Balancing Chemical Equations

This lesson covers how to balance chemical equations as required by the AQA GCSE Combined Science Trilogy specification (8464). You will learn why equations must be balanced, the rules for balancing, and how to tackle equations of increasing difficulty.

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Why Must Equations Be Balanced?

Because of the law of conservation of mass: no atoms are created or destroyed in a chemical reaction. A balanced equation has the same number of each type of atom on both sides of the arrow.

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Rules for Balancing

1. Never change the formulae of the substances — only adjust the coefficients (the large numbers in front of formulae).
2. Start with the element that appears in the fewest formulae.
3. Balance one element at a time.
4. Leave hydrogen and oxygen until last (they often appear in multiple compounds).
5. Check your answer by counting atoms on both sides.