Final answer:
When heat is removed from a substance at a critical temperature, it undergoes a phase change and the temperature remains constant until the phase change is complete. The energy removed is used for the phase transition rather than changing the temperature. (Option A)
Step-by-step explanation:
When heat is removed from a substance at a critical temperature, such as its boiling point or freezing point, the substance undergoes a phase change. During this process, the temperature of the substance will not change until the entire substance has completed the phase change. This means that as heat is removed, the substance releases heat but will not change its temperature until the state has completely changed from one phase to another. For example, when removing heat from a liquid at its boiling point, the liquid will turn into a gas, but the temperature will remain constant at the boiling point until all the liquid has vaporized into a gas.
Moreover, the relation Q = mcΔT, where m is the mass of the substance and c is its specific heat, describes the amount of heat Q absorbed or released by a substance with the change of temperature ΔT, and this relation does not apply when the substance is undergoing a phase transition. In a phase transition, the energy involved goes into changing the intermolecular attractions between the molecules rather than changing the temperature.