Chemical reactions can be exothermic or endothermic. You're probably more likely to see exothermic reactions, in which heat is released, during your day to day activities. Combustion is an exothermic reaction.
An endothermic reaction that's easy to observe is the reaction of sodium bicarbonate (baking soda) with aqueous acetic acid (found in vinegar). In addition to lots of bubbling, the temperature of the solution drops. Energy is absorbed from the surroundings in which the reaction takes place.
There are two factors that drive chemical reactions. One is the energy or heat consideration. Chemical reactions that are exothermic tend to occur spontaneously. However, due to the second factor, it's still possible for endothermic reactions to occur spontaneously. The second factor is that chemical reactions that result in an increase in entropy (randomness or disorder) tend to be spontaneous. Endothermic chemical reactions are spontaneous when the increase in entropy outweighs the energy change as a driving factor.
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