Hydrogen compounds of sulfur
Hydrogen sulfide H₂S is a polar covalent molecule (H–S–H), Mᵣ = 34. It is a colourless, very poisonous gas that at low concentration smells of rotten eggs; slightly heavier than air (D = 34 : 29 ≈ 1.17). At higher concentrations the sense of smell is lost, so smell cannot be trusted, and it must never be smelled deliberately. It dissolves in water (about 2.5 volumes in 1 volume), and the solution is a weak acid, hydrosulfuric acid. It is made by acting on a sulfide with acid: FeS + 2HCl → FeCl₂ + H₂S↑ (only the teacher, in a fume cupboard). H₂S burns: with enough oxygen 2H₂S + 3O₂ → 2SO₂ + 2H₂O, with little oxygen 2H₂S + O₂ → 2S + 2H₂O. Sulfur in it is –2, so it is a strong reducing agent: H₂S + Cl₂ → 2HCl + S. The reagent for the sulfide ion is lead nitrate: S²⁻ + Pb²⁺ → PbS↓ (black precipitate). H₂S appears in volcanic gases, some mineral springs and in decay.
“Hazard sheet”: make a safety sheet for H₂S: formula, properties, hazards (poisonous, flammable), first aid (move to fresh air, call a doctor), where it occurs. No gas is made; the experiment is done only in a fume cupboard by a specialist.