Why must chemical equations be balanced?

The law of conservation of mass: atoms are neither created nor destroyed in a chemical reaction. A balanced equation shows the same number of each type of atom on both sides.

What is the step-by-step method for balancing equations?

Write the unbalanced equation, then adjust coefficients (whole numbers in front of formulas) to make the atom counts equal on both sides. Never change subscripts — only add coefficients.

What is the difference between a coefficient and a subscript in a chemical formula?

A coefficient multiplies the entire formula unit; it can be adjusted when balancing. A subscript is part of the formula and defines the compound — it cannot be changed.

How do you balance ionic equations?

Write ionic equations by separating strong electrolytes into their ions. Cancel spectator ions (ions that appear unchanged on both sides). The net ionic equation shows only the species that actually change.

GUIDE Chemistry Beginner

Balancing Chemical Equations

A reliable method for making both sides of a chemical equation match — step by step, with worked examples from combustion to neutralisation.

Why balance equations? → Back to all guides

Primary sources: Khan Academy Chemistry · RSC Education Resources

01 · Why balance equations?

Atoms are conserved — equations must show it.

The law of conservation of mass, established by Antoine Lavoisier in the 1780s, states that in a chemical reaction atoms are neither created nor destroyed — they are rearranged into new substances. A balanced equation expresses this: the total number of each type of atom is the same on both sides of the arrow.

A balanced equation also gives the mole ratio of reactants to products — the foundation of all stoichiometric calculations. Once you know the ratio, you can calculate the mass of any reactant needed or product formed. See our moles and molar mass guide for the calculation framework.

02 · The method

Six steps, in order.

Write correct formulas for all reactants and products. Do not guess — use known compounds.
Count atoms on each side for every element.
Balance the most complex molecule first — the one with the most elements. Work outward from it.
Balance elements in one compound only — leave hydrogen and oxygen until last (they usually appear in multiple places).
Add coefficients (whole numbers in front of formulas) to make each element equal. Never change subscripts.
Verify and simplify — check every element, then divide all coefficients by their highest common factor to get the simplest ratio.

03 · Worked examples

Three common reaction types.

Combustion of methane

Unbalanced CH₄ + O₂ → CO₂ + H₂O

C: 1 = 1 ✓
H: 4 ≠ 2 — place 2 in front of H₂O
O: need 2 + 1 = 4 — place 2 in front of O₂

Balanced CH₄ + 2O₂ → CO₂ + 2H₂O

Formation of iron(III) oxide

Unbalanced Fe + O₂ → Fe₂O₃

Fe: 1 ≠ 2 — trial coefficient 4 for Fe, 2 for Fe₂O₃
O: 4×3/2 = 6 O atoms → 3O₂
Check: 4Fe + 3O₂ → 2Fe₂O₃

Balanced 4Fe + 3O₂ → 2Fe₂O₃

Neutralisation of HCl by NaOH

Unbalanced HCl + NaOH → NaCl + H₂O

H: 1+1=2 on left → 1+1=2 on right ✓
Cl: 1 = 1 ✓
Na: 1 = 1 ✓
O: 1 = 1 ✓

Balanced HCl + NaOH → NaCl + H₂O

04 · Ionic equations

Full, complete and net ionic equations.

When ionic compounds dissolve in water they split into their constituent ions. An ionic equation shows these individual ions rather than the full formula:

Full ionic equation — all dissolved strong electrolytes are written as separate ions.
Spectator ions — ions that appear unchanged on both sides (they "watch" the reaction).
Net ionic equation — spectator ions are cancelled, leaving only the species that change. For precipitation of silver chloride: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Charge must also be balanced: the total ionic charge on the left must equal the total on the right. See our acids and bases guide for how ionic equations are used in acid-base reactions, and our moles guide for converting balanced equations into mass calculations.

Questions about balancing equations

: The law of conservation of mass (Lavoisier, 1789) states that mass is conserved in a chemical reaction — atoms are rearranged, not created or destroyed. A balanced equation confirms this: the same atoms appear on both sides, just in different combinations. An unbalanced equation would imply matter is created or destroyed, which violates this fundamental law.
: Write the correct formulas for all reactants and products.
Count atoms on each side.
Add coefficients to balance — start with the most complex molecule.
Balance elements that appear in only one compound first; leave H and O until last.
Check all atoms and charges are balanced.
Ensure all coefficients are the smallest whole-number ratio.
Never change subscripts in a formula — that would change the identity of the substance.
: In 2H₂O: the 2 in front (coefficient) means two water molecules. The subscript 2 in H₂ means two hydrogen atoms per molecule. Changing the subscript (H₂O → H₃O) would create a different substance entirely (hydronium ion). Only coefficients can be changed during balancing.
: For full ionic equations, split all strong acids, strong bases and soluble ionic compounds into their constituent ions. Then cancel spectator ions — ions that appear identically on both sides. The resulting net ionic equation shows only the chemical change. Example: Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Also balance charges: total charge must be equal on both sides.

Put it into practice

Balanced equations give mole ratios — which feed directly into moles and molar mass calculations.

Moles guide →