What do you mean by change of state of matter? Explain :

(a) the change of a solid into a liquid at a constant temperature, and

(b) the change of a liquid into a gas at a constant temperature.

A change of state of matter refers to the physical transformation of a substance from one state (solid, liquid, or gas) to another due to a change in temperature.

(a) The process of changing of a substance from the solid state into its liquid state on absorption of heat at a particular temperature, called the melting point, is called melting in which the heat energy absorbed by the substance increases the amplitude of vibrations of the molecules of the solid and a stage is reached at the melting point when the molecules acquire sufficient energy to overcome the force of attraction between them and they become free to move. The solid thus changes into a liquid.

Solid $\xrightarrow[\text{Heat absorbed}]{\text{Melting}}$ Liquid

(b) The process of change of a substance from the liquid state to its gaseous state at a particular temperature, called the boiling point, is called boiling in which the heat energy absorbed by a substance in liquid state increases the energy of its molecules due to which they begin to move rapidly. Thus a liquid changes into a gas.

Liquid $\xrightarrow[\text{Heat absorbed}]{\text{Boiling}}$ Gas