An ionic equation shows only the particles that change. Write the full equation, split the aqueous ionic compounds, cancel the spectators, then check atoms and charges.
This lesson is part of SPM Chemistry acids, bases and salts. It follows writing neutralisation equations.
What are the four steps?
Work in this order every time.
- Write the balanced full equation with state symbols.
- Rewrite each (aq) ionic compound as its ions.
- Cross out ions that are identical on both sides.
- Add up atoms and charges on each side to check.
Worked example 1: a precipitation
Lead(II) nitrate solution reacts with potassium iodide solution.
Full equation: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Split the aqueous compounds: Pb²⁺ + 2NO₃⁻ + 2K⁺ + 2I⁻ → PbI₂(s) + 2K⁺ + 2NO₃⁻
K⁺ and NO₃⁻ appear on both sides, so they are spectators. The ionic equation is Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s).
Check: charge on the left is (+2) + 2(−1) = 0, and the right is 0. Atoms are also balanced.
Worked example 2: a metal with acid
Magnesium ribbon reacts with dilute hydrochloric acid. Full equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Split HCl and MgCl₂ into ions: Mg + 2H⁺ + 2Cl⁻ → Mg²⁺ + 2Cl⁻ + H₂. The chloride ions cancel.
The ionic equation is Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g). Both sides have a total charge of +2.
The mistake that loses marks
A student writes Pb²⁺ + I⁻ → PbI₂. Count the iodine: one on the left, two on the right. The charge is also wrong, +1 on the left and 0 on the right.
The fix is the coefficient 2 in front of I⁻, which repairs both counts together. A charge check catches this when an atom check alone may not.
Check yourself
Write the ionic equation for barium chloride solution reacting with sodium sulfate solution to give a white precipitate.
Answer
Full equation: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq).
Split: Ba²⁺ + 2Cl⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄(s) + 2Na⁺ + 2Cl⁻. Cancel Na⁺ and Cl⁻.
Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The charge is 0 on both sides.
What to study next
Put the skill to work in interpreting qualitative analysis observations, where each precipitate has an ionic equation. You can also review planning salt preparation.
The mole and stoichiometry steps tool checks quantities once the equation is right. To have a teacher read your ionic equations with you, see online one-to-one Chemistry tuition.