A spectator ion is an ion that is present in a reaction mixture but does not change during the reaction, and so is left out of the ionic equation.
Malay: Ion penonton · Chinese: 旁观离子
A spectator ion is an ion that is present in the reaction mixture but does not change during the reaction, so it is left out of the ionic equation. It is in the beaker and it stays dissolved, unchanged, from start to finish, it watches rather than takes part. Recognising spectator ions is the step that turns a full chemical equation into a clean ionic equation.
A worked example shows the cut. In the neutralisation NaOH + HCl → NaCl + H₂O, write every dissolved substance as ions: Na⁺ + OH⁻ + H⁺ + Cl⁻ → Na⁺ + Cl⁻ + H₂O. The sodium ion, Na⁺, and the chloride ion, Cl⁻, appear unchanged on both sides, so they are spectators and are cancelled, leaving the ionic equation H⁺ + OH⁻ → H₂O. In the precipitation of silver chloride, Ag⁺ + Cl⁻ → AgCl, the nitrate and sodium ions are the spectators left behind in solution.
The confusion to avoid is deleting an ion that actually reacts, or keeping one that does not. Only cross out ions that are identical and dissolved on both sides; an ion that becomes part of a precipitate, a gas or water is a reactant, not a spectator. A second slip is forgetting that spectator ions must still balance in the full equation, they are omitted from the ionic equation only, not ignored in the chemistry.
Within this chapter spectator ions are the key to writing the ionic equations for neutralisation and precipitation, connecting the ideas of ions, salts and reactions. Our teachers have you write the full equation first, then strike out the spectators, so the ionic equation is a result you can justify.
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