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What are Decomposition Reactions?

Decomposition reactions occur when a single compound breaks down into two or more simpler substances. They follow the pattern AB → A + B and are the opposite of synthesis reactions.

Short answer

A decomposition reaction breaks one compound into multiple simpler substances: AB → A + B. Examples include water splitting (2H₂O → 2H₂ + O₂) and the thermal breakdown of calcium carbonate (CaCO₃ → CaO + CO₂).

Decomposition Reaction: AB → A + B

1
Compound AB
Single reactant
↓
2
Energy applied
Heat, light, or electric current
↓
3
Bonds break
Chemical bonds separate
↓
4
Products A and B
Two or more simpler substances form

Step-by-step worked examples

Water breaks down into hydrogen and oxygen when electrolyzed. Write and balance this decomposition reaction.

Unbalanced: H₂O → H₂ + O₂
H: 2 on left, 2 on right ✓
O: 1 on left, 2 on right (unbalanced)
Balance O: need 2 H₂O on left
Balance H: 4 on left, need 2 H₂ on right
Balanced: 2H₂O → 2H₂ + O₂

Calcium carbonate (limestone) decomposes when heated to form calcium oxide and carbon dioxide. Balance the equation.

Unbalanced: CaCO₃ → CaO + CO₂
Ca: 1 on each side ✓
C: 1 on each side ✓
O: 3 on left, 1+2=3 on right ✓
Balanced: CaCO₃ → CaO + CO₂

Ammonium nitrate decomposes when heated. If N₂ + H₂O + O₂ form, write the balanced equation.

Unbalanced: NH₄NO₃ → N₂ + H₂O + O₂
N: 2 on left, 2 on right ✓
H: 4 on left, 2 on right (unbalanced)
O: 3 on left, 1+2=3 on right ✓
Balance H: need 2 H₂O
Balanced: 2NH₄NO₃ → N₂ + 4H₂O + O₂

Flashcards

Quick quiz

Q1.Which equation is a decomposition reaction?

Correct answer: B. 2H₂O → 2H₂ + O₂ breaks one compound into two products.

Q2.What is the general form of decomposition?

Correct answer: B. Decomposition: AB → A + B (one compound breaks into multiple).

Q3.Is CaCO₃ → CaO + CO₂ decomposition?

Correct answer: B. One compound (calcium carbonate) breaks into two products (calcium oxide and carbon dioxide).

Q4.Decomposition reactions often require…

Correct answer: B. Decomposition typically needs energy input like heat (thermal), light (photochemical), or electricity (electrolytic).

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Common mistakes

✗

All decomposition needs heat. — Correct: Decomposition can need heat (thermal), light (photochemical), or electricity (electrolytic).

✗

Decomposition always produces elements. — Correct: Decomposition produces simpler substances — could be elements or compounds. Example: CaCO₃ → CaO + CO₂ (one compound into another).

✗

Decomposition has one product. — Correct: Decomposition has at least two products. Example: 2H₂O → 2H₂ + O₂.

✗

Decomposition is the same as synthesis. — Correct: They are opposites. Synthesis combines (A + B → AB); decomposition breaks apart (AB → A + B).

FAQ

What is decomposition in chemistry?

Decomposition is when one compound breaks down into two or more simpler substances (AB → A + B).

Is water splitting decomposition?

Yes. Electrolysis of water is decomposition: 2H₂O → 2H₂ + O₂ splits water into hydrogen and oxygen.

What is an example of decomposition?

Heating calcium carbonate (limestone): CaCO₃ → CaO + CO₂. One compound breaks into two simpler substances.

How do I identify decomposition?

Look for one reactant on the left breaking into two or more products on the right. Pattern: AB → A + B (or more).