Question

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

Answer 1

When a solid is heated, it's temperature steadily increases up to its melting point. ... during boiling ,the phase of the substance is changing from liquid to vapour.The heat is used as latent heat of vapourisation, so the heat is used for phase change not for increasing the temperature.

Answer 2

because during its melting or boiling the number of heat is make in large quantity at the same temprature