A Level Chemistry OCR Practice Exam 2026 - Free Chemistry Practice Questions and Study Guide

Spectator ions are ions that are present in a solution during a chemical reaction but do not participate in the actual reaction itself. Their role is simply to maintain the charge balance and neutrality of the solution, essentially "watching" the reaction without being involved in it. This means that while they are present in the same phase as the reacting species and might appear in the overall balanced equation, they do not undergo any change in composition or concentration as a result of the reaction.

In the context of precipitation reactions or acid-base reactions, spectator ions are often the ions of soluble salts or the counterions that remain in solution unchanged despite the formation of precipitates or the transfer of protons in acid-base interactions. This property distinguishes them from the ions directly involved in forming products or changing their oxidation states, allowing for a clearer understanding of the reaction mechanisms at play.