Study of Hydrogen Chloride Study of Hydrogen Chloride (HCl)

Hydrogen chloride is a colourless gas with a sharp, pungent, irritating smell that makes you cough. It is about 1.28 times denser than air, so it is a heavy gas. Its most striking property is how greatly it dissolves in water: one volume of water can dissolve roughly 450 volumes of the gas, forming hydrochloric acid. In moist air the gas fumes, because it grabs water vapour and forms tiny droplets of acid mist. When dry, the pure gas is covalent and does not conduct electricity or turn dry litmus red; only after dissolving in water does it ionise and behave as a strong acid.

Hydrogen chloride is made by warming common salt (sodium chloride) with concentrated sulphuric acid in a flask. Cold or gentle heating gives sodium bisulphate: NaCl + H2SO4 gives NaHSO4 + HCl. On stronger heating above about 200 degrees Celsius the bisulphate reacts with more salt to give sodium sulphate: NaCl + NaHSO4 gives Na2SO4 + HCl. Concentrated sulphuric acid is chosen because it is non-volatile (high boiling point), so it stays behind and pushes out the volatile HCl, and because it does not oxidise HCl the way concentrated nitric acid would. The gas is then passed through concentrated sulphuric acid to dry it and collected by upward displacement of air.

HCl cannot be collected over water because it is far too soluble, it would simply dissolve. It is collected by upward displacement of air (downward delivery) since it is denser than air and sinks into the jar. To dry it we bubble it through concentrated sulphuric acid; we must NOT use the usual drying agents quicklime (CaO) or phosphorus pentoxide, because these are basic or react chemically with the acidic gas and would remove it. A trap or inverted funnel arrangement is used over the water surface in the fountain demonstration to prevent suck-back of water into the hot flask.

Dry hydrogen chloride gas is not acidic, but its solution in water, hydrochloric acid, is a strong monobasic acid. It turns blue litmus red, reacts with active metals such as zinc to give hydrogen, with carbonates to give carbon dioxide, and neutralises bases to give chlorides and water. Two simple tests identify the gas: it gives dense white fumes when a rod dipped in ammonia solution is held in it (forming ammonium chloride), and its solution gives a white precipitate of silver chloride with silver nitrate solution, insoluble in dilute nitric acid but soluble in ammonia. The aqueous acid is also part of aqua regia (3 parts conc. HCl to 1 part conc. HNO3), which dissolves gold and platinum.