Chemical Reactions and Equations — Revision Notes
Balancing equations and the five reaction types you must be able to recognise, in one revision pass.
About 8 min readReviewed by a CBSE Science teacherSyllabus 2024-25Updated 6/8/2026
A chemical equation is only correct when the number of atoms of each element is the same on both sides. Everything else in this chapter builds on that.
Balancing an equation
Hit-and-trial method
- Write the skeletal equation
- Count atoms of each element on both sides
- Balance the compound with the most atoms first
- Balance hydrogen and oxygen last
- Add the physical states and conditions
Balance Fe + H₂O → Fe₃O₄ + H₂
- 13Fe + 4H₂O → Fe₃O₄ + 4H₂
- 2Check: Fe 3 = 3, O 4 = 4, H 8 = 8
Types of reactions
| Type | What happens | Example |
|---|---|---|
| Combination | Two or more substances form one | CaO + H₂O → Ca(OH)₂ |
| Decomposition | One substance breaks into two or more | 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ |
| Displacement | A more reactive element displaces a less reactive one | Fe + CuSO₄ → FeSO₄ + Cu |
| Double displacement | Ions are exchanged between two compounds | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl |
| Redox | Oxidation and reduction happen together | CuO + H₂ → Cu + H₂O |
- Corrosion
- The gradual eating away of a metal surface by air, moisture or chemicals around it.
- Rancidity
- The oxidation of fats and oils in food, producing an unpleasant smell and taste.
Quick recap
- Balance by coefficients, never by subscripts
- Five reaction types: combination, decomposition, displacement, double displacement, redox
- Corrosion affects metals; rancidity affects fats and oils