In many reactions in solution, some ions are spectators: they sit in the mixture but are the same before and after. An ionic equation leaves them out and shows only the ions that change. This lesson builds on state symbols and balanced equations, because you need both to decide what to split.
Your syllabus decides whether this is Core or Supplement content. Check the current Cambridge 0620 page. This lesson belongs to the formulas and equations module, and the scenarios are original paper exercises.
Which substances are split into ions?
Split a substance into ions only if it is (aq) and ionic, for example a dissolved salt. Keep solids (s), liquids (l), gases (g) and water as whole formulas.
How to write an ionic equation, step by step
- Write the balanced full equation with state symbols.
- Split every (aq) ionic substance into its ions, keeping coefficients. For Na₂SO₄, 1 formula unit gives 2Na⁺ and 1 SO₄²⁻.
- Cancel ions that appear unchanged on both sides. These are the spectators.
- Write what remains as the ionic equation.
- Check that atoms and charges balance on both sides.
Worked example
Barium chloride solution is mixed with sodium sulfate solution. A white solid, barium sulfate, forms. (Illustrative scenario for writing practice.)
Step 1, full equation: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
Check atoms: Ba 1, Cl 2, Na 2, S 1, O 4 on both sides.
Step 2, split the (aq) substances: Ba²⁺(aq) + 2Cl⁻(aq) + 2Na⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) + 2Na⁺(aq) + 2Cl⁻(aq)
Step 3, cancel: Na⁺ and Cl⁻ appear unchanged on both sides. They are the spectator ions.
Step 4, ionic equation:
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Step 5, check: charge on the left is 2+ and 2−, which is 0. The right side is neutral, so 0. Atoms: Ba 1, S 1, O 4 on both sides.
The mistake to watch for
A common slip is to split the precipitate into ions.
Mistaken answer: Ba²⁺(aq) + SO₄²⁻(aq) → Ba²⁺(s) + SO₄²⁻(s)
The student treated the solid as separate ions, so nothing is left to show the reaction happening.
The solid stays as one formula. Only the dissolved ions are split, and the precipitate BaSO₄(s) stays together. The correct answer is Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Check yourself
1. Write the ionic equation for NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l), and name the spectator ions.
Show answer
Split the (aq) substances: Na⁺ + OH⁻ + H⁺ + Cl⁻ → Na⁺ + Cl⁻ + H₂O(l). Na⁺ and Cl⁻ cancel.
H⁺(aq) + OH⁻(aq) → H₂O(l). Spectators: Na⁺ and Cl⁻.
2. Write the ionic equation for Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s), and name the spectator ion.
Show answer
Split the (aq) substances: Mg(s) + Cu²⁺ + SO₄²⁻ → Mg²⁺ + SO₄²⁻ + Cu(s). SO₄²⁻ cancels.
Mg(s) + Cu²⁺(aq) → Mg²⁺(aq) + Cu(s). Charge: 2+ on both sides. Spectator: SO₄²⁻.
3. Write the ionic equation for Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq), and name the spectator ions.
Show answer
Split: Pb²⁺ + 2NO₃⁻ + 2K⁺ + 2I⁻ → PbI₂(s) + 2K⁺ + 2NO₃⁻. K⁺ and NO₃⁻ cancel.
Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s). Charge: 2+ and 2− gives 0, matching PbI₂. Spectators: K⁺ and NO₃⁻.
Where this leads next
Bring the five lessons together in the formulas and equations practice set. The mole and equation-ratio tutor shows how balanced ratios carry into calculations.
If cancelling ions feels uncertain, a teacher in online one-to-one Chemistry tuition can go through the splitting rules with your own examples.