Hydrogen chloride

Hydrogen chloride
Properties
Appearance	Colorless gas
Melting Point	−114.22 °C (−173.60 °F; 158.93 K)
Boiling Point	−85.05 °C (−121.09 °F; 188.10 K)
Density	1.49 g/L
pKa	-5.9

Hydrogen chloride is a colorless gas at room temperature. Its aqueous solution is usually called hydrochloric acid.

Production

Hydrogen chloride can be prepared in a few ways in the laboratory:
${\ce {H2SO4 + NaCl = NaHSO4 + HCl ^}}$
${\ce {NaCl + NaHSO4 = Na2SO4 + HCl ^}}$

Hydrogen chloride can also be made by directly combining hydrogen and chlorine:
${\ce {H2 + Cl2 = 2HCl}}$

Properties

Physical Properties

Hydrogen chloride exists as a gas at room temperature and standard pressure. It melts at −114.22 °C (−173.60 °F; 158.93 K), and boils at −85.05 °C (−121.09 °F; 188.10 K). Its density is 1.49g/L. It is soluble in water, ethanol and many other polar solvents. The concentrated hydrochloric acid used in chemical laboratories usually has a concentration of 38%.

Chemical Properties

Hydrogen chloride, just like other hydrogen halides, can be added to some unsaturated hydrocarbons to form chloroalkanes. This reaction is also called hydrohalogenation.
${\ce {C2H4 + HCl = C2H5Cl}}$

Hydrochloric acid is one of the best-known acids. It is a strong acid. It can react with most metals, with the exceptions of unreactive metals like copper and gold. It can also react with many metal oxides and with bases.
${\ce {Mg + 2HCl = MgCl2 + H2 ^}}$
${\ce {Al2O3 + 6HCl = 2AlCl3 + 3H2O}}$
${\ce {NaOH + 2HCl = NaCl + H2O}}$

Applications

Hydrogen chloride can be used to produce alkyl chlorides, like vinyl chloride, which is the monomer of PVC.

Hydrogen chloride is involved in the purification of elemental silicon. In the process, hydrogen chloride is reacted with raw silicon to form trichlorosilane, a precursor to ultrapure silicon:
${\ce {Si + 3HCl = SiHCl3 + H2}}$

Hydrochloric acid can be used to produce many metal chlorides:
${\ce {Zn + 2HCl = ZnCl2 + H2 ^}}$
${\ce {NiO + 2HCl = NiCl2 + H2O}}$

Hydrochloric acid can be used to remove rust from iron surfaces:
${\ce {Fe2O3 + 6HCl = 2FeCl3 + 3H2O}}$

Hydrochloric acid can also be reacted with calcium carbonate to produce carbon dioxide gas. This reaction is usually used in labs to produce carbon dioxide.
${\ce {CaCO3 + 2HCl = CaCl2 + CO2 ^ + H2O}}$

Presence in human body

Hydrochloric acid is one of the main components of stomach acid, which helps us digest food. It provides an acidic environment for pepsin, a protease, to work in.