Final answer:
During a phase change, a substance does not change temperature even when heat is added or removed because the energy is used to change the state of the substance, indicating latent heat. The process could be endothermic (heat absorbed) or exothermic (heat released).
Step-by-step explanation:
When a substance undergoes a phase change and the temperature does not change despite the continuous supply of heat, the process can be either endothermic or exothermic, depending on whether the system absorbs or releases heat, respectively. During such a phase change, energy is used to alter the state of the substance rather than increasing its temperature. For instance, during the melting of ice (endothermic), heat is absorbed from the surroundings to break the lattice structure of the solid water, resulting in a latent heat effect where the temperature remains constant until the phase change is complete. Similarly, during the condensation of steam (exothermic), heat is released into the surroundings as water vapor turns into liquid water, keeping the temperature stable during the transition.
An endothermic process absorbs heat, causing the surroundings to feel cooler, while an exothermic process releases heat, warming up the surroundings. Since the question mentions that the temperature is not changing even though the heat source is still on, this suggests that the substance is undergoing a phase change, and the process can be either endothermic or exothermic, depending on the direction of heat flow.