Endothermic reaction is any chemical reaction that absorbs heat from its environment. A reaction in which heat is released is called an exothermic reaction. When water is added to quicklime heat is evolved. It is an exothermic reaction in which heat energy is released. When quick lime (calcium oxide) is added to water, it reacts rapidly with water and produces slaked lime (calcium hydroxide) with the release of a large amount of heat.