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Ionic compounds react when they are dissolved. However, not all the ions react – some spectator ions do not react and remain unchanged. Spectator ions are not shown in an ionic equation.

There are two different types of ionic equations:

• Neutralisation
• Displacement

Neutralisation

When an acid reacts with an alkali, the acid’s hydrogen ions react with the alkali’s hydroxide ions to produce water. The other ions are left unchanged.

For example, when hydrochloric acid (HCl) is mixed with potassium hydroxide (KOH), the hydrogen ions (H⁺) react with the hydroxide ions (OH^-). The chloride (Cl^-) and potassium (K⁺) ions are spectator ions.

H⁺(aq) + OH^-(aq) → H₂O(l)

Displacement

In displacement reactions, the more reactive metal displaces (takes the place of) a less reactive metal from a metal compound.

For example, when magnesium (Mg) reacts with copper(II) sulfate (CuSO₄), the magnesium atoms react with the copper(II) ions in the copper(II) sulfate. The sulfate ions are spectator ions and do not take part in the reaction.

Mg(s) + Cu²⁺(aq) → Cu(s) + Mg²⁺(aq)