Vapour Pressure

During the process of evaporation, molecules escape from the surface of liquids in an enclosed container. These molecules accumulate on the surface of the liquid to form vapour.

The pressure exacted by this escaped molecules or vapour on the surface of the liquid is called vapour pressure of the liquid.

Vapour Pressure  is defined as the pressure exacted by the escaped molecules or vapour on the surface of the liquid ..

Saturated and unsaturated vapour

Saturated vapour is when the space above a liquid can no longer take additional escaped molecules of the liquid. At this stage the molecules is said to be in dynamics equilibrium. The number of molecules entering the liquid is the same as the number of molecules escaping from the liquid. The vapour is said to be saturated at this point.

The pressure exerted by a saturated vapour is called saturated vapour pressure.

On the other hands, unsaturated vapour is a vapour that can increase in number of molecules coming out of  the liquid to form more vapour above the liquid. At the stage of unsaturation, there is one directional movement of molecules which is out of the liquid to form more vapour. The molecules are coming out but not returning.

Unsaturated vapour can become saturated vapour when enough molecules have escaped from the surface of the liquid and the molecules are now in dynamic equilibrium.

A vapour is said to be in dynamic equilibrium when the amount of the molecules coming in is equal to the amount of molecules going out of the liquid.

The pressure exerted by an unsaturated vapour is called unsaturated vapour pressure.

Increase in temperature increases the saturation vapour pressure because increase the average Kinetic energy of the molecules and the saturation vapour pressure shift to a higher value.

Saturation vapour pressure does not obey Charles’ law and Boyles’ law, while unsaturated vapour pressure obey these two laws.

When the temperature of a liquid is increased until it reached the boiling point, it has been proved that the saturation vapour pressure of a liquid at its boiling point is equal to the atmospheric pressure.

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