Ionic equations and how to write them

An ionic equation shows only the ions and substances that actually change in a reaction, leaving out the spectator ions that stay the same on both sides. The best known is the ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l). Whichever acid and alkali you use, this is the reaction that really happens. In the US it is called a net ionic equation.

What is an ionic equation?

Many substances in a reaction are ionic compounds dissolved in water. When sodium chloride dissolves, it splits into separate Na⁺ and Cl⁻ ions moving freely in the solution. So a full symbol equation such as

hides what is going on. Written as ions, it becomes:

The Na⁺ and Cl⁻ ions are in solution at the start and still in solution at the end. They take no part, like spectators at a match, so they are called spectator ions. Cross them out and what is left is the ionic equation:

How to write ionic equations in 4 steps

1. Write the balanced symbol equation, with state symbols. The state symbols tell you what splits into ions.
2. Split every aqueous ionic compound into its ions. That means dissolved salts, alkalis and acids. Use the balancing numbers: 2NaCl(aq) becomes 2Na⁺ + 2Cl⁻.
3. Cross out the spectator ions, the ones that appear unchanged on both sides.
4. Write what is left, and check it balances. The atoms must balance, and so must the total charge on each side.

What to split and what to leave alone

You need to know ion charges to split compounds correctly: sulfate is SO₄²⁻, nitrate NO₃⁻, carbonate CO₃²⁻, ammonium NH₄⁺, and hydroxide OH⁻. If ion charges are shaky, this practice from the Chemical Bonding course will help. Build the formula so the charges balance, then press Mark to check it in the course.

Ionic equation examples

Example 1: neutralisation with sulfuric acid

Write the ionic equation for H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l).

Split the aqueous compounds into ions: 2H⁺ + SO₄²⁻ + 2K⁺ + 2OH⁻ → 2K⁺ + SO₄²⁻ + 2H₂O. K⁺ and SO₄²⁻ are spectators. That leaves 2H⁺ + 2OH⁻ → 2H₂O, which simplifies to:

The same equation as for hydrochloric acid and sodium hydroxide. That is the point of an ionic equation: every acid and alkali neutralisation is the same reaction underneath.

Example 2: a precipitation reaction

Silver nitrate solution is added to sodium chloride solution and a white precipitate forms. Write the ionic equation.

AgCl is a solid, so it stays together. Na⁺ and NO₃⁻ are spectators.

This is the test for chloride ions. Any soluble chloride gives the same precipitate with any soluble silver salt, and the ionic equation shows why.

Example 3: a metal hydroxide precipitate

Sodium hydroxide solution is added to iron(II) sulfate solution, and a green precipitate of iron(II) hydroxide forms.

Na⁺ and SO₄²⁻ are spectators, and the precipitate stays together:

Check the charge: 2+ and 2 × (1−) make 0 on the left, and the solid on the right has no overall charge. Balanced.

Example 4: a metal displacement reaction

Zinc is added to copper(II) sulfate solution. Copper forms and the blue colour fades.

Sulfate is the spectator. Zinc and copper are elements, so they stay as atoms:

This is a redox reaction: zinc atoms lose electrons and copper ions gain them. It can be split further into two half equations, Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.

Example 5: an acid and a carbonate

Write the ionic equation for marble chips reacting with hydrochloric acid.

Calcium carbonate is a solid, so it is not split. Only Cl⁻ is a spectator.

Charge check: 2+ on the left, 2+ on the right.

Example 6: halogen displacement

Chlorine water is added to potassium bromide solution, and the solution turns orange as bromine forms.

K⁺ is the spectator. Chlorine is more reactive than bromine, so it takes the electrons from the bromide ions.

Here is a quick check on spotting spectator ions.

Common mistakes

• Splitting a solid, liquid or gas. Only aqueous ionic compounds split. A precipitate stays as one formula.
• Splitting water. Water is covalent and hardly ionised, so it stays as H₂O.
• Wrong ion charges. Sulfate is SO₄²⁻, not SO₄⁻. A wrong charge makes the equation impossible to balance.
• Forgetting the balancing numbers when you split. 2NaOH gives 2Na⁺ and 2OH⁻.
• Leaving spectator ions in. If an ion appears unchanged on both sides, cross it out.
• Not checking the charge. The total charge on each side must be equal, just like the atoms.

Ionic equations at GCSE and A-level

In AQA's GCSE Chemistry specification, writing balanced half equations and ionic equations where appropriate is Higher tier only. The specification also says that in neutralisation between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water, H⁺(aq) + OH⁻(aq) → H₂O(l). At A-level, ionic equations are expected throughout, especially for redox reactions, tests for ions and transition metal chemistry. The Equation Balancer balances symbol and ionic equations and shows the working.

Practise in a short timed paper

GCSE Chemistry Higher and A-level Chemistry: Year 1 each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course. Ionic equations turn up in acids, electrolysis, tests for ions and redox questions.

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Chemistry · Bonding and structureWhat is an ionic bond? Ionic bonding, lattices, propertiesWhat an ionic bond is, how ions form, dot and cross diagrams, working out formulae, giant ionic lattices and the properties of ionic compounds.