Balancing Chemical Equations
Every chemical equation is a statement of balance: the atoms you start with are exactly the atoms you end with, just rearranged. Learn the method, work through examples, then choose one of four exams to check you've got it.
What is a chemical equation?
A chemical equation describes a reaction using the formulae of the substances involved. Reactants (what you start with) are written on the left; products (what you end up with) are written on the right, joined by an arrow.
Word equation: methane + oxygen → carbon dioxide + water
The small numbers in front of a formula (like the 2 before O2) are coefficients. They show how many particles of that substance react or form.
The law of conservation of mass
Atoms are never created or destroyed in a chemical reaction — they are only rearranged into new substances. This means the total mass of the reactants must equal the total mass of the products, and every atom present at the start must still be present at the end.
Why an unbalanced equation is a problem
Compare these two versions of the same reaction — magnesium burning in oxygen.
Unbalanced (incorrect)
Balanced (correct)
Change coefficients, never subscripts
It can be tempting to "balance" oxygen by changing H2O to H2O2. Don't. Changing a subscript changes what the substance is — hydrogen peroxide is a completely different compound from water. Only the big numbers in front of a formula (the coefficients) may be changed.
Elements that travel in pairs
Seven elements exist as two-atom (diatomic) molecules in their pure form, not as single atoms. Forgetting this is one of the most common counting errors in balancing.
A common way to remember them is the phrase "Have No Fear Of Ice Cold Beer" — the first letter of each word gives H, N, F, O, I, Cl, Br.
State symbols
A fully correct IGCSE equation often needs one more thing: a state symbol after every formula, showing what physical state that substance is in during the reaction. Leaving them out can cost marks even when the balancing itself is right.
| Symbol | Meaning |
|---|---|
| (s) | solid |
| (l) | liquid |
| (g) | gas |
| (aq) | aqueous — dissolved in water |
State symbols describe the substance, not its formula — they are never counted as atoms and never affect balancing.
Balancing a real equation together
Watch the scale as each step is applied to Fe + O2 → Fe2O3. It tips toward whichever side has more atoms, and settles flat only once the equation is fully balanced.
Two more fully worked examples
Most common equations you'll meet at IGCSE
These twelve reactions cover the reaction types that come up again and again on IGCSE papers. Each is already balanced — use them as a quick reference.
Common mistakes to avoid
- Changing a subscript instead of a coefficient (see rule 4 above).
- Forgetting that H2, N2, O2, F2, Cl2, Br2 and I2 exist as pairs of atoms.
- Stopping as soon as one element balances, without re-checking the others.
- Leaving coefficients that could still be simplified — e.g. writing 4H2 + 2O2 → 4H2O instead of the simplest ratio, 2H2 + O2 → 2H2O.
- Balancing a coefficient of 1 by writing "1" in front of a formula — it's correct chemistry, but IGCSE convention is to leave it blank.
- Leaving out state symbols, or using (aq) and (l) inconsistently — a dissolved substance is (aq), not (l).
Quick check before the exam
Fill in the missing coefficients. Leave a box blank if the coefficient is 1.