Liquid State: Vapour Pressure

What is Vapour Pressure?

When a liquid is placed in a vessel which gets continually heated, the molecules of the liquid are seen to be moving at varying speeds in different directions. This happens due to the different kinetic energies possessed by the molecules of the liquid. When the liquid is heated, the energy of the molecules rises; it becomes lighter and occupies the surface of the liquid. This process is known as ‘evaporation’. The molecules which can be seen on the liquid surface are called ‘vapour’. The evaporation continues at a constant rate the temperature of the liquid is kept constant.

When some molecules of the liquid in the vapor phase, strikes the walls of the containers or the surface of the liquid, it may get converted back to the liquid phase. This process is called condensation.

However, as time passes, the numbers of molecules in vapor phase increases while the rate of condensation also increases. It reaches a stage where the rate of evaporation is equal to the rate of condensation. This phase is called the stage of equilibrium. As represented by the manometer, at this point the pressure exerted by the molecules is called the vapor pressure of the liquid. Vapour pressure is defined as the pressure exerted by the vapor present above the liquid.

Vapour Pressure

Vapour Pressure

The process of evaporation depends on different factors:-

  1. Nature of the liquid:

    Liquids have weak intermolecular forces.Heating the molecules of the liquid can help change them to vapor phase and thus increase the vapor pressure of the liquid. For example, Acetone and benzene have higher vapor pressure than water at a particular temperature.

  1. Effect of temperature:

    The vapor pressure of the liquid increases with increase in its temperature. The molecules of the liquid have higher energy at higher temperatures.

What is a Boiling point?

As we go on increasing the temperature of the liquid, its vapor pressure increases proportionately. It reaches a stage where the vapor pressure of the liquid becomes equal to the atmospheric pressure. At this temperature, the vapors near the surface start escaping to the atmosphere and the liquid undergoes a phase transition. This temperature is defined as the boiling point of the liquid.

Standard boiling point of the liquid is given at:

Pressure of 1 atm = 102325 Pa or 1 bar = 105 Pa

What is a heat of vaporization?

As we provide heat to a liquid, its energy increases, which results in the increase of the overall temperature. At the boiling point, the additional heat is used up by the molecules to overcome the intermolecular force of attraction in the liquid and change to the gaseous state.

When 1 mole of liquid is transformed to a gaseous state, the amount of heat provided by this process is known as the Heat of Vaporization.

This was just a quick intro about the vapor pressure. To find out more about different states of liquid, please download BYJUS the learning app.


Practise This Question

Ashar has water at different temperatures.

(1)    25C                       (2) 50C                (3) 75C                         (4) 90C

He is wondering where vapour pressure of water will be highest.

Can you help him and choose the correct order of vapour pressure for him.