In a chemical reaction, heat energy is often transferred between the reacting chemicals and their surroundings. An exothermic change or reaction is one in which heat energy is given out to the surroundings. Because the heat energy is transferred to the surroundings, their temperature rises. A thermometer placed in the reacting solution shows a temperature rise.
Burning fuels is exothermic, and so is respiration in living cells. Neutralisation, where an acid reacts with an alkali, is also exothermic, so the solution gets warmer. Hand warmers and self-heating food packs use exothermic reactions to produce warmth.
