Hydrogen chloride is the anhydride of hydrochloric acid. It has the chemical formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry. Hydrochloric acid, the aqueous solution of hydrogen chloride, is also commonly given the formula HCl (aq).
HCl is a diatomic gas consisting of hydrogen and chlorine. The much greater electronegativity of chlorine causes the gas to be very polar. Due to it's polarity, it is highly soluble it water, and the gas is hygroscopic. Upon contact, H2O and HCl combine to form hydronium cations H3O+ and chloride anions Cl– through a reversible chemical reaction:
- HCl + H2O → H3O+ + Cl–
A simple test for hydrogen chloride fumes involves holding an open container of aqueous ammonia in a suspected area. If sufficient amounts of hydrogen chloride is present, it will react with the ammonia gas to form a dense white fog of ammonium chloride.
HCl is a colorless gas, which forms white fumes of hydrochloric acid in the presence of moisture. It has a strong, pungent smell and taste. HCl is extremely soluble in water (720 g/L at 20 °C), and more so at lower temperatures, as well as other solvents such as methanol, ethanol, diethyl ether, THF, but less soluble in non-polar solvents such as benzene, chloroform, carbon tetrachloride. It has a boiling point of −85.05 °C and a melting point of −114.22 °C.
Anhydrous hydrogen chloride is only available to industrial consumers. In US it is considered DEA List II chemical.
Direct synthesis: Industrially, HCl (and sodium hydroxide) is made from the chlor-alkali process, brine (mixture of sodium chloride and water) solution is electrolyzed producing chlorine (Cl2), sodium hydroxide, and hydrogen (H2). The pure chlorine gas can be combined with hydrogen to produce hydrogen chloride in the presence of UV light.
- Cl2(g) + H2(g) → 2 HCl(g)
As the reaction is exothermic, the installation is called an HCl oven or HCl burner. The resulting hydrogen chloride gas is absorbed in deionized water, resulting in chemically pure hydrochloric acid. This reaction can give a very pure product, e.g. for use in the food industry. This reaction occurs explosively in the presence of light.
Small amounts of HCl gas for laboratory use can be generated in a HCl generator by dehydrating hydrochloric acid with either sulfuric acid or anhydrous calcium chloride. The latter however, required lots of anhydrous calcium chloride. 
Alternatively, HCl can be generated by the reaction of sulfuric acid with sodium chloride:
- NaCl + H2SO4 → NaHSO4 + HCl
This reaction occurs at room temperature. Provided there is salt remaining in the generator and it is heated above 200 degrees Celsius, the reaction proceeds to;
- NaCl + NaHSO4 → HCl + Na2SO4
For such generators to function, the reagents should be dry.
- Make hydrochloric acid
- Anhydrous metal chloride synthesis
- Trichlorosilane synthesis
Hydrogen chloride forms corrosive hydrochloric acid on contact with water found in body tissue. Inhalation of the fumes can cause coughing, choking, inflammation of the nose, throat, and upper respiratory tract, and in severe cases, pulmonary edema, circulatory system failure, and death. Skin contact can cause redness, pain, and severe skin burns. Hydrogen chloride may cause severe burns to the eye and permanent eye damage.
The gas, being strongly hydrophillic, can be easily scrubbed from the exhaust gases of a reaction by bubbling it through water, producing useful hydrochloric acid as a byproduct.
Any equipment handling hydrogen chloride gas must be checked on a routine basis; particularly valve stems and regulators. The gas requires the use of specialized materials on all wetted parts of the flow path, as it will interact with or corrode numerous materials hydrochloric acid alone will not; such as stainless and regular polymers.
The Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health have established occupational exposure limits for hydrogen chloride at a ceiling of 5 ppm (7 mg/m3).
Hydrogen chloride cylinders should be kept away from any heat or light source. A special acid cabinet can be used, which should be lined with a scrubbing material to remove any leaking hydrogen chloride. The cylinders must be inspected periodically for any signs of corrosion.
Hydrogen chloride, like hydrochloric acid can be neutralized by reacting it with a base or a carbonate. Calcium hydroxide or carbonate are cheap and effective. Fumes can be neutralized with ammonia, though this method has the disadvantage of producing a a very thick mist of ammonium chloride that tends to persist.