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3.20: Decomposition Reactions

Difficulty Level: At Grade Created by: CK-12
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Hydrogen peroxide is a widely used disinfectant for minor cuts and scrapes. You may have a bottle of hydrogen peroxide like the one pictured here in your medicine cabinet at home. Did you ever notice that hydrogen peroxide is always kept in a dark brown bottle like this one? Do you know why? The reason is that hydrogen peroxide needs to be protected from light. If it isn’t, it will gradually break down to water and oxygen. This is a decomposition reaction.  

What Is a Decomposition Reaction?

A decomposition reaction occurs when one reactant breaks down into two or more products. It can be represented by the general equation:

AB → A + B

In this equation, AB represents the reactant that begins the reaction, and A and B represent the products of the reaction. The arrow shows the direction in which the reaction occurs.

Q: What is the chemical equation for the decomposition of hydrogen peroxide (H2O2) to water (H2O) and oxygen (O2)?

A: The equation for this decomposition reaction is:

2 H2O2 → 2 H2O + O2

Other Examples of Decomposition Reactions

Two more examples of decomposition reactions are described below. You can see additional examples at this URL: http://www.youtube.com/watch?v=dxlWtsFinTM

  • Carbonic acid (H2CO3) is an ingredient in soft drinks. A decomposition reaction takes place when carbonic acid breaks down to produce water (H2O) and carbon dioxide (CO2). This occurs when you open a can of soft drink and some of the carbon dioxide fizzes out. The equation for this reaction is:
H2CO3 → H2O + CO2
  • Another decomposition reaction occurs when water (H2O) breaks down to produce hydrogen (H2) and oxygen (O2) gases (see Figure below). This happens when an electric current passes through the water, as illustrated below. The equation for this reaction is:
2 H2O → 2 H2 + O2

Decomposition of water

Q: What ratio of hydrogen molecules (H2) to oxygen molecules (O2) is produced in the decomposition of water?

A: Two hydrogen molecules per oxygen molecule are produced because water (H2O) has a ratio of two hydrogen atoms to one oxygen atom.


  • A decomposition reaction occurs when one reactant breaks down into two or more products. This can be represented by the general equation: AB → A + B.
  • Examples of decomposition reactions include the breakdown of hydrogen peroxide to water and oxygen, and the breakdown of water to hydrogen and oxygen.


  • decomposition reaction: Chemical reaction in which one reactant breaks down into two or more products.


Watch the video about decomposition reactions at the following URL, and then answer the questions below.


  1. Why are decomposition reactions the opposite of synthesis reactions?
  2. What are the products of the decomposition of hydrogen chloride (HCl)?
  3. What is missing from each of these balanced equations for decomposition reactions?
    1. 2Ni2O3 → 4Ni + ?
    2. 2Al2O3 → ? + 3O2
  4. What is a general rule for the decomposition of metal hydrogen carbonates?


  1. What is a decomposition reaction? What is the general equation for a decomposition reaction?
  2. Describe the decomposition of hydrogen peroxide, and write a balanced chemical equation for this reaction.
  3. Which of the following chemical equations represents a decomposition reaction?
    1. H2SO3 → H2O + SO2
    2. 2Na + Cl2 → 2NaCl
    3. 2K + 2H2O → 2KOH + H2
    4. NaCl + AgF → NaF + AgCl


decomposition reaction

decomposition reaction

Chemical reaction in which one reactant breaks down into two or more products.

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Difficulty Level:
At Grade
7 , 8
Date Created:
Oct 31, 2012
Last Modified:
May 23, 2016
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