Question

When a solid melts, its temperature remains constant. Why ?

Answer 1

When solid melts, its temperature remains constant because, the heat is used in changing the state of matter, the heat is used to increase the kinetic energy of the particles of ice to such a limit that they break away from the rigid lattice structure of the solid form, and move around freely to turn into liquid form.
So, until a Solid melts, the heat absorbed does not raise the temperature of the Solid-liquid mixture. In the same way the temperature doesn't change while a liquid is boiling. This heat, which does not raise the temperature of the body (also called as, latent heat).