Curriculum fit: GCSE Chemistry and Combined Science (quantitative chemistry and chemical changes). US: high school physical science (conservation of matter in reactions).
On this page
What Is It?
Atoms are not created or destroyed in a chemical reaction, so the number of atoms of each element must be the same on both sides of an equation. This is the law of conservation of mass.
Balancing an equation means putting numbers (coefficients) in front of formulas until each element is balanced. You must never change the small numbers inside a formula.
Key Formulas and Ideas
- Count atoms of each element on both sides.
- Change only the large numbers in front of formulas.
- Balance elements that appear in only one substance first, and leave oxygen or hydrogen until last.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.
Worked Examples
Combustion of Hydrogen
Balance H₂ + O₂ → H₂O.
- Oxygen: 2 on the left and 1 on the right, so put 2 in front of H₂O.
- Now hydrogen is 4 on the right, so put 2 in front of H₂.
✓ Answer 2H₂ + O₂ → 2H₂O
Iron and Oxygen
Balance Fe + O₂ → Fe₂O₃.
- Oxygen: use 3 O₂ (6 atoms) and 2 Fe₂O₃ (6 atoms).
- Then iron: 2 × 2 = 4 on the right, so put 4 in front of Fe.
✓ Answer 4Fe + 3O₂ → 2Fe₂O₃
Propane
Balance C₃H₈ + O₂ → CO₂ + H₂O.
- Carbon: 3 CO₂.
- Hydrogen: 8 H atoms means 4 H₂O.
- Oxygen on the right: 6 + 4 = 10, so 5 O₂.
✓ Answer C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Common Mistakes to Avoid
- Changing a subscript, which changes the substance.
- Balancing oxygen first, which often has to be redone.
- Writing an unbalanced equation as the final answer.
- Forgetting to multiply a coefficient through the whole formula.
Practice Questions
Frequently Asked Questions
Why can't I change the small numbers?
They define the substance. Changing H₂O to H₂O₂ would make a different compound, hydrogen peroxide.
Can I use fractions in a balanced equation?
In GCSE equations use whole numbers. Multiply through to clear any fractions.
Keep Going With These Guides
Moles and Molar Mass: Calculations Explained Step by Step
Convert between mass and moles using Mr, use balanced equations to find reacting masses and calculate concentration, with worked examples.
GCSEA-LevelHigh SchoolAPAcids, Bases and the pH Scale: Reactions and Neutralisation
Read the pH scale, write neutralisation equations and name the salts acids make, with a titration calculation for Higher tier.
GCSEHigh SchoolAlkanes and Alkenes: Formulae, Combustion and Bromine Test
Learn the hydrocarbon families, their general formulae, how they burn and how bromine water tells them apart, with worked examples.
GCSEA-LevelHigh SchoolSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.