Final answer:
The heat absorbed or lost during a substance's phase change at critical temperature is known as the heat of fusion or vaporization, causing the temperature to remain constant.
Explanation:
The heat absorbed or lost by a substance while changing state at the critical temperature is called heat of fusion or heat of vaporization. An example is the heat required to melt ice or boil water, respectively. During these phase changes, the temperature remains constant until the process is complete.
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