Final answer:
The heat added at the melting point of a solid is used for the melting process, breaking intermolecular forces and transitioning the solid into a liquid without changing the temperature. If heat is removed at the melting point, the liquid undergoes freezing, releasing energy as it transitions back to a solid, again without a temperature change. The temperature of the substance will only change after all the material has completed the phase transition.
Step-by-step explanation:
When the temperature of a solid reaches its melting point and we continue to add heat, the temperature of the substance does not increase further. Instead, the added heat is used in the phase transition process, turning the solid into a liquid. This phase change is called melting, and during this process, the energy absorbed is used to break the attractive forces between the molecules within the solid, allowing them to move more freely in the liquid state. This heat absorption when a substance is at its melting point is known as the heat of fusion, signifying the change of state without a change in temperature.
Conversely, when a substance at its melting point releases heat, it undergoes the reverse process, freezing. The released energy is the same amount that was absorbed during melting. The energy is removed from the system in the form of heat, and the molecules come together, transitioning back to the solid phase. This process occurs until all of the liquid has solidified, and the phase change is complete.
Only once all the solid has melted or all the liquid has frozen, will any further addition or removal of heat change the temperature of the substance, leading to either temperature rise (for liquids) or temperature fall (for solids).