How do you balance a chemical equation?
Short answer
A balanced equation has the same number of each type of atom on both sides. Count the atoms of each element, then change the large numbers in front of formulas (coefficients) until the counts match. Never change the small numbers inside a formula, because that changes the substance.
Step by step
- Write the correct formula for every reactant and product.
- Count the atoms of each element on both sides. Treat a group like SO₄ as one unit if it appears unchanged on both sides.
- Balance one element at a time by placing a number in front of a formula. Leave hydrogen and oxygen until last.
- Recount every element after each change.
- Add state symbols (s), (l), (g) or (aq) if the question asks for them.
Worked example
Balance CH₄ + O₂ gives CO₂ + H₂O
- Carbon: 1 on each side, already balanced
- Hydrogen: 4 on the left, so write 2H₂O on the right
- Oxygen: now 2 + 2 = 4 on the right, so write 2O₂ on the left
- CH₄ + 2O₂ gives CO₂ + 2H₂O
Common mistakes
- Changing a formula, for example writing H₂O₂ instead of 2H₂O.
- Forgetting that a coefficient multiplies every atom in the formula: 2H₂O has 4 H and 2 O.
- Not recounting the element you balanced first after later changes.
Useful for:GCSEIGCSEWAECMatric
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