Question

why does the temperature remains constant until whole of the solid changes into liquid, though heat energy is supplied constantly ?​

Answer 1

Answer:

The temperature remains constant because the heat supplied is utilized to break the bonds between the solid molecules so that its state changes into liquid.

Answer 2

The temperature remains constant during the change of state of a solid to liquid, because the eneegy which is actually supplied to change the state is used up in breaking down the force of attraction between the particles of solid.

$\textbf{Then when temperature will rise of heat is supplied continuously ? }$

Tempertaure will only increase when the whole solid get melted to form liquid, then the heat supplied will start to break the force of attraction between the particles of liquid.