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How to Balance Chemical Equations

Learn a repeatable method for balancing reactions without changing the chemical substances.

◎ Clear explanations◉ Worked examples▤ Practice problems⬇ Free resources

Why equations must be balanced

A chemical equation records the identities and relative amounts of substances in a reaction. In an ordinary chemical reaction, atoms are rearranged rather than created or destroyed, so the number of each element’s atoms must match on both sides.

Illustrated How to Balance Chemical Equations study overview
IntoChemistry visual guide. Follow the worked examples below for the complete reasoning.

Five steps for balancing equations

  1. Write the correct formulas for every reactant and product.
  2. Count the atoms of each element on both sides.
  3. Adjust coefficients only, beginning with an element that appears in relatively few substances.
  4. Recount every element after each adjustment. If an unchanged polyatomic ion appears on both sides, you can sometimes treat it as a unit.
  5. Reduce to the smallest whole-number coefficients and verify all atom counts.

See conservation of atoms

Two H2 molecules and one O2 molecule rearrange into two H2O molecules, preserving 4 hydrogen and 2 oxygen atoms.
The equation 2 H₂ + O₂ → 2 H₂O has the same number of H and O atoms before and after the reaction.

Three worked examples

Worked example 1

Hydrogen and oxygen form water

  1. Start: H₂ + O₂ → H₂O. Left O = 2; right O = 1.
  2. Put 2 before H₂O: H₂ + O₂ → 2 H₂O. Now right H = 4.
  3. Put 2 before H₂: 2 H₂ + O₂ → 2 H₂O.
  4. Check: both sides contain 4 H and 2 O atoms.

Balanced: 2 H₂ + O₂ → 2 H₂O.

Worked example 2

Iron reacts with oxygen

  1. Start: Fe + O₂ → Fe₂O₃.
  2. Balance oxygen using a multiple of 6: Fe + 3 O₂ → 2 Fe₂O₃.
  3. Now the product has 4 Fe atoms; put 4 before Fe.

Balanced: 4 Fe + 3 O₂ → 2 Fe₂O₃.

Worked example 3

Propane combustion

  1. Start: C₃H₈ + O₂ → CO₂ + H₂O.
  2. Balance carbon: 3 CO₂. Balance hydrogen: 4 H₂O.
  3. Right side now contains 10 oxygen atoms, requiring 5 O₂.

Balanced: C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O.

Mistakes to avoid

  • Changing subscripts: Turning H₂O into H₂O₂ changes water into another chemical substance.
  • Forgetting diatomic elements: Elemental H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ are often written as pairs under standard conditions.
  • Stopping at a multiple: 4 H₂ + 2 O₂ → 4 H₂O balances, but reduce the coefficients to 2:1:2.
  • Ignoring parentheses: Ca(OH)₂ contains 2 oxygen and 2 hydrogen atoms from hydroxide.

Check your understanding

Why can’t you change O₂ to O₃ to balance a reaction?

Because the subscript identifies the chemical species. O₂ and O₃ are different substances; changing a formula changes the reaction.

Does the equation need equal numbers of molecules on each side?

No. It must conserve the number of atoms of every element, not the total number of molecules.

Interactive How to Balance Chemical Equations Practice

Apply what you have learned using immediate feedback.

Interactive chemistry practice

Balance the equation

Type the smallest positive whole-number coefficients. Do not change chemical formulas.