Definition: A spectator ion is an ion that exists in the same form on both the reactant and product sides of a chemical reaction.
Examples:Consider the reaction between sodium chloride (NaCl) and copper sulfate (CuSO4) in aqueous solution.
2 NaCl (aq) + CuSO4 (aq) → 2 Na+ (aq) + SO42- (aq) + CuCl2 (s)
The ionic form of this reaction is: 2 Na+ (aq) + 2 Cl- (aq) + Cu2+ (aq) + SO42- (aq) → 2 Na+ (aq) + SO42- (aq) + CuCl2 (s)
The sodium ions and sulfate ion are the spectator ions in this reaction. They appear unchanged in both the product and reactant side of the equation. These ions just 'spectate' while the other ions form the copper chloride.