Hydrogen Chloride

Hydrogen Chloride

Hydrogen chloride was first prepared by Glauber in 1648, who heated common salt with concentrated sulphuric acid. It took until 1810 for Davy to establish that it was a compound made purely of hydrogen and chlorine.

Preparation

In the laboratory, hydrogen chloride is generated by heating sodium chloride with concentrated sulphuric acid, in two stages at different temperatures:

The moist HCl gas produced can be dried by passing it through concentrated sulphuric acid.

Properties

Hydrogen chloride is a colourless, pungent-smelling gas. It liquefies easily to a colourless liquid (boiling point 189 K) and freezes to a white crystalline solid (freezing point 159 K). It is extremely soluble in water, where it ionises almost completely:

Its aqueous solution is what we call hydrochloric acid; the enormous value of $K_a$ tells us this is a strong acid in water. Gaseous HCl also reacts directly with ammonia to give dense white fumes of ammonium chloride:

Mixing three parts of concentrated HCl with one part of concentrated $HNO_3$ produces aqua regia, famous for dissolving noble metals like gold and platinum that resist ordinary acids:

Being a strong acid, hydrochloric acid readily decomposes the salts of weaker acids — carbonates, hydrogencarbonates, sulphites and the like — releasing their weaker acid or its decomposition products:

$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2$ …