Hydrogen chloride (HCl)

Physical and chemical characteristics

HCl gas is highly soluble in water; dissolution is strongly exothermic and produces the corrosive solution known as hydrochloric acid. The gas is denser than air and can form acidic mists when moisture is present. As a molecular gas it reacts readily with bases and many metals, releasing hydrogen chloride fumes or forming metal chlorides. A familiar laboratory demonstration is the white smoke produced when hydrogen chloride reacts with ammonia vapor to form solid ammonium chloride; see a description at ammonia reactions and the product ammonium chloride.

Preparation and industrial production

Hydrogen chloride can be generated in several ways. Laboratory preparations commonly use the reaction of sodium chloride with concentrated sulfuric acid to evolve the gas; detailed procedure notes are summarized at salt–sulfuric acid method. On an industrial scale HCl is often obtained as a byproduct of chlorination processes or made by direct synthesis from hydrogen and chlorine gases. The direct combination of H2 and Cl2 is highly exothermic and must be controlled because it can be violent in the presence of light; more information is available via hydrogen and chlorine reaction technology.

Uses and applications

Both the gas and its aqueous form have widespread applications. Major uses include:

• Production of inorganic and organic chloride compounds and intermediates for plastics and chemicals.
• Pickling and cleaning of metal surfaces, where acid removes oxides and scale.
• pH control and neutralization in industrial effluents and process streams.
• Laboratory reagent for synthesis and titrations in analytical chemistry.

Technical-grade aqueous HCl is frequently called muriatic acid in trade and older texts. For background on the name and common industrial roles see hydrochloric acid overview.

History, names and notable facts

Historically the substance or its solutions were known as 'spirit of salt' following early preparations; Johann Rudolf Glauber and other alchemists and chemists of the 17th century described salts and acid vapors in this context. The modern distinction is that hydrogen chloride denotes the anhydrous gas while hydrochloric acid refers to its aqueous solutions.

Safety and handling

Hydrogen chloride and concentrated hydrochloric acid are corrosive and can damage skin, eyes and the respiratory tract. Exposure to vapors or mists can cause coughing, irritation and chemical burns; because the gas is heavier than air it can collect in low-lying spaces. Proper ventilation, respiratory protection and corrosion-resistant materials are required for storage and transfer. Emergency response and first-aid guidance should be obtained from safety data sheets and qualified sources before working with this chemical.

For further technical literature and safety references consult dedicated chemical handbooks and material safety data sheets linked through resources such as technical data and industry guidance portals referenced at acid resources.