Final answer:
In the reaction mg²⁺(aq) + cu(s) → cu²⁺(aq) + mg(s), mg²⁺(aq) is identified as a reactant. It is involved in a single-replacement or metal replacement reaction, where magnesium replaces copper.
Step-by-step explanation:
In the reaction mg²⁺(aq) + cu(s) → cu²⁺(aq) + mg(s), mg²⁺(aq) is a reactant. This reaction is a type of single-replacement reaction specifically known as a metal replacement reaction, where magnesium is replacing copper. Related information from chemical equations such as Mg(s) + Cu(NO3)2 (aq) → Mg(NO3)2 (aq) + Cu(s) and Cu(s) + 2 Ag+ (aq) → Cu²⁺ (aq) + 2 Ag(s), demonstrate where metals switch places. In these reactions, one metal is oxidized while the other is reduced, resulting in the formation of new ionic compounds.