Chemical Decomposition Reactions: Examples and Types

Chemical Decomposition

Erupted lava (magma) made up of tectonically melting natural topographic material undergoing chemical decomposition
Erupted lava (magma) constituted of tectonically melting natural topographic material undergoing chemical decomposition

Chemical decomposition is the breaking down of a single unit of a chemical substance, material, or compound into two or more individual and simpler units at the molecular level.

The process involves a chemical reaction in which there is basically one reactant that decays or fragments into dual or multiple products. This chemical reaction is termed a decomposition reaction.

Chemical decomposition can also be defined as the separation or degeneration (gradual reduction in overall mass) of a complex chemical compound or depletion by the removal of bonds through a set of reactions into smaller compounds or elements.

It is described as being the direct opposite process to that of chemical synthesis (the constitution of single complex chemical compounds from two or multiple simpler ones) through combination/synthesis reactions.

The stability of compounds can become compromised from environmental exposure to the elements that can weaken their bonds and render them susceptible to chemical decomposition that will require less energy to break the bonds as a result.

This is an important factor in the metabolic processes that recycle mineral resources and nutrients in an ecosystem making them available for reabsorption.

Most decomposition reactions are typically endothermic (absorbing ambient energy or heat, warming the internal core, and cooling the surroundings) because of their usual requirement of energy to sever the existing chemical bonds, however, there are some instances where the reactions may be exothermic (generating and releasing internal heat cooling the core while radiating the surrounding)

Types of Chemical Decomposition Reactions

Decomposition is generally a biochemical process when it involves organic material and the activity of organic microorganisms. All forms of decomposition whether organic or inorganic require chemical decomposition reactions to occur and are not possible without the chemical alteration of matter.

There are three main types of decomposition that all other existing forms fall under. They are as follows:

  1. Thermal decomposition Reaction
  2. Electrolytic decomposition reaction
  3. Photolytic decomposition reaction

Thermal Decomposition Reaction

Butane burning flame chemically decomposing silver
Butane burning flame chemically decomposing silver

Thermal decomposition is a chemical decomposition reaction in which an individual complex compound or single substance is broken down and reduced into two or more simpler compounds or substances as a result of the application of, or exposure to heat.

Thermal decomposition reactions are also termed thermolysis and are typically endothermic as externally-derived thermal energy (heat) is required to break the structural bonds of the substance undergoing thermolysis.

Electrolytic Decomposition Reaction

Electricity passing through a medium
Electricity passing through a medium

An electrolytic decomposition reaction is a chemical decomposition reaction in which a single complex compound or substance is broken down and reduced to two or more simpler compounds or substances with electricity as the reaction’s activation energy.

Photolytic Decomposition Reaction

Natural sunlight
Natural sunlight

Photolytic decomposition reactions also termed photolysis, photodecomposition, and photochemical decomposition, are chemical decomposition reactions in which an individual complex compound or substance is broken down and reduced to two or more simpler compounds or substances with exposure to photons or light/photolytic energy being the reaction’s activation energy.

Examples of Decomposition Reactions

Below are some examples of basic types of decomposition reactions.

Thermal Decomposition

An example of a thermal decomposition reaction application is in the manufacture of quick lime (calcium oxide {CaO} also called burnt lime). Calcium carbonate is heated to decompose into calcium oxide and carbon dioxide. The equation is represented below:

CaCO3 →CaO + CO2


Combustion is the burning of tangible substances that is also a high-temperature exothermic reduction chemical reaction that occurs between a reductant fuel and an oxidant which is usually atmospheric oxygen-producing oxidized and gaseous derivatives.

Flames are typically visible when material and substances undergoing combustion vaporize. However, there is no flame when there is no vaporization but there is sometimes visible incandescent light from the intense heat of the reaction.

At the initial stage of combustion reactions, there is a thermal decomposition of molecules into free radicals. Represented by the equation;

CH4 → CH3 + H + (several others)

The propagation stage follows with the free radicals going on to react with certain molecules producing more free radicals and prolonging the reaction in the process.

H + O2 → HO + O

Finally, in the termination step, there is a combination of free radicals to constitute stable molecules like water.

H + HO → H2O

Combustion, therefore, involves both the thermal decomposition of molecules into free radicals that keep the reaction going and the combination of free radicals that help end the chemical reaction.

Solid forms of fuel materials like wood and coal initially undergo endothermic pyrolysis that produces impure gaseous fume fuels whose subsequent combustion generates the heat that multiplies them.


Pyrolysis also termed devolatilization (a chemical decomposition process that ejects volatile substances from hydrocarbon materials) is the thermal decomposition of certain materials at very high temperatures between two hundred and fifty to three hundred degrees Celsius (250°C-300°C) in a chemically inactive environment.

The process of pyrolysis alters the chemical composition of reactant compounds by reducing volatile components and leaving a more stable product. The pyrolysis of organic material produces volatile discharge material and residual char (dark carbon residue that remains after gases and tar are ejected from the combustion of a carbonaceous material).

Electrolytic Decomposition

The electrolysis of water decomposes it to produce oxygen and hydrogen gas when an electrical current is passed through it is an example of an electrolytic decomposition reaction and is represented by the chemical equation:

2H2O→2H2 + O2

Photolytic Decomposition

An example of photolysis, photodecomposition, or photolytic decomposition reaction is the decomposition of silver chloride after exposure to sunlight to produce silver and chlorine gas. This reaction is illustrated in the equation below:

2AgCl(s) sunlight→  2Ag(s) +Cl2 (g)

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