Hydrochloric acid

Hydrochloric acid was the second of the great mineral acids, synthesized by alchemists who heated common salt with sulfuric acid. Jabir ibn Hayyan described the process in the 8th century; European alchemists called the resulting fuming liquid "spirit of salt" or "muriatic acid" from the Latin muria (brine). It became essential to chemical industry.

The synthesis required sulfuric acid—the first mineral acid to be discovered. Heating salt (sodium chloride) with sulfuric acid released hydrogen chloride gas, which dissolved in water to form hydrochloric acid. The reaction demonstrated how one acid could be used to produce another, a principle fundamental to industrial chemistry.

Alchemical uses focused on dissolving metals. Hydrochloric acid attacks most metals, producing hydrogen gas and metal chlorides. Combined with nitric acid, it forms aqua regia ("royal water"), capable of dissolving gold—the only liquid mixture that can attack the noble metal.

Industrial applications multiplied in the 19th century. The Leblanc process for producing sodium carbonate generated hydrochloric acid as a byproduct, initially released into the atmosphere as pollution until regulations forced capture and sale. Steel pickling—removing oxide scale from metal surfaces—consumed vast quantities. PVC plastic production, gelatin manufacture, and food processing all required hydrochloric acid.

The human body produces hydrochloric acid naturally. Stomach acid, at pH 1-2, kills bacteria and activates digestive enzymes. What alchemists synthesized in laboratories was the same compound our cells produce for digestion.

From alchemical curiosity to industrial commodity, hydrochloric acid followed the typical trajectory of chemical substances: discovered through manipulation of other known materials, studied for its reactive properties, then scaled to industrial production when demand emerged. The sulfuric acid that enabled its synthesis remained the more important chemical; hydrochloric acid was its first major derivative.