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Decomposition Reaction

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

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

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

  2. 2

    Then show how to use it

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

  3. 3

    Add one concrete example

    Heating calcium carbonate gives calcium oxide and carbon dioxide.

  4. 4

    Avoid this incomplete answer

    Calling it combination because the products are shown together. That is wrong because combination starts with many reactants, not one.

Definition

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

Example

Heating calcium carbonate gives calcium oxide and carbon dioxide.

Rule to remember

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

Memory hook

Decompose means break apart.

Examples and method

Worked example

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

Method to apply

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

Diagram support

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

How CBSE asks it

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

Avoid common mistakes

Common confusion

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

Common wrong answer

Calling it combination because the products are shown together. That is wrong because combination starts with many reactants, not one.

Exam tip

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

Quick check

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

It is a decomposition reaction because one compound breaks into simpler substances.

Answer writing and exam use

1-mark answer

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

2-mark answer

A decomposition reaction is a reaction in which one compound breaks into two or more simpler substances. General pattern: AB → A + B, often needing heat, light, or electricity. Heating calcium carbonate gives calcium oxide and carbon dioxide.

3-mark answer

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. General pattern: AB → A + B, often needing heat, light, or electricity. CaCO3 → CaO + CO2 on heating. One compound splits into two simpler substances, so it is decomposition. Students may be asked to identify decomposition from a thermally broken compound or from electrolysis. Calling it combination because the products are shown together. That is wrong because combination starts with many reactants, not one.
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