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Balancing chemical equations is really just accounting — the Law of Conservation of Mass says atoms are never created or destroyed in a chemical reaction, only rearranged, so the number of atoms of each element must match exactly on both sides of the equation. These 10 examples walk through the balancing process across four common reaction types: combination, decomposition, displacement, and combustion.

Why Balancing Matters Beyond Just "Getting It Right"

An unbalanced equation isn't just untidy — it's chemically meaningless, because it implies atoms appeared or vanished during the reaction, which never actually happens. Balanced equations are also the foundation for stoichiometry (calculating the exact masses or volumes of reactants and products), so a mistake here carries forward into every calculation that depends on it later in the syllabus.

Simple Combination Reactions (Examples 1–3)

1. Balance: H₂ + O₂ → H₂O
Oxygen is unbalanced (2 on left, 1 on right). Add a coefficient of 2 to H₂O: H₂ + O₂ → 2H₂O. Now H is unbalanced (2 vs 4) — add a coefficient of 2 to H₂: 2H₂ + O₂ → 2H₂O.

2. Balance: N₂ + H₂ → NH₃
Balance N first: N₂ + H₂ → 2NH₃. Now balance H (2 vs 6): N₂ + 3H₂ → 2NH₃.

3. Balance: Mg + O₂ → MgO
Balance O first: Mg + O₂ → 2MgO. Now balance Mg: 2Mg + O₂ → 2MgO.

Decomposition Reactions (Examples 4–5)

4. Balance: CaCO₃ → CaO + CO₂
Already balanced as written — one Ca, one C, and three O on each side. CaCO₃ → CaO + CO₂.

5. Balance: KClO₃ → KCl + O₂
Balance O: 2KClO₃ → 2KCl + 3O₂. Check K and Cl — both balanced. 2KClO₃ → 2KCl + 3O₂.

Displacement Reactions (Examples 6–7)

6. Balance: Zn + HCl → ZnCl₂ + H₂
Balance Cl: Zn + 2HCl → ZnCl₂ + H₂. Check H — 2 on each side. Zn + 2HCl → ZnCl₂ + H₂.

7. Balance: Fe + CuSO₄ → FeSO₄ + Cu
Already balanced — one Fe, one Cu, one S, four O on each side. Fe + CuSO₄ → FeSO₄ + Cu.

Combustion Reactions (Examples 8–10)

8. Balance: CH₄ + O₂ → CO₂ + H₂O
Balance C (already 1:1), then H: CH₄ + O₂ → CO₂ + 2H₂O. Now balance O (2 vs 4) — add coefficient 2 to O₂: CH₄ + 2O₂ → CO₂ + 2H₂O.

9. Balance: C₂H₆ + O₂ → CO₂ + H₂O
Balance C: → 2CO₂. Balance H: → 3H₂O. Now count O on the right (4+3=7) and set O₂ coefficient to 7/2: C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O, or multiply everything by 2: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.

10. Balance: C₃H₈ + O₂ → CO₂ + H₂O
Balance C: → 3CO₂. Balance H: → 4H₂O. Count O on right (6+4=10), so O₂ coefficient = 5: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

General Method Recap

1. Balance metals first, then non-metals, then hydrogen, then oxygen last.
2. Never change subscripts inside a formula — only adjust the coefficients in front.
3. Double-check every element's count on both sides before finalizing.

Common Mistakes Students Make

• Changing subscripts instead of coefficients. Turning H₂O into H₂O₂ to balance oxygen is one of the most common errors — this actually changes the substance (water into hydrogen peroxide) rather than balancing the equation. Only the numbers in front of a formula can be changed.
• Balancing oxygen or hydrogen too early. These elements often appear in multiple compounds on the same side, so balancing them first tends to create a moving target. Save them for last, as shown in the recap above.
• Leaving fractional coefficients in the final answer. As in example 9, a fractional coefficient like 7/2 O₂ is mathematically valid but conventionally cleaned up by multiplying every coefficient in the equation by 2 to get whole numbers.
• Not double-checking after the "final" answer. It's easy to balance one element, feel confident, and forget to re-verify the ones you balanced earlier are still correct. Always recount every element after the last adjustment.

Frequently Asked Questions

Why can't I just change the subscript to balance an equation?
Because the subscript defines what the compound actually is — changing H₂O to H₂O₂ turns water into an entirely different chemical (hydrogen peroxide), which isn't what the original reaction produces. Balancing must be done only with coefficients placed in front of the formula.

Is there a fixed order I should always balance elements in?
A generally reliable order is: metals first, then non-metals (other than H and O), then hydrogen, then oxygen last — since oxygen and hydrogen tend to appear in the most compounds and are easiest to fix once everything else is set.

What do I do if I end up with a fractional coefficient?
Multiply every single coefficient in the entire equation by the denominator of the fraction (2, in the case of a half) to convert everything to whole numbers, as shown in example 9.