Final answer:
During a phase change, the energy being added is used solely to change the substance from one phase to another, and not to increase its temperature. For example, when ice is being heated, it undergoes a phase change from solid to liquid at 0°C. The temperature remains constant until all the ice has melted into water.
Step-by-step explanation:
The process described, where the temperature of a substance does not change even though the heat source is still active, is called a phase change. During a phase change, the energy being added is used solely to change the substance from one phase to another, and not to increase its temperature. This is why the temperature remains constant.
For example, when ice is being heated, it undergoes a phase change from solid to liquid at 0°C. During this phase change, the added heat energy is used to break the bonds between the water molecules, converting them from a solid to a liquid state. The temperature remains at 0°C until all the ice has melted into water. Only then does the added heat energy contribute to raising the temperature of the water.