2HCl

2HCl denotes two molecules of hydrogen chloride and is not a distinct chemical compound. In chemical equations and stoichiometry, a coefficient such as 2 indicates two units of the species involved, for example in the reaction 2 HCl + Zn → ZnCl2 + H2.

Hydrogen chloride (HCl) is a diatomic molecule consisting of hydrogen and chlorine. When dissolved in water, it forms hydrochloric acid, a strong acid that dissociates completely in solution. In the gaseous state, HCl is a colorless, corrosive gas with a characteristic sharp, pungent odor. It is substantially more soluble in water than in many other solvents, and the aqueous solution is widely used in laboratory and industrial chemistry.

Physical properties vary with phase. Gaseous HCl has a boiling point of about −85°C and a melting point around −114°C; as a liquid, it forms a highly acidic solution. Concentrated hydrochloric acid is typically around 37% HCl by weight in water and is highly corrosive to metals and tissue. The compound is denser than air in gas form and reacts with many substances, sometimes vigorously.

Production and handling: HCl is commercially produced by methods such as direct synthesis of H2 and Cl2 followed by combination, or by reacting sodium chloride with sulfuric acid to generate HCl gas that is absorbed in water. It is used in metal pickling, chemical synthesis, pH control, and as a catalyst or reagent in various processes. Safety concerns center on its corrosivity and the irritant nature of the fumes; appropriate protective equipment and ventilation are essential.