How to balance chemical equations for GCSE chemistry
Balancing chemical equations is one of those skills that looks tricky at first but becomes second nature once you know the method. The good news is that there is a method – a reliable, repeatable set of steps you can follow every single time.
This guide walks you through exactly how to balance equations, starting from the basics and building up to harder examples. By the end, you should feel confident tackling any balancing question your GCSE Chemistry exam throws at you.
Every topic
of GCSE Chemistry tests your ability to read, write, or balance equations – they appear across the whole specification, from acids and bases to electrolysis to combustion
What does balancing actually mean?
A chemical equation describes a reaction. On the left you have the reactants (what you start with) and on the right you have the products (what you end up with). An arrow sits between them, meaning "reacts to form".
Balancing an equation means making sure the same number of atoms of each element appears on both sides. This has to be the case because of the law of conservation of mass – atoms are not created or destroyed during a chemical reaction. They just rearrange.
If your equation has two oxygen atoms on the left but three on the right, something is wrong. Those atoms have to come from somewhere. Balancing fixes this by adjusting the numbers in front of each formula (called coefficients) until every element matches on both sides.
The systematic balancing method
Rather than guessing and hoping for the best, use this step-by-step approach. It works for simple equations and complicated ones alike.
How to balance any equation
Follow these steps in order every time you need to balance a chemical equation.
- Write out the unbalanced equation with the correct chemical formulae
- List every element that appears in the equation
- Count the atoms of each element on both sides
- Pick the most complex molecule and balance its elements first
- Adjust coefficients (the big numbers in front) one element at a time
- Leave simple molecules like O₂ and H₂ until last – they are easiest to adjust
- Do a final atom count on both sides to check everything matches
Only ever change the coefficients (the big numbers in front of a formula). Never change the subscript numbers inside a formula – that would turn it into a completely different substance. Changing H₂O to H₂O₂ does not balance your equation. It turns water into hydrogen peroxide.
Worked balancing equation examples
Example 1 – a simple one to start
Magnesium reacts with oxygen to form magnesium oxide.
Unbalanced: Mg + O₂ → MgO
Start by counting atoms on each side.
Left side: 1 Mg, 2 O Right side: 1 Mg, 1 O
Magnesium is already balanced (1 on each side), but oxygen is not. There are 2 oxygen atoms on the left and only 1 on the right. Putting a 2 in front of MgO gives 2 oxygen atoms on the right – but now there are 2 Mg on the right and only 1 on the left. So put a 2 in front of Mg as well.
Balanced: 2Mg + O₂ → 2MgO
Final check: 2 Mg on each side, 2 O on each side. Done.
Example 2 – combustion of methane
Methane burns in oxygen to produce carbon dioxide and water.
Unbalanced: CH₄ + O₂ → CO₂ + H₂O
Count atoms.
Left side: 1 C, 4 H, 2 O Right side: 1 C, 2 H, 3 O
Carbon is balanced already (1 each side). Hydrogen is not – 4 on the left, 2 on the right. Put a 2 in front of H₂O to get 4 hydrogen atoms on the right.
CH₄ + O₂ → CO₂ + 2H₂O
Now recount oxygen on the right: 2 from CO₂ plus 2 from 2H₂O = 4. The left side has only 2. Put a 2 in front of O₂.
Balanced: CH₄ + 2O₂ → CO₂ + 2H₂O
Final check: 1 C, 4 H, 4 O on each side. Done.
Example 3 – a trickier one
Iron reacts with oxygen to form iron oxide (Fe₂O₃).
Unbalanced: Fe + O₂ → Fe₂O₃
Count atoms.
Left side: 1 Fe, 2 O Right side: 2 Fe, 3 O
Start with the most complex molecule – Fe₂O₃. There are 2 Fe on the right, so put a 2 in front of Fe on the left. Oxygen is trickier: 3 on the right but 2 on the left. The lowest common multiple of 2 and 3 is 6. So use 3O₂ on the left (giving 6 O) and 2Fe₂O₃ on the right (giving 6 O). But now Fe on the right is 2 × 2 = 4, so adjust the left to 4Fe.
Balanced: 4Fe + 3O₂ → 2Fe₂O₃
Final check: 4 Fe on each side, 6 O on each side. Done.
Coefficients vs subscripts – the most common mistake
This is by far the most common error students make. It is worth repeating because it costs marks every exam series.
A coefficient is the big number written in front of a chemical formula. It multiplies everything in that formula. 2H₂O means two molecules of water – that is 4 hydrogen atoms and 2 oxygen atoms in total.
A subscript is the small number written below and to the right of an element symbol within the formula. It tells you how many atoms of that element are in one molecule. The 2 in H₂O means each molecule of water contains 2 hydrogen atoms.
When you balance an equation, you are only allowed to change the coefficients. Changing subscripts would change the substance itself. H₂O is water. H₂O₂ is hydrogen peroxide. They are completely different chemicals.
Common equations to practise
Here are ten equations that come up regularly in GCSE Chemistry. Try balancing each one yourself before checking the balanced version.
| Reaction | Unbalanced | Balanced |
|---|---|---|
| Magnesium + oxygen | Mg + O₂ → MgO | 2Mg + O₂ → 2MgO |
| Hydrogen + oxygen | H₂ + O₂ → H₂O | 2H₂ + O₂ → 2H₂O |
| Methane combustion | CH₄ + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Sodium + water | Na + H₂O → NaOH + H₂ | 2Na + 2H₂O → 2NaOH + H₂ |
| Iron + oxygen | Fe + O₂ → Fe₂O₃ | 4Fe + 3O₂ → 2Fe₂O₃ |
| Neutralisation (HCl + NaOH) | HCl + NaOH → NaCl + H₂O | HCl + NaOH → NaCl + H₂O |
| Thermal decomposition of CaCO₃ | CaCO₃ → CaO + CO₂ | CaCO₃ → CaO + CO₂ |
| Ethane combustion | C₂H₆ + O₂ → CO₂ + H₂O | 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O |
| Zinc + hydrochloric acid | Zn + HCl → ZnCl₂ + H₂ | Zn + 2HCl → ZnCl₂ + H₂ |
| Aluminium + oxygen | Al + O₂ → Al₂O₃ | 4Al + 3O₂ → 2Al₂O₃ |
Some equations are already balanced as written – like neutralisation of HCl + NaOH and thermal decomposition of CaCO₃. Do not force extra coefficients in where none are needed. Always check by counting atoms first.
Tips for getting faster
Speed comes from practice, but there are a few shortcuts that help.
Always deal with the most complex molecule first. It contains the most elements, so balancing it first reduces the number of things left to adjust.
Leave hydrogen and oxygen until last whenever possible. They appear in many compounds, so adjusting them early often creates new imbalances elsewhere.
If you are stuck with an odd number on one side and an even number on the other, try doubling everything. For example, if you have 3 oxygen atoms on one side and O₂ on the other, you cannot get 3 from O₂ alone. But you can get 6 from 3O₂ – so double the product side too.
Finally, always do a complete atom count at the end. It takes ten seconds and catches mistakes before they cost you marks.
