Final answer:
The temperature remains constant during the melting and boiling phase changes of a substance, because these changes are isothermal (constant temperature) processes. The heat energy added is used to break molecular bonds rather than increase temperature.
Step-by-step explanation:
When investigating melting and boiling points using the Gizmo, or any other method of observation, one can observe that the temperature does not change during these phase changes. This is because phase changes are isothermal, meaning they occur at a constant temperature. For example, when ice melts to become water, or when water boils to become steam, the temperature at which these processes occur remains at 0°C and 100°C respectively.
The correct answer to the student's question is 'b) No, the temperature remains constant during both phase changes'. As long as any ice is present, it will hold a temperature of 0°C during melting. Similarly, during boiling, as long as any liquid water remains, the system's temperature stays at 100°C. The heat energy added during these processes goes into breaking the intermolecular forces that hold the water molecules together in a solid or liquid state, rather than increasing the temperature.