A decomposition reaction is a type of chemical reaction in which a substance degrades into two or more basic components. It is a process in direct contrast to that of chemical synthesis, which involves molecular bonding so that two or more components may join to form a single chemical compound. The decomposition reaction often requires exposure to a catalyst, such as a heat source or solvent. A simple example of such a reaction is the conversion of water to hydrogen and oxygen gas when heated to a temperature of 212 degrees Fahrenheit (100 degrees Celsius), which is the boiling point of water.

Decomposition chemical reactions are generally classified into three main types, which are catalytic, electrolytic, or thermal. In catalytic reactions, an agent is introduced to cause a chemical reaction. For example, hydrogen peroxide readily degrades into water and oxygen in the presence of manganese dioxide. An electrolytic decomposition reaction is achieved by exposing an aqueous solution or molten compound to an electrical current. Thermal decomposition occurs when a compound is affected by direct heat or radiation.

Regardless of what triggers the event, a decomposition reaction always involves an exchange of energy. The type of energy exchange is classified as either endothermic or exothermic, which describes the manner in which the chemical bonds of a substance have been broken. In an endothermic reaction, energy is absorbed and is usually prompted by a heat or electrical source. An exothermic reaction, on the other hand, results in a release of energy. An example of this type of decomposition reaction is combustion, which occurs when a material or substance is burned.

The specific products obtained from a chemical compound undergoing a decomposition reaction is generally governed by anions, or the molecule groups that contain atoms with a negative electrical charge. The outcome of the reaction may also be determined by ammonium ions, if applicable. Other factors that influence the products produced from decomposing reactants include ammonium salts, oxoanions, and oxoanion salts and acids, which tend to generate oxidation-reduction reactions. This means that elements involved in the reaction will experience a change in oxidation number, which indicates how many electrons have been shared, gained, or lost in the decomposition process.

There are some scientific applications that depend on decomposition reactions in order to complete an analysis. For example, mass spectrometry is a method used to detect and target certain ions in a sample by subjecting it to an electron beam to promote an electrolytic decomposition reaction in order to determine its chemical composition. This permits the technician to identify the sample in question, as well as determine the concentration of each substance contained in the sample.