The 1st group of anions consist of CO, HCO, CHCOO, S, SO, Thiosulfate| and NO. The reagent for Group 1 anions is dilute hydrochloric acid (HCl) or dilute sulfuric acid (HSO).
* Carbonates give a brisk effervescence with dilute HSO due to the release of CO, a colorless gas which turns limewater milky due to formation of CaCO (carbonatation). The milkiness disappears on passing an excess of the gas through the lime water, due to formation of Ca(HCO).
* Acetates give the vinegar-like smell of CHCOOH when treated with dilute HSO and heated. A blood red colouration is produced upon addition of yellow FeCl, due to formation of iron(III) acetate.
* Sulfides give the rotten egg smell of HS when treated with dilute HSO. The presence of sulfide is confirmed by adding lead(II) acetate paper, which turns black due to the formation of PbS. Sulfides also turn solutions of red sodium nitroprusside purple.
* Sulfites produce SO gas, which smells of burning sulfur, when treated with dilute acid. They turn acidified KCrO from orange to green.
* Thiosulfates produce SO gas when treated with dilute acid. In addition, they form a cloudy precipitate of sulfur.
* Nitrites give reddish-brown fumes of NO when treated with dilute HSO. These fumes cause a solution of potassium iodide (KI) and starch to turn blue.