Balancing equations: how to balance a chemical equation
To balance a chemical equation, you make the number of atoms of each element the same on both sides. You do it by writing a number in front of a formula. You never change the small numbers inside a formula, because that would make it a different substance. Atoms are not lost or made in a reaction, so a balanced equation shows that mass is conserved.
Why an equation has to balance
In a chemical reaction, the atoms swap partners. KS3 science calls this the rearrangement of atoms. No atoms vanish and no new ones appear, so the mass of the products equals the mass of the reactants. This is the law of conservation of mass. A balanced symbol equation shows it, with the same count of each element on both sides.
Big numbers and small numbers
An equation uses two kinds of number, and AQA asks you to know the difference.
- A small number low inside a formula, a subscript, counts the atoms of the element just before it. H₂O has 2 hydrogen atoms and 1 oxygen atom.
- A full-size number in front multiplies the whole formula. 2H₂O has 4 hydrogen atoms and 2 oxygen atoms.
When you balance, you only change the numbers in front. The balancer lets you type them and watch the atom count on each side.
A method that works every time
- Write the correct formula for every reactant and product.
- Count the atoms of each element on both sides.
- Pick an element that does not match, and put a number in front of a formula to fix it.
- Count every element again, because one change can upset another.
- Repeat until all the counts match. An element on its own, such as O₂, is often easiest to leave until last.
To get the formulae right for compounds of metals and non-metals, you need the charges on their ions. The page on ionic bonding shows how the group number gives you the charge.
State symbols
A state symbol in brackets after a formula shows its state: (s) solid, (l) liquid, (g) gas and (aq) aqueous, which means dissolved in water. All three boards include state symbols in writing balanced equations.
What each board asks
Every GCSE course here asks for balanced symbol equations at both tiers. Edexcel lists balanced ionic equations as a separate skill. OCR Gateway also asks for half equations, and OCR Twenty First Century asks you to use ratios when balancing.
An ionic equation shows only the ions that change. Writing your own is Higher tier on AQA, Edexcel and OCR Gateway. AQA uses them for displacement, covered on the reactivity series page. KS3 asks you to show reactions with formulae and equations, and balancing them starts at GCSE.
Worked examples
atoms on each side
- Count the atoms. Left: 2 H and 2 O. Right: 2 H and 1 O.
- Oxygen does not match. Put a 2 in front of the water: H₂ + O₂ → 2H₂O
- Now the right has 4 H. Put a 2 in front of the hydrogen: 2H₂ + O₂ → 2H₂O
- Check: 4 H and 2 O on each side.
6 atoms on each side (4 H and 2 O)
atoms on each side
- Count the atoms. Left: 1 Mg, 1 H and 1 Cl. Right: 1 Mg, 2 H and 2 Cl.
- Hydrogen and chlorine are both one short on the left, and both come from HCl. Put a 2 in front of it: Mg + 2HCl → MgCl₂ + H₂
- Check: 1 Mg, 2 H and 2 Cl on each side.
5 atoms on each side (1 Mg, 2 H and 2 Cl)
atoms on each side
- Methane burning. Left: 1 C, 4 H and 2 O. Right: 1 C, 2 H and 3 O.
- Fix hydrogen with a 2 in front of the water: CH₄ + O₂ → CO₂ + 2H₂O. The right now has 4 O.
- Fix oxygen last with a 2 in front of O₂: CH₄ + 2O₂ → CO₂ + 2H₂O
- Check: 1 C, 4 H and 4 O on each side.
9 atoms on each side (1 C, 4 H and 4 O)
Common mistakes
- Changing a subscript, such as turning H₂O into H₂O₂. That makes a different substance.
- Forgetting that the number in front multiplies every atom in the formula.
- Saying mass is conserved without checking the atoms. OCR notes that many learners know mass stays the same but forget that the atoms do too.
Balance one equation after another in Balance It. The hydrogen made in the second example is the gas that gives a pop in the test for hydrogen, and the chemistry hub lists the rest of this set.
Questions people ask
How do I know I have finished?
Every element has the same count on both sides.
Do I have to include state symbols?
Include them when the question asks. AQA, Edexcel and OCR all list state symbols as part of writing balanced equations.
How is a word equation different from a symbol equation?
A word equation uses names, such as magnesium + hydrochloric acid → magnesium chloride + hydrogen. A symbol equation uses formulae. Only the symbol equation shows atoms, so that is the one you balance.