When evacuated containers are filled partially with some liquid, then a part of the liquid tends to evaporate in order to fill the remainder of the volume of the container with gaseous substance (commonly referred to as vapour). The pressure exerted by this gaseous vapour on the walls of the container is called vapour pressure. Eventually, the amount of gaseous vapour and the amount of liquid in the container reaches a state of equilibrium. It is important to note that the vapour pressure of a liquid is dependent on the temperature – a liquid can exhibit different vapour pressures at different temperatures.
The boiling point of a liquid can also be defined in terms of its vapour pressure as – the point of temperature at which the vapour pressure of the liquid becomes equal to the vapour pressure of its surroundings.