Hydrochloric acid (HCl) is a solution of hydrogen chloride gas dissolved in water. A given volume of water can dissolve up to 1,000 times its volume of hydrogen chloride gas. Hydrochloric acid, one of the strongest of all acids, is colorless and very corrosive. It is a clear liquid that may be colored yellow by trace amounts of iron, chlorine or organic matter that occurs in most water sources. It is extremely caustic and can be dangerous. On contact, it causes severe burns that should be flushed immediately with water. In air, the acid gives off strong fumes that irritate the throat and lungs if inhaled. Because it does not contain carbon, hydrochloric acid is categorized as an inorganic acid. It is often referred to by its traditional name, muriatic acid.
Centuries after hydrochloric acid was first prepared, scientists have still not completely explained how the delicate lining of the human stomach protects itself against this corrosive liquid. Hydrochloric acid is involved in digesting our food, and having the right amount of acid in our stomachs is vital to our health. Too much hydrochloric acid can upset the stomach and cause heartburn or even ulcers, while too little acid leads to indigestion.
During the Middle Ages, alchemists began formulating new acids and using them to decompose metals and other substances. Hydrochloric acid was probably discovered during the fifteenth century although the actual date and the identity of the discoverer are unknown. During the late 1500s and early 1600s a German alchemist and physician named Andreas Libavius (1540?-1616) published a comprehensive study of alchemy in which he gave a clear description of how to prepare hydrochloric acid and other strong acids. Around 1625 German chemist Johann Rudolf Glauber (1604-1668) discovered a much more convenient method of manufacturing hydrochloric acid. In one of his earliest experiments, he found that common salt ( sodium chloride) would produce hydrochloric acid when dissolved in strong sulfuric acid. He also discovered that the other product of this reaction, sodium sulfate, is a gentle laxative. Known even today as "Glauber's salt," he called it sal mirabile (wonderful salt) and sold it as a cure-all.
For more than a century, chemists remained ignorant of hydrochloric acid's formula. During the late 1700s, Antoine-Laurent Lavoisier observed that acids are formed when oxygen compounds are dissolved in water, which led him to suggest that all acids contain oxygen. This theory was disproved in the early 1800s when chemists began using electrolysis to break down compounds into elements. In this way, Humphry Davy demonstrated in 1810 that hydrochloric acid does not contain oxygen. Instead, when decomposed, it produces hydrogen and a greenish-colored gas that Davy named chlorine. A few years later, in 1824, William Prout discovered small quantities of hydrochloric acid in stomach secretions. This was the first indication that the acid is involved in the human digestive process.
The acid is produced through the combustion of hydrogen in chlorine or by combining common salt and sulfuric acid in a process similar to that used by Glauber. During the nineteenth and twentieth centuries, hydrochloric acid became an important industrial chemical, which is still used in great quantities today. For example, steel is cleaned in a bath of hydrochloric acid before being galvanized (coated with zinc). The acid is used extensively in metallurgy and food processing and in the manufacture of synthetic rubber and many chemical compounds. It is predicted that use of hydrochloric acid will reach 1.5 million tons by the year 2001.
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