Balancing equations · Year 10
A chemical equation shows what reacts and what is made. A word equation uses names; a symbol equation uses formulae. Atoms are only rearranged in a reaction, never made or destroyed, so a symbol equation must have the same number of each kind of atom on both sides. You balance it by putting numbers in front of the formulae, never by changing the formulae themselves.
Key facts
- Reactants are on the left of the arrow; products are on the right: magnesium + oxygen → magnesium oxide.
- A balanced equation has the same number of atoms of each element on both sides. This is why mass is conserved.
- Balance by changing the big numbers in front (2H2O), never the small numbers in a formula: H2O2 would be a different substance.
- A number in front multiplies the whole formula: 2H2O has 4 hydrogen atoms and 2 oxygen atoms.
- A good method: count each element on both sides, balance one element at a time (leave elements that appear on their own, like O2, until last), then check every element again.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
- An ionic equation shows only the ions that change; spectator ions are left out. Every neutralisation is H⁺(aq) + OH⁻(aq) → H2O(l).