hydrogen chloride

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hydrogen chloride

hydrogen chloride, chemical compound, HCl, a colorless, poisonous gas with an unpleasant, acrid odor. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity. Hydrogen chloride is prepared commercially by the reaction of sulfuric acid with sodium chloride (common salt); niter cake, a mixture of sodium bisulfite and sulfuric acid that is a byproduct of nitric acid manufacture, is sometimes used in place of sulfuric acid. Hydrogen chloride is also produced as a byproduct of the manufacture of chlorinated organic chemicals. It can be prepared directly by reaction of hydrogen and chlorine gases; the reaction is very exothermic and takes place readily in sunlight or at elevated temperatures. Although anhydrous (water-free) hydrogen chloride is commercially available as a high-pressure compressed gas in steel cylinders, most of the gas produced is dissolved in water to form hydrochloric acid (see acids and bases), a commercially important chemical. Pure grades of hydrochloric acid are colorless, but technical grades, commonly called muriatic acid, are often yellow-colored because of impurities such as dissolved metals. Most hydrochloric acid produced has a concentration of 30% to 35% hydrogen chloride by weight. The major use of hydrochloric acid is in the manufacture of other chemicals. It is also used in large amounts in pickling (cleaning) metal surfaces, e.g., iron before galvanizing. It reacts with most common metals, releasing hydrogen and forming the metal chloride; with most metal oxides and hydroxides it reacts to form water and the metal chloride. Hydrochloric acid is also used in small amounts in processing glucose and other foods and for various other uses. Concentrated solutions are strong acids and highly corrosive. Hydrochloric acid is not an oxidizing agent but can be oxidized by very strong oxidizing agents, liberating chlorine gas. In dilute solutions of the acid the hydrogen chloride is almost completely dissociated into hydrogen and chloride ions. A solution containing 20.24% hydrogen chloride by weight is azeotropic, boiling at a constant temperature of 110℃ at atmospheric pressure. Hydrogen chloride also forms monohydrates, dihydrates, and trihydrates that are liquids at room temperature.
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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Hydrogen Chloride


HCl, under ordinary conditions, a colorless gas, with a pungent odor. Hydrogen chloride fumes upon the absorption of moisture, the vapor consisting of tiny droplets of hydrochloric acid. The mass of 1 liter (l) of hydrogen chloride at 0°C and a pressure of 0.1 meganewton/m2 (1 kilogram-force/cm2) is 1.6391 g. The density relative to air is 1.268; the density of liquid hydrogen chloride at –60°C is 1.12 g/cm3. The melting point is –114.2°C, and the boiling point, –85.1°C.

Hydrogen chloride is a stable compound, which at 1800°C only slightly dissociates into Cl2 and H2. It has low chemical reactivity in the absence of water. In gaseous form, it does not react with carbon, phosphorus, sulfur, and iron. Upon heating with oxygen to 400°–500°C in the presence of the catalyst CuCl2, hydrogen chloride is oxidized to chlorine: 4HCl + O2 = 2H2O + 2Cl2. With alkali metals, aluminum, and magnesium, it reacts with the liberation of hydrogen, especially readily upon heating; for example, 2Al + 6HCl = 2AlCl3 + 3H2. With ammonia, it reacts with the formation of a smoke consisting of solid particles of ammonium chloride, NH4Cl. In the presence of catalysts, it adds to unsaturated organic compounds; for example, C2H4 + HCl → C2H5Cl.

Hydrogen chloride is obtained by direct synthesis from its elements or by the reaction of sulfuric acid with sodium chloride. Large amounts of hydrogen chloride are produced as a by-product in the chlorination of organic compounds: RH + Cl2 = RCl + HCl, where R is a radical. Large amounts are also produced in the hydrolysis of magnesium chloride: MgCl2 + H2O = MgO + 2HCl. The world production of hydrogen chloride in 1975 was about 10 million tons.

Hydrogen chloride is used in the production of hydrochloric acid and the synthesis of organic compounds, such as vinyl chloride.


Iakimenko, L. M. Proizvodstvo khlora, kausticheskoi sody i neorganicheskikh khlorproduktov. Moscow, 1974.
Iakimenko, L. M., and M. I. Pasmanik. Spravochnik po proizvodstvu khlora, kausticheskoi sody i osnovnykh khlorproduktov, 2nd ed. Moscow, 1976.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.

hydrogen chloride

[′hī·drə·jən ′klȯr‚īd]
(inorganic chemistry)
HCl A fuming, highly toxic, colorless gas soluble in water, alcohol, and ether; used in the production of vinyl chloride and alkyl chloride, and in polymerization, isomerization, and other reactions.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.