Question

1. When a substance starts melting, its temperature does not rise till whole the substance melts. Explain.
2. What happens to the heat energy supplied during melting?
3. What is the common name of this energy in (ii)?

Answer 1

1. During melting, the heat supplied is used to overcome the intermolecular forces and increase the intermolecular spaces. This energy increases the potential energy of the molecules, not their average kinetic energy. Since temperature measures the average kinetic energy of molecules, it remains constant until the whole solid changes into a liquid.
2. The heat energy supplied during melting is stored in the substance as potential energy of its molecules. It is used only to change the state of the substance from solid to liquid at constant temperature.
3. This energy is called latent heat of fusion. When calculated for unit mass, it is called specific latent heat of fusion.