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Physical changes of state and chemical reactions may be either exothermic or endothermic. Exothermic reactions release heat to the surroundings while endothermic reactions absorb heat from the surroundings. With the Measuring Energy Changes: Heat of Fusion—ChemTopic™ Lab Activity, determine the temperature and heat changes that occur when ice melts. Obtain a heating curve by melting ice in a beaker on a hot plate, measuring temperature as a function of time, then determine the heat of fusion of water (the heat required to melt ice).