Final answer:
The spectator ions in this reaction are Mg²⁺ and Cl-.
Step-by-step explanation:
In the reaction between Mg(OH)₂(aq) and HCl(aq), the spectator ions are Mg²⁺ and Cl-. The balanced chemical equation is 2 HCl(aq) + Mg(OH)₂(aq) → 2 H₂O(l) + MgCl₂(aq). In this reaction, magnesium hydroxide (Mg(OH)₂) reacts with hydrochloric acid (HCl) to form water (H₂O) and magnesium chloride (MgCl₂), while the Mg²⁺ and Cl- ions remain unchanged.