Sulphite and sulphide
Two more anions belong to the dilute HCl group because their acids are weak and unstable. Sulphite gives sulphur dioxide, SO₂, and sulphide gives hydrogen sulphide, H₂S. Both gases are poisonous, so they are never made or tested by smell at home or in school; trained people handle them in a laboratory, and on this page everything stays on paper. The book recognises the gas with a strip of paper wet with a reagent: SO₂ turns paper wet with acidified dichromate from orange to green, and H₂S turns lead acetate paper black, because black lead sulphide forms. A confirmation test follows. Silver nitrate gives a white precipitate of silver sulphite with a sulphite, which turns black on heating, and a black precipitate of silver sulphide at once with a sulphide. One trap: SO₂ also turns lime water milky, exactly as CO₂ does, so lime water alone can never tell a carbonate from a sulphite.
| Anion | Gas with dilute HCl | Paper test for the gas | With silver nitrate |
|---|---|---|---|
| sulphite SO₃²⁻ | SO₂ | acidified dichromate paper turns green | white Ag₂SO₃, black on heating |
| sulphide S²⁻ | H₂S | lead acetate paper turns black | black Ag₂S at once |
| Gas | Lime water | Dichromate paper | Lead acetate paper |
|---|---|---|---|
| CO₂ | milky | no change | no change |
| SO₂ | milky | green | no change |
| H₂S | no change | no change | black |
Gas detective cards, paper only. Make nine cards: three anions (carbonate, sulphite, sulphide), three gases (CO₂, SO₂, H₂S) and three test results (milky, green, black). Shuffle them and match each anion to its gas and to the result that tells the gas apart. Then add a tenth card that says “lime water alone” and explain to a friend why it is not enough. Nothing is mixed, smelled or heated: these gases are poisonous and are only for trained people in a laboratory.