When zinc in its metallic form and hydrochloric acid are combined, they form hydrogen gas and an aqueous solution of zinc chloride. This type of a reaction is called an oxidation reduction reaction.
The chemical equation for this reaction is written: Zn(s) + 2HCl(aq) ? H2(g) + ZnCl2(aq).
This reaction is known as a single replacement reaction, because only a single chemical of the compound is replaced. The hydrogen is replaced in its combination with the zinc.
The reaction is called an oxidation reaction because one element loses electrons and another gains electrons. The process of gaining an electron is known as reduction. The process of losing an electron is known as oxidation. In the reaction between zinc and hydrochloric acid, the zinc atoms are oxidized and lose two electrons. Since the zinc is oxidized and the reaction needs to remain balanced, the hydrogen atoms are reduced.
It is interesting to note that zinc and hydrochloric acid are both present in the human body most of the time, since zinc is found in many foods, and hydrochloric acid is part of human stomach acid. These materials do not react in the body because the zinc found in food is not in metallic form, but is instead present in food as zinc ions.