spectator ion

Meaning: A soluble ionic compound dissolves into aqueous ions. In a reaction, some ions combine into a precipitate, water, or a gas, while others stay dissolved and unchanged. Those unchanged ions are spectators. Remove them from the complete ionic equation and the net ionic equation remains. Example: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq). Na⁺ and NO₃⁻ are unchanged, so the net equation is Ag⁺(aq) + Cl⁻(aq) → AgCl(s).

On the MCAT: You may be asked to write a net ionic equation, use solubility rules to predict which ions form a precipitate, or name the spectators. In a strong acid–strong base titration, the base's cation and the acid's anion are spectators, and the net reaction is H⁺ + OH⁻ → H₂O. In a galvanic cell, salt bridge ions that join neither half-reaction act as spectators while keeping charge balanced.

Common confusion: A spectator is defined by the specific reaction, not by the ion itself. Na⁺ is a spectator when NaCl meets AgNO₃, but a different reaction could involve it. Spectators still carry current and balance charge, so they are not absent from solution. Conjugate bases of weak acids, such as CH₃COO⁻, can react with H⁺ or water, so they often do not count as spectators.