Hydrochloric acid, abbreviation HCl(aq), is a common acid both in the body and in the lab. It is, for example, a major component of gastric acid (pH 1-2, 0.5% w/v HCl). In experiments, it is used among other things to set the pH in buffers (e.g. Tris) and to reveal antigens (e.g. BrdU). Chemically speaking, it is a solution of the gas hydrogen chloride = HCl(g) in water.