Exothermic reactions transfer energy to the surroundings, making the surroundings warmer. Endothermic reactions take energy from the surroundings, making the surroundings cooler. The system means the reacting chemicals; the surroundings are everything around them.
Energy is conserved. An exothermic reaction leaves the products with less chemical energy than the reactants. That energy has gone to the surroundings. An endothermic reaction leaves the products with more chemical energy. That energy came from the surroundings.
EXO = energy EXITS. ENDO = energy ENTERS. Always say which way energy moves. “It gets hot” on its own is an observation, not a full explanation.
Exothermic: energy out, surroundings warm. Endothermic: energy in, surroundings cool. Energy is conserved.
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