What Is a Decomposition Reaction? Definition and Examples

Decomposition Reaction
A decomposition reaction or analysis reaction occurs when one reactant breaks into two or more products.

A decomposition reaction is one of the four main types of chemical reactions. This type of reaction is also called an analysis reaction or breakdown reaction. Here is the decomposition reaction definition, examples of the reaction, and how to recognize a decomposition reaction.

Decomposition Reaction Definition

A decomposition reaction is a chemical reaction with a single reactant that forms two or more products.

The general form of a decomposition reaction is:

AB → A + B

A decomposition reaction forms smaller molecules, often including pure elements.

Opposite of a Decomposition

The opposite of a decomposition reaction is a synthesis reaction, which is also called a combination reaction. In a synthesis reaction, two or more reactants combine, forming a more complex product.

Decomposition Reaction Examples

Decomposition reactions are common in daily life. One example is the electrolysis of water to form oxygen gas and hydrogen gas:

2 H2O → 2 H2 + O2

Another example is the decomposition of hydrogen peroxide to form water and oxygen:

2 H2O2 → 2 H2O + O2

Soft drinks get their carbonation from a decomposition. Carbonic acid breaks into water and carbon dioxide:

H2CO3 → H2O + CO2

Endothermic or Exothermic?

Most decomposition reactions are endothermic. In other words, it takes more energy to break the chemical bonds in the reactant than is released when new chemical bonds form to make the products. These reactions absorb energy from their environment in order to proceed. For example, the decomposition of mercury(II) oxide into mercury and oxygen (like the decomposition of most metal oxides) requires the input of heat and is endothermic:

2HgO → 2Hg + O2

However, a few decomposition reactions are exothermic. They release more heat than they absorb. For example, the decomposition of nitrous oxide into nitrogen and oxygen is exothermic:

2NO → N2 + O2

How to Recognize a Decomposition Reaction

The easiest way to identify a decomposition reaction is to look for a reaction that starts with a single reactant and yields multiple products. Also, it helps to recognize familiar examples. Metal oxides form metals and oxygen, carbonates typically yield oxides and carbon dioxide, and so on.

Types of Decomposition Reactions

The three main types of decomposition reactions are thermal decomposition, electrolytic decomposition, and photolytic decomposition.

  • Thermal decomposition: Heat activates a thermal decomposition reaction. These reactions tend to be endothermic. An example is the decomposition of calcium carbonate to form calcium oxide and carbon dioxide:
    CaCO3 → CaO + CO2
  • Electrolytic decomposition: Electrical energy supplies the activation energy for the reactant to decompose into products. An example is water electrolysis into hydrogen and oxygen:
    2H2O→ 2H2 + O2
  • Photolytic decomposition: The reactant absorbs energy from light (photons) to break chemical bonds and form products. An example is the decomposition of ozone to form oxygen:
    O3 + hν → O2 + O.

Catalysts may aid decomposition reactions. These reactions are termed catalytic decompositions.

Uses of Decomposition Reactions

Sometimes decomposition reactions are undesirable, but they have several important applications.

  • To make quicklime (CaO) for cement and other applications.
  • To weld via the thermite reaction.
  • To extract pure metals from their ores, oxides, chlorides, and sufides.
  • To treat acid indigestion.
  • To obtain hydrogen, which is usually bound in compounds.
  • To identify the identity of a sample based on its decomposition products.


  • Brown, T.L.; LeMay, H.E.; Burston, B.E. (2017). Chemistry: The Central Science (14th ed.). Pearson. ISBN 9780134414232.
  • McNaught, A.; Wilkinson, A. (1997). “Chemical decomposition”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”)”. Blackwell Scientific Publications. doi:10.1351/goldbook.C01020