Balancing Chemical Equations Step by Step
A clear method for balancing chemical equations in IGCSE Chemistry 0620, with worked examples and the errors that lose marks.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Balancing equations is a skill that appears on every 0620 paper. Paper 2 may give you four equations and ask which is balanced. Paper 4 asks you to write and balance equations from scratch. The method below works for every equation the exam can set, from simple combinations to complex organic combustion. The broader equation-writing process is covered in how to write chemical equations.
The golden rule
You may only change the large number in front of a formula (the coefficient). You must never change the small number within a formula (the subscript). Changing H2O to H2O2 does not balance an equation; it creates a completely different substance.
The method
Step 1: Write the unbalanced equation with correct formulae
Before you can balance, every formula must be correct. This is where most errors start. Check:
- Diatomic elements: H2, O2, N2, Cl2, Br2, I2, F2 (not H, O, N, etc.).
- Ionic compound formulae: the charges must cancel. MgCl2 not MgCl; Al2O3 not AlO.
- Brackets are used correctly: Ca(OH)2 means two OH groups.
Step 2: Count atoms on each side
Make a quick table, either in your head or in the margin:
Example: Fe + O2 -> Fe2O3 (unbalanced)
| Element | Left | Right |
|---|---|---|
| Fe | 1 | 2 |
| O | 2 | 3 |
Neither element balances. That tells you coefficients are needed.
Step 3: Balance metals first, then non-metals, then H and O
Balance iron: There are 2 Fe on the right, so put 2 in front of Fe on the left. 2Fe + O2 -> Fe2O3
Balance oxygen: There are 3 O on the right and 2 on the left. You cannot easily make 2 become 3 with a whole number. When oxygen is awkward, try doubling everything first.
Multiply the right side by 2: 2Fe + O2 -> 2Fe2O3. Now the right has 4 Fe and 6 O. Adjust the left: 4Fe + 3O2 -> 2Fe2O3.
Check: 4 Fe on each side, 6 O on each side. Balanced.
Step 4: Final check
Count every element one more time. It takes ten seconds and catches errors that would cost marks.
Worked examples
Example 1: simple neutralisation
NaOH + H2SO4 -> Na2SO4 + H2O
Count: Na: 1 left, 2 right. H: 3 left, 2 right. O: 5 left, 5 right. S: 1 left, 1 right.
Balance Na: put 2 in front of NaOH. 2NaOH + H2SO4 -> Na2SO4 + H2O
Recount: Na: 2, 2. H: 4 left (2 from NaOH + 2 from H2SO4), 2 right. O: 6 left, 5 right.
Balance H and O: put 2 in front of H2O. 2NaOH + H2SO4 -> Na2SO4 + 2H2O
Check: Na: 2, 2. O: 6, 6. H: 4, 4. S: 1, 1. Balanced.
Example 2: combustion of ethanol
C2H5OH + O2 -> CO2 + H2O
Count C: 2 left, 1 right. Put 2 in front of CO2. C2H5OH + O2 -> 2CO2 + H2O
Count H: 6 left (5 + 1 from OH), 2 right. Put 3 in front of H2O. C2H5OH + O2 -> 2CO2 + 3H2O
Count O: 1 (from OH) + 2 (from O2) = 3 left. 4 (from 2CO2) + 3 (from 3H2O) = 7 right. Need 7 total O on left. Already have 1 from ethanol, so need 6 more from O2. Put 3 in front of O2. C2H5OH + 3O2 -> 2CO2 + 3H2O
Check: C: 2, 2. H: 6, 6. O: 1 + 6 = 7, 4 + 3 = 7. Balanced.
Example 3: dealing with fractions
Sometimes balancing produces a fraction. For instance, CH4 + O2 -> CO2 + H2O gives:
C: 1, 1. H: 4, 2. Put 2 in front of H2O. CH4 + O2 -> CO2 + 2H2O
O: 2 left, 2 + 2 = 4 right. Need 4 O on the left, so put 2 in front of O2. CH4 + 2O2 -> CO2 + 2H2O
Check: C: 1, 1. H: 4, 4. O: 4, 4. Balanced. In this case no fractions were needed.
But if the equation were 2C2H6 + 7O2 -> 4CO2 + 6H2O, and you had started with C2H6 + 3.5O2 -> 2CO2 + 3H2O, the fraction (3.5) is removed by multiplying everything by 2. The exam expects whole-number coefficients.
Balancing with brackets
When a formula contains brackets, count the atoms inside by multiplying:
Ca(OH)2 contains: 1 Ca, 2 O, 2 H. Al2(SO4)3 contains: 2 Al, 3 S, 12 O. Mg(NO3)2 contains: 1 Mg, 2 N, 6 O.
Then balance as normal, treating each element individually.
The errors that lose marks
- Changing subscripts instead of coefficients. This is the most penalised error because it changes the chemistry.
- Leaving the equation unbalanced. An unbalanced equation loses the balancing mark, even if the formulae are correct.
- Forgetting diatomic elements. Writing O instead of O2 means your oxygen count is wrong from the start.
- Not checking. A five-second recount catches most errors.
- Forgetting products. Acid + carbonate reactions produce three products (salt + water + CO2). Missing CO2 means missing atoms that cannot be balanced.
Practice strategy
Take ten unbalanced equations from past papers and balance them without looking at the answers. Time yourself: you should be able to balance a standard equation in under two minutes. If it takes longer, the issue is usually in step 2 (counting atoms in complex formulae) rather than step 3 (adjusting coefficients).
If balancing equations is a persistent difficulty, a trial lesson can identify whether the root cause is formula writing, atom counting, or the balancing logic itself.