We are independent & ad-supported. We may earn a commission for purchases made through our links.
Advertiser Disclosure
Our website is an independent, advertising-supported platform. We provide our content free of charge to our readers, and to keep it that way, we rely on revenue generated through advertisements and affiliate partnerships. This means that when you click on certain links on our site and make a purchase, we may earn a commission. Learn more.
How We Make Money
We sustain our operations through affiliate commissions and advertising. If you click on an affiliate link and make a purchase, we may receive a commission from the merchant at no additional cost to you. We also display advertisements on our website, which help generate revenue to support our work and keep our content free for readers. Our editorial team operates independently of our advertising and affiliate partnerships to ensure that our content remains unbiased and focused on providing you with the best information and recommendations based on thorough research and honest evaluations. To remain transparent, we’ve provided a list of our current affiliate partners here.

Our Promise to you

Founded in 2002, our company has been a trusted resource for readers seeking informative and engaging content. Our dedication to quality remains unwavering—and will never change. We follow a strict editorial policy, ensuring that our content is authored by highly qualified professionals and edited by subject matter experts. This guarantees that everything we publish is objective, accurate, and trustworthy.

Over the years, we've refined our approach to cover a wide range of topics, providing readers with reliable and practical advice to enhance their knowledge and skills. That's why millions of readers turn to us each year. Join us in celebrating the joy of learning, guided by standards you can trust.

What is a Decomposition Reaction?

Karyn Maier
Updated: May 21, 2024

A decomposition reaction is a type of chemical reaction in which a compound is broken down into simpler components. It is the opposite of chemical synthesis, in which elements or relatively simple compounds combine to produce one that is more complex. Since a decomposition reaction involves the breaking of chemical bonds, it requires the addition of energy; this may come from heat, an electrical current or other sources. Sometimes, a catalyst will speed up the reaction or allow it to take place at a lower temperature. These reactions are used industrially in the production of some elements — especially reactive metals — and in the laboratory for the analysis of samples.

Decomposition by Heat

Heat is commonly used to bring about a decomposition reaction. When a compound heats up, its atoms move about more vigorously, and this movement can break chemical bonds. For example, if calcium carbonate (CaCO3) is strongly heated, it will decompose into calcium oxide (CaO) and carbon dioxide (CO2). The temperature required to decompose a compound depends on the strength of the bonds that hold it together. In this example, calcium carbonate loses an atom of carbon and two atoms of oxygen as CO2, but the calcium holds on to one oxygen atom because the calcium-oxygen bond is very strong and cannot be broken by heating to any easily achievable temperature.

The more reactive elements tend to form stronger bonds and are, therefore, more difficult to separate from their compounds. In contrast to the above example, the oxides of less reactive metals, such as silver and mercury, can be decomposed by relatively moderate heating, releasing oxygen and leaving the pure metal. Highly reactive metals, such as sodium and potassium, cannot be separated from their compounds by heating alone.


In the liquid state, elements can be separated from a compound by the application of a direct electric current in a process known as electrolysis. The current flows through electrodes, which are placed in the liquid. Negatively charged electrons flow into one electrode, known as the cathode, and out of the other, which is known as the anode. The cathode therefore has a negative charge, and the anode, a positive charge. Ions in the liquid move toward the oppositely charged electrode, allowing the current to flow.

An example is the decomposition of water into hydrogen and oxygen by electrolysis. Pure water is a very poor conductor, but the introduction of even a very small amount of an ionic compound, such as sodium sulfate, greatly improves its conductivity and allows electrolysis to take place. At the cathode, water (H2O) is split into hydrogen gas (H2) and hydroxide (OH-) ions, which are attracted to the positively charged anode. At the anode, water is split into oxygen gas and hydrogen (H+) ions, which are attracted to the cathode.

Other Factors

In some compounds, the energy needed for decomposition is small and can be supplied by a minor shock, such as a physical impact. One such compound is lead azide (Pb(N3)2), which decomposes explosively into lead and nitrogen gas if subjected to a fairly small impact. Sodium azide is a similar, but slightly less sensitive, compound that is used to inflate car airbags in a collision.

Light can cause the decomposition of some compounds. For example, silver chloride is converted to silver and chlorine gas on exposure to light. This phenomenon was crucial to the development of photography.


In many cases, a decomposition reaction can be prompted or speeded up by the use of a catalyst. These substances do not take part in the reaction, and are therefore unchanged by it, but they encourage the reaction to take place. A good example is the decomposition of dilute solutions of hydrogen peroxide (H2O2) into water and oxygen. This reaction can be promoted by the addition of powdered manganese dioxide, which acts as a catalyst to produce oxygen gas.


Thermal decomposition is used in the industrial production of quicklime for cement manufacture and various other purposes. Electrolysis is used in the production of reactive metals. For example, sodium is produced by the electrolysis of molten salt (sodium chloride). This also produces chlorine gas, which has many industrial uses, although most chlorine is produced by the electrolysis of salt solutions in water. Decomposition reactions involving electrolysis are also used to make the extremely reactive element fluorine, and as a “clean” way of generating hydrogen for fuel.

There are some scientific applications that depend on decomposition reactions in order to analyze materials. In mass spectrometry, for example, a small sample of the material of interest is split into ions, which are separated according to their charges and masses. The composition of the material can then be determined.

All The Science is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Karyn Maier
By Karyn Maier
Contributing articles to All The Science is just one of Karyn Maier's many professional pursuits. Based in New York's Catskill Mountain region, Karyn is also a magazine writer, columnist, and author of four books. She specializes in topics related to green living and botanical medicine, drawing from her extensive knowledge to create informative and engaging content for readers.
Discussion Comments
By anon291978 — On Sep 17, 2012

Mgco3(s) → mgo(s) + co2(g)

By anon170357 — On Apr 26, 2011

This really helped me understand chemistry a little bit more.

By dkarnowski — On Jul 11, 2010

If I understand the concept of decomposition reactions correctly, then electrolysis of water to create hydrogen fuel is an example of this type of chemical reaction. This type of fuel generator is becoming more popular as hydrogen fuel cells are starting to come to the consumer market as a means to generate electricity.

By anon94994 — On Jul 11, 2010

Thanks a lot! This has been really helpful. Keep up the good work. You have really saved my chemistry project! Way to go, wisegeek!

Karyn Maier
Karyn Maier
Contributing articles to All The Science is just one of Karyn Maier's many professional pursuits. Based in New York's...
Learn more
All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.

All The Science, in your inbox

Our latest articles, guides, and more, delivered daily.