How to Balance Chemical Equations: Step-by-Step Guide

Balancing chemical equations enforces the law of conservation of mass: atoms are neither created nor destroyed, so each element must have equal counts on both sides.

Why Do We Balance Equations?

An unbalanced equation like:

H₂ + O₂ → H₂O

isn't wrong — it just says hydrogen and oxygen react to form water. But it doesn't tell us how much of each. The balanced version:

2H₂ + O₂ → 2H₂O

tells us: 2 molecules of hydrogen gas react with 1 molecule of oxygen to produce 2 molecules of water.

Step-by-Step: The Inspection Method

This works for most equations you'll encounter in high school chemistry.

Step 1: Write the Unbalanced Equation

Start by writing out reactants and products with correct formulas. Don't change any subscripts — you can only add coefficients in front.

Example: Iron reacts with oxygen to form iron(III) oxide.

Fe + O₂ → Fe₂O₃

Step 2: Count Atoms on Each Side

Neither is balanced.

Step 3: Balance One Element at a Time

Start with elements that appear in the fewest compounds. Here, balance Fe first.

Put a 4 in front of Fe on the left (we'll see why in a moment):

4Fe + 3O₂ → 2Fe₂O₃

Now re-count:

Balanced!

Worked Example 2: Combustion Reaction

Balance the combustion of propane (C₃H₈):

C₃H₈ + O₂ → CO₂ + H₂O

Step 1: Balance carbon. There are 3 C on the left, so put 3 in front of CO₂: C₃H₈ + O₂ → 3CO₂ + H₂O

Step 2: Balance hydrogen. There are 8 H on the left, H₂O has 2 H each, so put 4 in front of H₂O: C₃H₈ + O₂ → 3CO₂ + 4H₂O

Step 3: Balance oxygen. Right side: 3×2 + 4×1 = 10 O atoms. Left side needs 10/2 = 5 O₂: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Final check:

Balanced: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O ✓

The Half-Reaction Method (for Redox Equations)

For oxidation-reduction (redox) reactions in acidic or basic solution, the inspection method gets complicated. Use half-reactions instead.

Example: Balance in acidic solution: MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺

Step 1 — Separate into half-reactions:

• Oxidation: Fe²⁺ → Fe³⁺
• Reduction: MnO₄⁻ → Mn²⁺

Step 2 — Balance atoms (not O, not H yet): Both are already balanced for main atoms.

Step 3 — Balance O using H₂O: MnO₄⁻ → Mn²⁺ + 4H₂O

Step 4 — Balance H using H⁺ (acidic solution): 8H⁺ + MnO₄⁻ → Mn²⁺ + 4H₂O

Step 5 — Balance charge using electrons:

• Oxidation: Fe²⁺ → Fe³⁺ + e⁻
• Reduction: 5e⁻ + 8H⁺ + MnO₄⁻ → Mn²⁺ + 4H₂O

Step 6 — Equal electrons in both half-reactions: Multiply oxidation ×5: 5Fe²⁺ → 5Fe³⁺ + 5e⁻

Step 7 — Add and cancel: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺ ✓

Common Mistakes

Changing subscripts — Never change subscripts (e.g., H₂O to H₃O). Only add coefficients in front of whole formulas.

Forgetting polyatomic ions — If a polyatomic ion like SO₄²⁻ appears unchanged on both sides, balance it as a unit.

Skipping the check — Always recount every atom after you think it's balanced.

Leaving fractions — If you end up with fractional coefficients (like ½), multiply everything through to get whole numbers.

Try the Balancer

Paste an unbalanced equation below and get whole-number coefficients instantly.

Tips for Success

Save oxygen and hydrogen for last — they appear in many compounds, so balance them after everything else.

Use trial and error confidently — it's expected. Start with a guess, adjust, and check.

Odd-even trick — if you have an odd count on one side and even on the other, double the odd side's coefficient.

For a complete reference, check our Chemistry Cheatsheets or use our Stoichiometry Calculator to check your balanced equations instantly.