It may be defined as a state in all changes or reactions carried in a closed vessel, where the measurable properties of the system such as colour, density, vapour pressure, concentration, temperature etc. do not show any further noticeable change.

(i) Evaporation of water in a closed vessel.

(ii) Dissolution of a gas in a liquid under pressure in a closed vessel.

The molecules having high kinetic energies can evaporate.

Factor at which vapours pressure of a liquid depends on:(i) Temperature (ii) Nature of liquid.