pressure. The vapour can become a liquid again. This is called condensation. All vapours have a saturation pressure which is the pressure of the vapour in equilibrium with its liquid. When the pressure in a system is higher than the saturation pressure, the vapour condenses, causing the pressure to return to the saturation value. Vice versa, as long as there is still a liquid in the system, the pressure cannot be reduced (by pumping) lower than the saturation pressure, because the liquid continues to evaporate.

In a container, in which a liquid and its vapour are present and the pressure is equal to the saturation vapour pressure, evaporation and condensation occur simultaneously. Water and its vapour pressure in vacuum systems need our special attention, because water is always present as humidity in the air and water is difficult to remove from vacuum systems.

We call gases vapour, when they can condense at normal temperatures. Strictly speaking all gases can condense, it depends only how much the temperature is reduced.

Vapour pressure of water at different temperatures.

Table 3

Vapour pressure of some liquids at 20° C

Table 4