Mg\(_{(s)}\) + 2HCl\(_{(aq)}\) → MgCl\(_2\)\(_{(aq)}\) + H\(_2\)\(_{(g)}\) In the reaction above, magnesium is a reducing agent because

A reducing agent is a substance that causes another substance to be reduced by donating electrons to it. In the process of donating electrons, the reducing agent itself is oxidized (its oxidation state increases).

In the reaction:

\[\text{Mg}_{(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{MgCl}_{2(aq)} + \text{H}_{2(g)}\]

Magnesium changes from an oxidation state of 0 (as the free element) to +2 (in MgCl₂). It loses two electrons:

\[\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^-\]

These electrons are gained by hydrogen ions (H⁺), which are reduced from +1 to 0 (as H₂ gas). Magnesium is the reducing agent because it supplies the electrons that reduce the hydrogen ions.

The defining characteristic of a reducing agent is that it is oxidized. Being a metal or being a solid are physical properties that do not explain why magnesium acts as a reducing agent in this reaction.