During a phase change, the temperature of a substance is what?  

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During a phase change, the temperature of a substance remains constant. We commonly observe phase changes from solid to liquid, such as the melting of ice. Liquid to gas phase changes are also commonly seen, such as during the boiling of water.

Ice melts to form water at 0 degree Celsius. When we supply heat to the ice, its temperature rises until it reaches 0 degrees Celsius. At that temperature, ice has a phase change and becomes liquid water, without any change in temperature. This is because the amount of heat that is supplied to the ice molecules is used to increase their kinetic energy, which is reflected in temperature increase. Once they have sufficient energy to overcome the bonds that are holding them together, we observe the phase change. Thus, we can see that the heat supplied during the phase change (at constant temperature) is used to break these bonds rather than provide kinetic energy to the molecules. Hence the phase change reaction takes place at a constant temperature.

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