Balancing Chemical Equations: KS3 Chemistry Guide

A balanced chemical equation shows that atoms are neither created nor destroyed in a chemical reaction — the number of each type of atom is the same on both sides of the arrow. Balancing equations is a core KS3 chemistry skill tested in Year 8 and Year 9.

Why equations must be balanced

The law of conservation of mass states that mass is conserved in any chemical reaction. This means the total number of each type of atom on the left-hand side (reactants) must equal the total number of that atom on the right-hand side (products).

You balance an equation by adding coefficients (whole numbers placed in front of a formula). You must never change the formula of a substance — only adjust the numbers in front.

Unbalanced: H₂ + O₂ → H₂O
Count: H: 2 = 2 ✓, O: 2 ≠ 1 ✗ (not balanced)

Balanced: 2H₂ + O₂ → 2H₂O
Count: H: 4 = 4 ✓, O: 2 = 2 ✓

How to balance a chemical equation: step-by-step

Step 1 — Write the unbalanced equation with correct formulas.
Step 2 — List and count all atoms on each side.
Step 3 — Add coefficients to make the atom counts equal. Start with the most complex molecule or the element that appears in the fewest places.
Step 4 — Recount all atoms to check.
Step 5 — Ensure all coefficients are whole numbers with no common factor greater than 1.

Worked examples

Worked example 1: formation of water

Balance H₂ + O₂ → H₂O.

Atom | Left side | Right side | Balanced?
H | 2 | 2 | ✓
O | 2 | 1 | ✗

To fix oxygen: put a 2 in front of H₂O → H₂ + O₂ → 2H₂O.

Now hydrogen: 2 on the left, 4 on the right. Put a 2 in front of H₂ → 2H₂ + O₂ → 2H₂O.

Atom | Left side | Right side | Balanced?
H | 4 | 4 | ✓
O | 2 | 2 | ✓

Answer: 2H₂ + O₂ → 2H₂O

Worked example 2: combustion of methane

Balance CH₄ + O₂ → CO₂ + H₂O.

Start with carbon (appears in one place each side):

Atom | Left | Right
C | 1 | 1 ✓
H | 4 | 2 ✗
O | 2 | 3 ✗

Fix hydrogen: put 2 in front of H₂O → CH₄ + O₂ → CO₂ + 2H₂O.

Recount oxygen on right: 2 (from CO₂) + 2 (from 2H₂O) = 4. Left has 2. Put 2 in front of O₂ → CH₄ + 2O₂ → CO₂ + 2H₂O.

Final check:

Atom | Left | Right
C | 1 | 1 ✓
H | 4 | 4 ✓
O | 4 | 4 ✓

Answer: CH₄ + 2O₂ → CO₂ + 2H₂O

Worked example 3: iron reacting with oxygen (rust)

Balance Fe + O₂ → Fe₂O₃.

Atom | Left | Right
Fe | 1 | 2 ✗
O | 2 | 3 ✗

The challenge here: O₂ gives 2 at a time, but Fe₂O₃ needs 3 oxygen atoms. Find the LCM of 2 and 3 = 6.

Put 3 in front of O₂ (gives 6 oxygen atoms) and 2 in front of Fe₂O₃ (gives 6 oxygen atoms): Fe + 3O₂ → 2Fe₂O₃.

Now fix iron: right side has 4 Fe. Put 4 in front of Fe → 4Fe + 3O₂ → 2Fe₂O₃.

Atom | Left | Right
Fe | 4 | 4 ✓
O | 6 | 6 ✓

Answer: 4Fe + 3O₂ → 2Fe₂O₃

Worked example 4: decomposition of calcium carbonate

Balance CaCO₃ → CaO + CO₂.

Atom | Left | Right
Ca | 1 | 1 ✓
C | 1 | 1 ✓
O | 3 | 3 ✓

This equation is already balanced. Sometimes no coefficients need to be added.

Answer: CaCO₃ → CaO + CO₂ (already balanced)

Worked example 5: neutralisation of hydrochloric acid with sodium hydroxide

Balance HCl + NaOH → NaCl + H₂O.

Atom | Left | Right
H | 2 | 2 ✓
Cl | 1 | 1 ✓
Na | 1 | 1 ✓
O | 1 | 1 ✓

Already balanced — all 1:1:1:1.

Answer: HCl + NaOH → NaCl + H₂O

State symbols

Fully written equations also include state symbols in brackets to show the physical state of each substance:

(s) = solid

(l) = liquid

(g) = gas

(aq) = aqueous (dissolved in water)

Example with state symbols:

2H₂(g) + O₂(g) → 2H₂O(l)

State symbols are required in GCSE chemistry and are introduced in Year 9 at many schools.

Common mistakes

Mistake 1 — Changing the subscripts.
You must never change the subscripts inside a formula. Changing H₂O to H₂O₂ changes the substance (from water to hydrogen peroxide). Only the coefficient (the number in front) may be changed.

Mistake 2 — Forgetting that a subscript multiplies.
In Fe₂O₃, there are 2 iron atoms and 3 oxygen atoms per formula unit. If there are 2 formula units (coefficient 2), that is 4 iron atoms and 6 oxygen atoms: 2 × Fe₂O₃ = 4 Fe + 6 O.

Mistake 3 — Not recounting after adding a coefficient.
Each time you add a coefficient, you change the count of every atom in that formula. Always recount all atoms after each change.

Mistake 4 — Using fractions in the final answer.
Coefficients must be whole (integer) numbers. If you get a fractional coefficient, multiply all coefficients by the denominator to clear the fraction.

How balancing equations fits the KS3 national curriculum

The Department for Education's KS3 Science Programme of Study requires pupils to "represent chemical reactions using formulae and using equations," including an understanding that atoms are conserved in reactions. BBC Bitesize KS3 Chemistry covers balancing equations as core Year 8 and Year 9 content, directly preparing pupils for GCSE Chemistry Unit 1.

Frequently asked questions

Why can I not just change the subscripts to balance an equation?

Subscripts are part of a chemical formula and cannot be changed without changing the substance entirely. H₂O is water; H₂O₂ is hydrogen peroxide — a completely different compound. Balancing is achieved by choosing how many molecules of each correct formula take part, not by altering the molecules themselves.

Do I have to balance elements in a particular order?

A common strategy is to balance any element that appears in only one reactant and one product first, saving elements like oxygen (which often appears in multiple compounds) until last. There is no single rule — with practice you develop a feel for which approach works fastest on any given equation.

What is the difference between a word equation and a symbol equation?

A word equation uses the names of reactants and products: hydrogen + oxygen → water. A symbol equation uses chemical formulas: 2H₂ + O₂ → 2H₂O. Symbol equations can be balanced; word equations cannot — they always show a 1:1 relationship by their nature.

How many equations do I need to know for KS3?

At KS3 you are expected to balance given equations rather than recall specific ones. Common equations used in exercises include combustion of hydrocarbons, metal oxide + acid neutralisation, and simple decomposition reactions. At GCSE, a set of equations for specific reactions must be memorised.

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