Hydrogen chloride
Hydrogen chloride HCl is a polar covalent molecule (H–Cl) with relative molecular mass Mᵣ = 1 + 35.5 = 36.5. It is a colourless, sharp-smelling, choking gas; its density relative to air is 36.5 : 29 ≈ 1.26, i.e. slightly heavier than air. It fumes in moist air because its vapour forms a fine mist with the moisture. It dissolves extremely well in water: 1 volume of water takes about 500 volumes of HCl, and this solution is hydrochloric acid. In industry it is made from hydrogen and chlorine: H₂ + Cl₂ → 2HCl; in the laboratory conc. H₂SO₄ is heated with dry NaCl: 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl (only a teacher does this, in a fume cupboard). Dry hydrogen chloride does not react with metals or metal oxides, which sets it apart from hydrochloric acid. Its chief use is making hydrochloric acid.
“Fountain” as a drawing: in your notebook sketch the HCl dissolving experiment: a flask of gas, a tube, water. Write why water shoots in (the gas dissolves and the pressure drops). Only the teacher demonstrates; no gas is made at home.