A balanced equation has the same number of each type of atom on both sides, because atoms are not created or destroyed in a chemical reaction (conservation of mass). You balance by changing coefficients, never subscripts.
The method
- Write the unbalanced equation with correct formulas.
- Count atoms of each element on both sides.
- Balance elements that appear in only one compound per side first; leave H and O (or free elements) for last.
- Recount everything, and reduce coefficients to the smallest whole numbers.
Example 1: Burning methane
Start: CH₄ + O₂ → CO₂ + H₂O
- Carbon: 1 and 1, already balanced.
- Hydrogen: 4 on the left, so put 2 in front of water:
CH₄ + O₂ → CO₂ + 2H₂O - Oxygen: the right now has 2 + 2 = 4, so use 2O₂:
CH₄ + 2O₂ → CO₂ + 2H₂O
Example 2: Iron rusting
Start: Fe + O₂ → Fe₂O₃. Oxygen comes in pairs on the left and threes on the right, so use the least common multiple, 6: Fe + 3O₂ → 2Fe₂O₃. Now iron is 4 on the right: 4Fe + 3O₂ → 2Fe₂O₃. Check: Fe 4 = 4, O 6 = 6 ✓.
Common mistakes
- Changing a subscript (H₂O to H₂O₂ is a different substance).
- Forgetting to multiply a coefficient through every atom in the formula.
- Leaving fractions or non-reduced coefficients in the final answer.