Everything in this world eventually decomposes. In living organisms, decomposition is simply the breaking down of dead, organic matter. In chemistry terms, it is when a single compound breaks down into at least two simpler products.
From the rotting of organic matter to the industrial breakdown of chemicals, decomposition is an essential phenomenon that impacts everything from ecosystems to everyday technology. But how does it work, why does it happen, and what kinds of decomposition exist?
In this post:
Definition of Decomposition
When bonds in a single chemical compound are broken, it results in it breaking apart to form two or more products. When this happens, it is known as chemical decomposition and it takes place as a chemical reaction.
Why Does Decomposition Happen?
Decomposition happens as substances seek a more stable state, driven by the laws of thermodynamics. Complex compounds often contain stored energy within their chemical bonds.
When external forces such as heat or electrical energy are applied, these bonds can break, releasing the stored energy or forming new substances. Every chemical compound is limited to certain conditions, whether it’s heat, radiation or acidity. For instance, organic matter like dead plants decomposes faster in warm, humid conditions.
In industrial and laboratory settings, decomposition is often controlled to achieve desired outcomes and for chemical testing purposes, including:
- Mass Spectrometry
- Gravimetric Analysis
- Thermogravimetric Analysis
How Does A Decomposition Reaction Work?
The general formula of a chemical decomposition reaction shows that when the bonds in a compound are broken, it causes it to decompose into either its elemental part or simpler compounds. The equation can be written as:
AB → A + B
In order for this reaction to occur, a lot of energy is required to break the existing bonds in the compound. This energy usually comes from heat, making the majority of decomposition reactions endothermic. However, this energy can also be supplied by other things like an electric current or an acid.
Therefore, there are 3 main types of chemical decomposition reactions based on these energy sources:
- Electrolytic: where electricity supplies the energy
- Thermal: where heat supplies the energy
- Catalytic: where a catalyst, like an acid, supplies the energy
Types of Decomposition
Decomposition can be classified into several types based on the energy source or mechanism driving the reaction. Here are some key examples:
Electrolytic Decomposition
When electricity is used to decompose a molecule or compound, it is known as electrolytic decomposition. It usually takes place in aqueous solutions when an electric current is sent through them. A common example of this type of reaction is the electrolysis of water, which decomposes into hydrogen and oxygen:
2H20 + electricity → 2H2 + O2
The reason why electricity is used to break the bonds in water is that heat is not effective at causing water to dissociate. Passing an electric current through it, however, supplies enough energy to cause the solution to separate into its elemental products.
Decomposition of Water
The decomposition of water requires an input of energy because the bonds between hydrogen and oxygen in water are strong. To overcome this, an electric current is passed through water that contains an electrolyte like sodium chloride or dilute sulphuric acid to increase its conductivity.
During this process, water molecules split into their constituent elements, which are then collected separately at the electrodes.
This method is driven by the instability of water molecules in the presence of an electric current. When electrodes are submerged in water with an electrolyte, the water molecules dissociate into H⁺ and OH⁻ ions. At the cathode (negative electrode), H⁺ ions gain electrons (also known as reduction) to form hydrogen gas:
2H+ + 2e– → H2
At the anode (positive electrode), OH⁻ ions lose electrons (oxidation) to form oxygen gas and water:
4OH− → O2 + 2H2O + 4e−
The overall reaction is represented as:
2H2O → 2H2 + O2
Thermal Decomposition
When a chemical decomposes as a result of heat, it is called thermal decomposition or thermolysis. This is an endothermic reaction that breaks apart the chemical bonds in a compound by causing friction between molecules. Thermal energy can cause compounds to decompose in two ways, physical decomposition and chemical decomposition:

Physical Decomposition
Physical decomposition involves the breaking apart of substances due to physical factors, such as changes in temperature, pressure, or mechanical forces.
For example, rocks breaking apart into smaller pieces due to freeze-thaw cycles represent physical decomposition.
Unlike chemical decomposition, physical decomposition does not alter the molecular composition of a substance, but it can play a significant role in preparing materials for further chemical reactions.
Chemical Decomposition
If, after it has decomposed, a compound has been broken down into simpler components, it is a chemical decomposition reaction because it is no longer the same physical substance.
One example of a chemical decomposition reaction caused by thermal energy is when potassium permanganate decomposes to form potassium manganate, magnesium oxide and oxygen:
CuCO3 (s) + heat → CuO (s) + CO2 (g)
Decomposition of Copper Sulphate
Copper sulphate (CuSO₄) is a common inorganic compound that undergoes decomposition when subjected to heat. This reaction demonstrates the principles of thermal decomposition, when heat energy breaks down a compound into more simple components.
When heated, copper sulphate crystals lose their water, turning from vibrant blue crystals into a pale, white powder. This transformation marks the first stage of decomposition as the compound transitions from hydrated to anhydrous copper sulphate (CuSO₄).
If heating continues, the anhydrous copper sulphate undergoes further decomposition. The compound breaks down into copper oxide (CuO), sulphur dioxide (SO₂), and oxygen (O₂).
This reaction not only changes the colour of the substance to black, reflecting the presence of copper oxide, but also releases gases, making it an ideal example of a thermal decomposition process in chemistry education and research. The decomposition is represented by the chemical equation:
2CuSO₄ → 2CuO + 2SO2 + O2
Catalytic Decomposition
When a catalyst is used to cause a chemical to break apart, it does not take part in the reaction but simply makes it happen more quickly by lowering the activation energy. Hydrogen peroxide, for example, is actually decomposing all the time but at a very slow rate. By introducing a catalyst, however, we can speed up the reaction.
Decomposition of Hydrogen Peroxide
Hydrogen peroxide (H2O2) is constantly decomposing because its chemical structure contains an incredibly weak and unstable peroxide bond (single oxygen-oxygen bond). Because this bond is so weak, it can easily break, causing hydrogen peroxide to decompose into water and oxygen.
This shows that the decomposition of hydrogen peroxide happens naturally and gives it a finite shelf-life. This is why it is stored very carefully in dark, plastic containers to help with stability and longevity. Still, when a catalyst is added, the decomposition reaction is sped up and has actually become one of the most popular science experiments in and out of the classroom.
Several catalysts can be added to hydrogen peroxide in order to speed up this reaction:
- Manganese (IV) Oxide
- Potassium Iodide
- Sodium Iodide
- Iron (III) Chloride
- Lead Dioxide
- Catalase
When a catalyst is added to hydrogen peroxide, the solution begins fizzing very quickly and will rise out of the container like a big cylinder of foam. This happens because of the rapid formation of oxygen gas, which forms bubbles.
As a practical example, if you add dish soap and food colouring to the solution, the foam will be thicker and colourful, making an attractive experiment known as Elephant’s Toothpaste. The equation for the decomposition of hydrogen peroxide is:
2H2O2 + catalyst → 2H2O + O2
Another example of a catalytic decomposition reaction is when sugar is catalysed by a strong, concentrated acid like sulphuric acid. This causes sugar to decompose into carbon and water, and the reaction causes the sugar to change from white to black.
Conclusion
Decomposition is a vital concept in chemistry, explaining how substances break down into simpler forms through various processes. Practical examples like the decomposition of water, copper sulphate, and hydrogen peroxide highlight its importance in both natural and industrial contexts.
By understanding decomposition, we understand the transformative nature of chemistry, such as how reactions like thermal breakdown and catalytic processes shape the materials and energy systems we depend on.