Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digestive systems of most animal species, including humans. Hydrochloric acid is an important laboratory reagent and industrial chemical. History In the early tenth century, the Persian physician and alchemist Abu Bakr al-Razi ( 865–925, Latin: Rhazes) conducted experiments with sal ammoniac (ammonium chloride) and vitriol (hydrated sulfates of various metals), which he distilled together, thus producing the gas hydrogen chloride. In doing so, al-Razi may have stumbled upon a primitive method for producing hydrochloric acid, as perhaps manifested in the following recipe from his ("The Book of Secrets"): However, it appears that in most of his experiments al-Razi disregarded the gaseous products, concentrating instead on the color changes that could be effected in the residue. According to Robert P. Multhauf, hydrogen chloride was produced many times without clear recognition that by dissolving it in water hydrochloric acid may be produced. Drawing on al-Razi's experiments, the ("On Alums and Salts"), an eleventh- or twelfth century Arabic text falsely attributed to al-Razi and translated into Latin by Gerard of Cremona (1144-1187), described the heating of metals with various salts, which in the case of mercury resulted in the production of mercury(II) chloride (corrosive sublimate). In this process, hydrochloric acid actually started to form, but it immediately reacted with the mercury to produce corrosive sublimate. Thirteenth-century Latin alchemists, for whom the De aluminibus et salibus was one of the main reference works, were fascinated by the chlorinating properties of corrosive sublimate, and they soon discovered that when the metals are eliminated from the process of heating vitriols, alums, and salts, strong mineral acids can directly be distilled. One important invention that resulted from the discovery of the mineral acids is aqua regia, a mixture of nitric acid and hydrochloric acid in a 1:3 proportion, capable of dissolving gold. This was first described in pseudo-Geber's De inventione veritatis ("On the Discovery of Truth", after c. 1300), where aqua regia was prepared by adding ammonium chloride to nitric acid. However, the production of hydrochloric acid itself (i.e., as an isolated substance rather than as already mixed with nitric acid) depended on the use of more efficient cooling apparatus, which would only develop in subsequent