Ionic equations appear across acid–base chemistry, precipitation and redox. Understanding them is essential for O-Level Paper 2 structured questions and data-based questions.
A net ionic equation is written by splitting soluble ionic species into their ions and cancelling the spectator ions, which take no part in the reaction. These notes cover the step-by-step method, the solubility rules for predicting precipitates, and the standard net ionic equations for precipitation, neutralisation, acids with carbonates and metals, and metal displacement.
What these notes cover
- •Writing ionic equations: step-by-step
- •Spectator ions
- •Solubility rules
- •Precipitation
- •Acid–base neutralisation
- •Acid + carbonate
- •Acid + metal
- •Metal displacement
- •Worked examples
- •Common mistakes
- •Summary
Key definitions
- Spectator ion
- An ion that takes no part in the reaction. It appears unchanged on both sides of the full ionic equation, is cancelled, and does not appear in the final net ionic equation.
- Full ionic equation
- The balanced equation with every ionic compound or strong acid in (aq) state split into its separate ions. Solids, liquids, gases, weak acids and insoluble compounds are left intact.
- Net ionic equation
- The final equation left after the spectator ions have been cancelled from the full ionic equation.
- Precipitate
- An insoluble compound formed when solutions are mixed. It is always written as (s) and must never be split into ions.
- Weak acid
- An acid such as CH₃COOH(aq) that is only partially ionised in water. It stays intact and is not split in an ionic equation.
Key formulas
Precipitation (silver chloride)
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
AgCl is a white precipitate. The precipitate is always written as (s) and is never split.
Neutralisation
H⁺(aq) + OH⁻(aq) → H₂O(l)
Applies to any strong acid reacting with any strong base; the net equation is the same whichever strong acid and strong base are used.
Acid + carbonate
2H⁺(aq) + CO₃²⁻(aq) → H₂O(l) + CO₂(g)
Carbonate ion carries a 2− charge, so two H⁺ are needed. H₂O(l) and CO₂(g) are not split.
Acid + metal
Zn(s) + 2H⁺(aq) → Zn²⁺(aq) + H₂(g)
Shown for a divalent metal such as Zn. Adjust the charge and coefficient to match the metal’s valency.
Metal displacement
Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s)
Fe displaces Cu from solution. The solid metals Fe and Cu are not split.
Worked example
Write the ionic equation for mixing silver nitrate solution with sodium chloride solution.
- 1Write the balanced molecular equation with state symbols: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
- 2Split all (aq) ionic species to give the full ionic equation: Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq).
- 3Cancel the spectator ions Na⁺ and NO₃⁻, which appear unchanged on both sides: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
- 4Check the charges: (1+) + (1−) = 0 on the left and AgCl is neutral on the right, so the equation balances; AgCl is a white precipitate.
Watch out: AgCl(s) is a precipitate, so it stays as AgCl(s) — never split a solid into ions.
Common mistakes
Splitting a precipitate
Wrong: Splitting BaSO₄(s) into its ions.
Right: BaSO₄(s) must stay as BaSO₄(s) — never split a solid.
Splitting a weak acid
Wrong: Splitting CH₃COOH(aq) into ions.
Right: CH₃COOH(aq) stays intact because it is only partially ionised in water.
Wrong ionic charges
Wrong: Writing SO₄⁻ for sulphate or CO₃⁻ for carbonate.
Right: Sulphate is SO₄²⁻ and carbonate is CO₃²⁻.
Questions students ask
How do you write an ionic equation step by step?
Write the balanced molecular equation with correct state symbols, then split every ionic compound or strong acid in (aq) state into its separate ions. Do not split solids, liquids, gases, weak acids or insoluble compounds. Finally, cancel the spectator ions that appear identically on both sides.
What is a spectator ion in chemistry?
A spectator ion takes no part in the reaction and appears unchanged on both sides of the full ionic equation. It is crossed out, so it does not appear in the final net ionic equation.
What is the ionic equation for neutralisation of a strong acid and strong base?
Any strong acid reacting with any strong base gives H⁺(aq) + OH⁻(aq) → H₂O(l). For sulphuric acid and potassium hydroxide, 2K⁺ and SO₄²⁻ are cancelled and the equation is divided by 2 to reach the same result.
What is the ionic equation for iron and copper(II) sulphate?
Iron displaces copper: Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s), after the spectator SO₄²⁻ ions are cancelled. The blue colour fades and a red-brown solid (copper) deposits on the iron.
In the full PDF
- •Full solubility rules table for nitrates, chlorides, sulphates, carbonates, hydroxides and sodium, potassium and ammonium salts
- •Worked example 2: sulphuric acid with potassium hydroxide, with the full ionic equation and dividing by 2
- •Worked example 3: iron with copper(II) sulphate, with full ionic equation and observations
- •Three further common mistakes: missing state symbols, charge imbalance and splitting a gas
- •Five-point summary with a final check for atoms, state symbols and charges