One breaks into many
Decomposition: the opposite of combination.
Heating limestone
Limestone (calcium carbonate, CaCO₃) is heated strongly in a kiln. It makes quicklime, which is used to make cement.
The reactant is CaCO₃, one substance. What would you guess about the products?
What this lesson covers
The idea
A reaction in which a single reactant breaks down to give two or more simpler products is a decomposition reaction; it is the opposite of a combination reaction.
Heating limestone
Limestone (calcium carbonate, CaCO₃) is heated strongly in a kiln. It makes quicklime, which is used to make cement.
The reactant is CaCO₃, one substance. What would you guess about the products?
- There will be two or more simpler substances
- There will be one bigger substance
- There will be no new substance
Break them down
Press the button. Watch one block split. Try all three reactions.
One in, many out
A reaction in which a single reactant breaks down to give two or more simpler products is a decomposition reaction. It is the opposite of a combination reaction.
2H₂ + O₂ → 2H₂O is a combination. 2H₂O → 2H₂ + O₂ is its opposite: a decomposition.
- Combination | Decomposition
- A + B → AB | AB → A + B
- Many reactants, one product | One reactant, many products
Notes
A reaction in which a single reactant breaks down to give two or more simpler products is a decomposition reaction; it is the opposite of a combination reaction.
Check yourself
Which one is a decomposition reaction?
When ferrous sulphate (FeSO₄) is heated it gives ferric oxide, SO₂ and SO₃. How many products?
Answer: 3
Three products from one reactant: a decomposition reaction.
2H₂O → 2H₂ + O₂ is the opposite of which reaction?
- C + O₂ → CO₂. Two reactants join into one: a combination.
- CaCO₃ → CaO + CO₂ — correct. Yes! One reactant breaks into two simpler products.
- Fe + CuSO₄ → FeSO₄ + Cu. Two reactants here: it is not a decomposition.
- C + O₂ → CO₂. That is a combination too, but it is not the exact reverse of this one.
- Fe + CuSO₄ → FeSO₄ + Cu. That is a different kind of reaction (displacement).
- 2H₂ + O₂ → 2H₂O, a combination reaction — correct. Yes! Decomposition is the opposite of combination.