Question

Why does the temperature of a substance remain constant during its melting point or boiling point?

Answer 1

When a substance melts, it absorbs heat for the conversion of the solid state into a liquid state. As we continue heating, the heat supplied is used up in converting the solid state into a liquid state by overcoming the forces of attraction between the particles and there is no change in temperature till the whole solid is converted into liquid. This heat absorbed by the solid which does not result in an increase in temperature is called the latent heat of fusion. When a liquid is heated, it starts converting into vapours.

Further heat given to the liquid is used in changing the state and there is no increase in the temperature till the liquid starts boiling. This heat is known as the latent heat of vaporisation. Hence, the temperature of a substance remains constant at its melting point or boiling point until all the substance melts or boils.