When a substance undergoes a change of state from solid to liquid or from liquid to gas, the temperature of the substance remains constant even though heat is being added. This phenomenon is known as the latent heat of fusion or vaporization.


The heat that is absorbed or released during a change of state is known as latent heat. This heat is required to break the intermolecular bonds between the particles of the substance.


During melting, the intermolecular forces between the particles of the solid are broken. The heat that is added is used to break these forces, and the temperature of the substance remains constant until all the solid has melted.


The energy that is added to the substance is used for breaking the intermolecular bonds rather than increasing the temperature. This energy is stored in the substance in the form of potential energy.


Similarly, during boiling, the intermolecular forces between the particles of the liquid are broken. The heat that is added is used to break these forces, and the temperature of the substance remains constant until all the liquid has boiled off.


The energy that is added to the substance is used for vaporization, which is the process of changing a liquid into a gas. The energy is used to break the intermolecular forces between the particles of the liquid and to overcome the pressure of the atmosphere.


In conclusion, the temperature of a substance remains constant during melting or boiling because the heat that is added is used to break the intermolecular forces between the particles of the substance rather than increasing the temperature. The energy is stored in the substance in the form of potential energy.

Answer: The temperature of a substance remains constant at its melting and boiling points until all the substance melts or boils because, the heat supplied is continuously used up in changing the state of the substance by overcoming the forces of attraction between the particles