How to Balance Chemical Equations
By the Periodixy Editorial Team · Last reviewed July 10, 2026
A chemical equation is balanced when every element has the same number of atoms on both sides. This is not a bookkeeping formality — it is the law of conservation of mass: atoms are never created or destroyed in a reaction, only rearranged.
Balancing means adjusting the big numbers in front of formulas (coefficients) — never the little subscripts inside them.
The golden rule
You may change
Coefficients: 2H₂O ✓ Never subscripts: H₂O → H₂O₂ ✗
Changing a subscript changes the substance. H₂O is water; H₂O₂ is hydrogen peroxide, a completely different (hair-bleaching) compound. Coefficients just say how many of each molecule take part.
The counting method, step by step
- Write the unbalanced equation with correct formulas.
- Count atoms of each element on both sides.
- Balance elements that appear in only one substance per side first.
- Balance oxygen and hydrogen last (they often appear in several substances).
- Recount everything. If a coefficient ends up fractional, multiply the whole equation to clear it.
Example 1: Hydrogen + oxygen
Balance H₂ + O₂ → H₂O.
- Count: left has 2 H, 2 O; right has 2 H, 1 O. Oxygen is short.
- Put a 2 before H₂O: now right has 4 H, 2 O.
- Hydrogen is now short on the left — put a 2 before H₂.
- Final count: 4 H and 2 O on each side. Balanced.
Answer: 2H₂ + O₂ → 2H₂O
Example 2: Propane combustion
Balance C₃H₈ + O₂ → CO₂ + H₂O.
- Balance carbon: 3 C on the left → put 3 before CO₂.
- Balance hydrogen: 8 H on the left → put 4 before H₂O.
- Count oxygen on the right: (3 × 2) + (4 × 1) = 10 → put 5 before O₂.
Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Example 3: Metal + acid
Balance Al + HCl → AlCl₃ + H₂.
- Chlorine: 3 needed on the left → 3HCl. But now H is 3 (odd) and H₂ needs even numbers.
- Double everything involving H and Cl: 6HCl → 2AlCl₃ + 3H₂.
- Balance aluminium: 2Al.
Answer: 2Al + 6HCl → 2AlCl₃ + 3H₂
Common mistakes to avoid
- Changing subscripts instead of coefficients — the number one error.
- Forgetting a coefficient multiplies the whole formula: 3Ca(OH)₂ contains 3 Ca, 6 O and 6 H.
- Leaving the equation with a common factor: 4H₂ + 2O₂ → 4H₂O should be simplified to 2H₂ + O₂ → 2H₂O.
- Miscounting polyatomic ions — if SO₄ appears unchanged on both sides, count it as one unit to save effort.
Check any equation instantly with the Chemical Equation Balancer — it shows the atom tally on both sides so you can see the conservation of mass at work.
Summary
- Balanced equations have equal atoms of every element on both sides — the law of conservation of mass.
- Only coefficients may change; subscripts define the substances themselves.
- Balance single-appearance elements first; save H and O for last.
- Always simplify to the smallest whole-number coefficients.