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Decomposition Reaction

A decomposition reaction is a reaction in which one compound breaks into two or more simpler substances.

In one line: Decompose means break apart.

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Understand it

Here, a single compound does not stay whole. It splits into simpler products because of heat, light, or electricity in many examples. The change is the opposite of combination.

Example

Heating calcium carbonate gives calcium oxide and carbon dioxide.

Rule to remember

General pattern: AB → A + B, often needing heat, light, or electricity.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

  1. 1CaCO3 → CaO + CO2 on heating.
  2. 2One compound splits into two simpler substances, so it is decomposition.

Method you can reuse

  1. 1Check whether there is only one compound at the start.
  2. 2See whether it splits into simpler substances.
  3. 3If yes, write decomposition.

Sketch it in your answer

Use one reactant arrowing to two or more products, with heat, light, or electricity written above.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

When calcium carbonate is heated and forms calcium oxide and carbon dioxide, what reaction type is this?

Think first, then show answer
It is a decomposition reaction because one compound breaks into simpler substances.

Common wrong answer

Calling it combination because the products are shown together.

Why it loses marks: Combination starts with many reactants, not one.

Don't mix this up

Students sometimes think any reaction with one reactant is decomposition. The reactant must break into simpler products.

Exam tip

Look for one compound on the left and more than one product on the right.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Students may be asked to identify decomposition from a thermally broken compound or from electrolysis.
  1. 1

    Enough for 1 mark

    A decomposition reaction is a reaction in which one compound breaks into two or more simpler substances.

  2. 2

    Add for 2 marks

    Example: Heating calcium carbonate gives calcium oxide and carbon dioxide.

  3. 3

    Add for 3 marks

    Here, a single compound does not stay whole.

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