Spectator Ion
An ion present in a reaction mixture that undergoes no chemical change and appears identically on both sides of the equation.
Unit
Dimensionless
In depth
Spectator ions remain dissolved and dissociated throughout, contributing to ionic strength but taking no part in the chemistry. Cancelling them from the complete ionic equation produces the net ionic equation, which shows the actual reaction stripped of irrelevant detail.
Examples in the real world
Na⁺ and NO₃⁻ when silver nitrate solution is mixed with sodium chloride solution to precipitate AgCl.