Cherylanne mixes together Chemical A and Chemical B in a test tube, forming Chemical C. The mixture instantly turns cool to the touch. She then adds Chemical D, and the reaction becomes hot and explodes.

When A and B react, the mixture gets cooler, meaning the reaction absorbs heat from its surroundings. A reaction that absorbs heat is endothermic. After D is added, the reaction becomes hot, meaning it releases heat; a heat-releasing reaction is exothermic. Therefore, A and B form an endothermic reaction, while C and D form an exothermic reaction.

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