Spectator ion

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A spectator ion is an ion that exists as a reactant and a product in a chemical equation. Spectator ions can be observed in the reaction of aqueous solutions of sodium chloride and copper (II) sulfate:

2Na⁺ + 2Cl^- + Cu²⁺ + SO₄^2- → 2Na⁺ + SO₄^2- + CuCl_{2 (solid)}

The Na⁺ and SO₄^2- ions are spectator ions since they remain unchanged on both sides of the equation. They simply "watch" the other ions react, hence the name. In reaction stoichiometry, spectator ions are removed from a complete ionic equation to form a net ionic equation. Common spectator ions include Na⁺, K⁺, and NO₃^-.

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