An endothermic reaction is a chemical reaction that absorbs heat from the surroundin endothermic reaction, heat energy is absorbed from the surroundings, the heat energy is changed into chemical energy, temperature of the surroundings decreases, the reacting mixture and the container become cold reactions are denoted by a positive heat flow(heat flows into the reaction) an increase enthalply(AH When sodium hydrogen carbonate is added to hydrochloric acid in a test tube, a reaction and the test tube becomes cold. This is because heat has been absorbed by the reactants the surroundings(the solvent and the container). Thus, the reaction is endothermic.