HCl

HCl is the chemical formula for hydrogen chloride, a diatomic molecule. In the gas phase at room temperature it is a colorless, highly soluble gas with a sharp, irritating odor. When dissolved in water, it forms hydrochloric acid, a strong mineral acid widely used in industry and laboratories. Hydrogen chloride gas is denser than air and can form white fumes in humid air.

Properties include a molecular weight of 36.46 g/mol, a boiling point of -85.05 °C, and a melting point of -114.2 °C. It is highly soluble in water and, in aqueous solution, dissociates completely into hydronium (H3O+) and chloride (Cl-) ions, giving a strongly acidic solution. It reacts with metals to release hydrogen gas and with bases to form chlorides.

Industrial production of HCl is typically by direct synthesis from hydrogen and chlorine: H2 + Cl2 → 2 HCl. Another common route is the reaction of concentrated sulfuric acid with sodium chloride to generate HCl gas, which is then absorbed in water to form hydrochloric acid. HCl is also produced as a byproduct in various chemical processes and is often handled as concentrated hydrochloric acid or as a gas for absorption into water.

Applications include metal surface treatment (pickling), pH control and neutralization of basic wastes, production of inorganic chlorides and vinyl chloride monomer precursors, and use as a reagent in organic synthesis to convert alcohols to alkyl chlorides. It is also used in electronics cleaning and wafer fabrication for etching and surface preparation.

Safety and handling require attention to its corrosivity: concentrated HCl solutions and gaseous HCl are irritating and corrosive to skin, eyes, and respiratory mucosa. Proper ventilation, containment, and personal protective equipment are essential.