Hydrochloric acid

Laboratory preparation: hydrochloric acid is prepared by the action of sodium chloride on concentrated sulphuric acid (NaCl + H2SO4 -> NaHSO4 + HCl at moderate temperature, and NaHSO4 + NaCl -> Na2SO4 + HCl at higher temperature, giving overall 2NaCl + H2SO4 -> Na2SO4 + 2HCl). The moist gas evolved is dried by passing it through more concentrated sulphuric acid.

Physical properties: hydrogen chloride is a colourless, pungent-smelling gas, easily liquefied (boiling point 189 K) and frozen to a white crystalline solid (melting point 159 K); it is extremely soluble in water, dissolving with essentially complete ionisation to give hydronium and chloride ions (HCl(g) + H2O(l) -> H3O+ + Cl-).

Chemical properties: like all strong acids, hydrochloric acid liberates hydrogen gas from reactive metals and carbon dioxide from carbonates and bicarbonates (Zn + 2HCl -> ZnCl2 + H2; Mg + 2HCl -> MgCl2 + H2; Na2CO3 + 2HCl -> 2NaCl + CO2 + H2O; CaCO3 + 2HCl -> CaCl2 + CO2 + H2O; NaHCO3 + HCl -> NaCl + CO2 + H2O), and it liberates sulphur dioxide from sodium sulphite (Na2SO3 + 2HCl -> 2NaCl + H2O + SO2). When three parts of concentrated hydrochloric acid are mixed with one part of concentrated nitric acid, the powerful mixture aqua regia ("royal water") is obtained, capable of dissolving even gold and platinum, which resist attack by either acid alone: Au + 4H+ + NO3- + 4Cl- -> AuCl4- + NO + 2H2O; 3Pt + 16H+ + 4NO3- + 18Cl- -> 3[PtCl6]2- + 4NO …