Guides
The Four Basic Reaction Types (combination, decomposition, displacement, metathesis)
Introductory chemistry groups many inorganic reactions into four basic types. Learn each one's general form and signature, and you can quickly recognize and write reactions. These types classify reactions from a different angle than redox does.
The four types
| Type | General form | Signature |
|---|---|---|
| Combination | A + B → AB | many into one |
| Decomposition | AB → A + B | one into many |
| Single displacement | A + BC → AC + B | element + compound → new element + new compound |
| Double displacement | AB + CD → AD + CB | two compounds swap partners |
Typical equations
- Combination: 2H₂ + O₂ → 2H₂O; CaO + H₂O → Ca(OH)₂
- Decomposition: 2H₂O₂ → 2H₂O + O₂↑; CaCO₃ → CaO + CO₂↑ (heat)
- Single displacement: Zn + H₂SO₄ → ZnSO₄ + H₂↑; Fe + CuSO₄ → FeSO₄ + Cu
- Double displacement: NaOH + HCl → NaCl + H₂O; AgNO₃ + NaCl → AgCl↓ + NaNO₃
How to judge them
- Whether a single displacement happens depends on the activity series: a metal displaces those below it.
- A double displacement proceeds if it forms a precipitate, gas or water (any one); check precipitates with the solubility rules.
- These four types and redox are two separate schemes: single displacement is always redox; double displacement never is.
For more, see common equations and verify them in the reactions balancer.