When iron reacts with acid, it undergoes a chemical change. The iron atoms in the metal react with the hydrogen ions in the acid to form hydrogen gas (H2) and iron ions. This is a chemical reaction because the starting materials (iron and acid) are transformed into new substances (hydrogen gas and iron ions). The chemical equation for this reaction is:

Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)

In this equation, Fe(s) represents solid iron, HCl(aq) represents hydrochloric acid, FeCl2(aq) represents iron(II) chloride, and H2(g) represents hydrogen gas.