The four patterns side by side
We have now met all four types. Set side by side they become far easier to tell apart.
| Type | General reaction | How to spot it | Example |
|---|---|---|---|
| Combination | A + B → C | one product | 2Mg + O2 → 2MgO |
| Decomposition | A → B + C | one reactant | CaCO3 → CaO + CO2 |
| Single displacement | A + BC → AC + B | a lone element on each side | Zn + CuSO4 → ZnSO4 + Cu |
| Double displacement | AB + CD → AD + CB | compounds only on both sides | BaCl2 + Na2SO4 → BaSO4 + 2NaCl |
How to decide
Two questions settle the type
- Is there only one substance on one side? On the left it is a decomposition; on the right, a combination.
- Is there a lone element? If so it is a single displacement; if not, a double displacement.
Counting is enough
Deciding the type needs no chemical knowledge of the reaction — counting the substances on each side and looking for a lone element is enough.