Final answer:
In the reaction Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g), the species that is reduced is H⁺ in HCl. It's evidenced by the hydrogen's oxidation state changing from +1 to 0, indicating a gain of electrons, which constitutes reduction.
Step-by-step explanation:
In the reaction Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g), the species that is reduced is H⁺ in HCl. This can be determined by looking at the changes in oxidation states of the elements involved in the reaction. Initially, hydrogen in HCl has an oxidation state of +1 and it becomes 0 in H₂ gas, showing a reduction in the oxidation state, hence the reduction process. Magnesium (Mg) starts with an oxidation state of 0 and ends with an oxidation state of +2 when it becomes MgCl₂, indicating that Mg is oxidized.
In a reduction-oxidation reaction or redox, the species that gives electrons is oxidized, while the species that gains electrons is reduced. Thus, H⁺ from HCl gains electrons and is reduced, and Mg loses electrons and is oxidized making it the reducing agent. Consequently, HCl which gets converted to H₂ acts as the oxidizing agent.