What is Vapour Pressure? – Definition, Raoult’s Law, Formula

What is Vapour Pressure? – Definition, Raoult’s Law, Formula

Generally, liquids have a tendency to evaporate. If the kinetic energy of molecules in the liquid state overcomes the intermolecular force of attraction between them, then the molecules will escape from the liquid state. This process is called ‘evaporation’ and it happens on the surface of the liquid.

If evaporation is carried out in a closed container then the vapour remains in contact with the surface of the liquid. These vapour molecules are in continuous random motion during which they collide with each other and also with the walls of the container. As the collision is inelastic, they lose their energy and as result the vapour returns back to liquid state.

Vapour Pressure of Liquid

This process is called as ‘condensation’. Evaporation and condensation are continuous processes. If the process is carried out in a closed system, a stage is reached when the rate of evaporation becomes equal to the rate of condensation. This, an equilibrium is established between liquid and its vapour.

The pressure of the vapour in equilibrium with its liquid is called vapour pressure of the liquid at the given temperature. The vapour pressure of a liquid depends on its nature, temperature and the surface area. The following simple apparatus demonstrates the measurement of vapour pressure of a liquid.

Vapour Pressure of Liquid img 1

The vapour pressure of a liquid is the point at which equilibrium pressure is reached, in a closed container, between molecules leaving the liquid and going into the gaseous phase and molecules leaving the gaseous phase and entering the liquid phase.

It is important to note that when a liquid is boiling, its vapour pressure is equal to the external pressure. For example, as water boils at sea level, its vapour pressure is 1 atmosphere because the external pressure is also 1 atmosphere.

Vapour Pressure of Liquid

Vapour pressure is the pressure exerted by vapours when they are in equilibrium with the liquid phase at a given temperature. It depends on the nature of the liquid and temperature. Vapour pressure of pure liquid is always greater than the vapour pressure of its solution.

The major difference between the vapour pressure and partial pressure of a gas is that vapour pressure is the pressure exerted by the gas over its condensed phase, whereas partial pressure is the pressure exerted by the gas in volume, which is occupied by the mixture of gases.

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