Key Takeaways
- Balancing means making the atom count equal on both sides. Atoms cannot be created or destroyed (conservation of mass).
- Only change the big numbers (coefficients). Never change the small subscript numbers. That would change the substance.
- Balance one element at a time. Start with the element that appears in the fewest formulas. Leave O and H until last.
- Always double-check. Count every atom on both sides when you think you are finished.
In This Article
Why do equations need balancing?
In any chemical reaction, atoms are rearranged but never created or destroyed. This is the law of conservation of mass. The total mass of the products always equals the total mass of the reactants.
An unbalanced equation breaks this law. For example:
H2 + O2 → H2O (unbalanced)
On the left there are 2 oxygen atoms (O2), but on the right there is only 1 (H2O). An oxygen atom has vanished, which is impossible. Balancing the equation fixes this:
2H2 + O2 → 2H2O (balanced)
Now there are 4 H atoms and 2 O atoms on each side. Mass is conserved.
The step-by-step method
How to balance any equation
- Write the unbalanced equation. Make sure all the chemical formulas are correct first.
- Count the atoms of each element on both sides. Make a tally.
- Start with the element that appears in the fewest formulas. Put a coefficient in front of the formula that needs adjusting.
- Leave H and O until last. They usually appear in multiple compounds, so they are easier to balance once everything else is done.
- If you get a fraction (like 2.5 O2), multiply all coefficients by 2 to remove it.
- Double-check. Count every atom on both sides. If they match, the equation is balanced.
You can only change the coefficients (the big numbers in front). Never change the subscripts (the small numbers inside the formula). Changing H2O to H2O2 does not balance the equation: it changes the substance from water to hydrogen peroxide.
5 Worked examples
Example 1: Magnesium and oxygen
Unbalanced: Mg + O2 → MgO
2Mg + O2 → 2MgO
Example 2: Iron + chlorine
Unbalanced: Fe + Cl2 → FeCl3
2Fe + 3Cl2 → 2FeCl3
Example 3: Combustion of ethane
Unbalanced: C2H6 + O2 → CO2 + H2O
2C2H6 + 7O2 → 4CO2 + 6H2O
Example 4: Neutralisation (acid + alkali)
Unbalanced: NaOH + H2SO4 → Na2SO4 + H2O
2NaOH + H2SO4 → Na2SO4 + 2H2O
Example 5: Thermal decomposition of calcium carbonate
Unbalanced: CaCO3 → CaO + CO2
CaCO3 → CaO + CO2 (already balanced)
Not every equation needs adjusting. Always check first before adding coefficients.
Want to practise? Our Balancing Act tool gives you random equations to balance with instant feedback.
Common mistakes
Mistakes that lose marks
- Changing the subscripts. Never do this. 2H2O is fine (2 molecules of water). H2O2 is hydrogen peroxide, a completely different substance.
- Only balancing some elements. Always do a final check of every element before writing your answer.
- Forgetting to balance oxygen last. If you balance O first, you often have to redo it when other elements change. Save it for last.
- Writing "1" as a coefficient. If there is only 1 molecule, you do not write a coefficient at all. Write Fe, not 1Fe.
- Not showing state symbols when asked. If the question says "include state symbols," add (s), (l), (g), or (aq) after each formula.
Tips for tricky equations
- Polyatomic ions: if a group like SO4 or NO3 appears unchanged on both sides, treat it as a single unit. This makes counting easier.
- Combustion: always balance C first, then H, then O last. This order works for every combustion equation.
- Fractions: if you get a half-coefficient on O2 (like 3.5 O2), multiply every coefficient in the equation by 2. Problem solved.
- Practice builds speed. The more equations you balance, the faster you get. Use our Balancing Act tool for unlimited practice with instant checks.
For more on equations and quantitative chemistry, see our AQA Topic 3: Quantitative Chemistry notes.
Frequently Asked Questions
What does it mean to balance a chemical equation?
It means adjusting the coefficients (the big numbers in front of formulas) so that there are the same number of atoms of each element on both sides. This reflects the law of conservation of mass: no atoms are created or destroyed.
Why can I not change the small (subscript) numbers?
The subscript numbers define what the substance is. Changing H2O to H2O2 changes water into hydrogen peroxide. You must only change the coefficients, which control how many molecules there are, not what the molecules are.
What should I balance first?
Start with the element that appears in the fewest formulas (often a metal or carbon). Leave hydrogen and oxygen until last because they appear in many compounds and are easier to fix at the end. For combustion, always go in the order: C, then H, then O.
What if I get a fraction?
Multiply all coefficients by the denominator to remove the fraction. If you have 2.5 O2, multiply everything by 2. If you have 1.5 O2, multiply everything by 2. The goal is whole-number coefficients throughout.
Practise balancing equations
Random equations with instant feedback. Build speed and confidence before the exam.