Decomposition Reaction Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Decomposition Reaction.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A chemical reaction in which a single compound breaks down into two or more simpler substances.

The reverse of synthesis — taking apart a complex structure into simpler pieces.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Pattern: AB → A + B. One reactant breaks into two or more products. Often requires energy input (heat, electricity, light).

Common stuck point: Decomposition is the exact opposite of synthesis. One compound breaks into simpler pieces.

Worked Examples

Example 1

easy
Write the balanced equation for the decomposition of water by electrolysis.

Solution

  1. 1
    Decomposition: one compound breaks into two or more simpler substances (\text{AB} \rightarrow \text{A} + \text{B}).
  2. 2
    \text{H}_2\text{O} \rightarrow \text{H}_2 + \text{O}_2 (unbalanced).
  3. 3
    Balanced: 2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2. Check: H 4=4, O 2=2. ✓

Answer

2\text{H}_2\text{O} \xrightarrow{\text{electricity}} 2\text{H}_2 + \text{O}_2
Decomposition reactions are the reverse of synthesis reactions. Many require energy input (heat, electricity, or light) to proceed, making them typically endothermic.

Example 2

medium
When heated, calcium carbonate decomposes: \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2. Classify this reaction and explain why heat is needed.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
Classify: 2\text{KClO}_3 \xrightarrow{\Delta} 2\text{KCl} + 3\text{O}_2.

Example 2

hard
Hydrogen peroxide (\text{H}_2\text{O}_2) slowly decomposes into water and oxygen gas. Write the balanced equation. Adding manganese dioxide (\text{MnO}_2) speeds up the decomposition dramatically — what role does \text{MnO}_2 play, and is it consumed?
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Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reactionsynthesis reaction