Medical College Admission Test (MCAT) General Chemistry Practice Exam 2026 - Free MCAT Chemistry Practice Questions and Study Guide

Spectator ions are ions that do not participate in the actual chemical reaction taking place in a solution; they remain unchanged and are not involved in the formation of products. When a reaction occurs in an aqueous solution, these ions are present but do not affect the outcome or the equilibrium state of the reaction.

For example, in the reaction between sodium chloride (NaCl) and silver nitrate (AgNO3) to form silver chloride (AgCl) and sodium nitrate (NaNO3), the sodium ions (Na+) and nitrate ions (NO3-) are spectator ions. They remain in solution and do not react chemically in the formation of the precipitate, AgCl.

Understanding the role of spectator ions is crucial in chemical equations, particularly in net ionic equations where only the species that undergo a change are included. The incorrect choices highlight different concepts: some refer to ions that influence the equilibrium solution or are involved in the reaction process, which does not apply to spectator ions. Others suggest states of matter that do not pertain directly to the definition of these ions.