When chemical reactions occur, they are accompanied by a transfer of energy, sometimes to and sometimes from the surroundings, which can often take place in the form of a temperature change. An endothermic reaction is one that results in the absorption of heat, and an exothermic one is the exact opposite, a chemical reaction that releases heat. In this type, the resultant product has higher energy than the reactants. To make up for the energy deficit, the product absorbs heat from the surroundings, resulting in a drop in temperature. Essentially, in an exothermic reaction, there is surplus energy that is released in the form of heat once the reaction is complete. Exothermic reactions may occur spontaneously, and can also be used under controlled situations to produce explosions. Endothermic reactions, on the other hand, are those in which the energy required for the reaction to take place is insufficient. In this case, the heat is absorbed to make up for this deficit (these reactions should not be confused with endergonic reactions, in which the energy absorbed need not be in the form of heat). Examples of Endothermic Reactions Photosynthesis It is one of the best illustrations of endothermic reactions that occur in nature. In this process, plants use chlorophyll in the presence of sunlight to convert carbon dioxide and water to glucose and oxygen. Melting Ice When ice melts, it draws heat from the surroundings rendering the solid form unstable. Evaporation of Water Another natural example of an endothermic reaction. Water in the liquid form uses heat to convert into the gaseous form of vapor. Electrolysis It involves the separation or decomposition of the original compounds, which occurs as a result of the application of an electric current. Examples of Exothermic Reactions They release energy on their completion, and the reactants contain more energy than the products. This surplus energy is released in the form of heat. These reactions are often spontaneous, as against exothermic reactions. Combustion/Burning When you light a match or a fire, the compounds that burn release heat into the surroundings causing an increase in temperature. This is the most basic illustration of an exothermic reaction. Combustion of fuel is another such example. Neutralization This type of reaction takes place between an acid and a base to produce salt and water. For example, mixing sodium hydroxide (a base) with hydrochloric acid (the acid) will give you a solution of sodium chloride and water, accompanied by an increase in temperature. Rusting Rusting of iron is also an example of a spontaneous exothermic reaction.