Spectator ion

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A spectator ion is an ion that is present in a solution where a chemical reaction happens, but is not part of the reaction itself. They do not change their formula or precipitate out of the solution.

Example

Potassium iodide and silver nitrate both dissolve in water. One solution contains the ions K+
and I−
, and the other contains Ag+
and NO−
3. Mixing the solutions causes a precipitate to form, with the total ionic equation:

K+
(aq) + I−
(aq) + Ag+
(aq) + NO−
3 (aq) → AgI (s) + K+
(aq) + NO−
3 (aq)

The insoluble silver iodide comes out of solution, but the potassium and nitrate ions are still dissolved. In this reaction, K+
and NO−
3 are spectators, because they are in the same state they were before the reaction.