chemistry

CLASS A (I):

ANIONS WHICH EVOLVE GASES ON REACTION WITH DIL HCL / DIL H2SO4

It includes - CO32- , SO32- , S2- , NO22- , CH3COO- , S2O32-

Carbonate (CO32-) : gives effervescence , due to evolution of carbon dioxide .
CO32- + 2H+ → CO2 ↑ + H2O

Sulphites (SO32-) : decomposes with the evolution of sulphur dioxide .
SO32- + 2H+ → SO2 + H2O
The gas have suffocating odour of burning sulphur

Sulphide (S2-) : Pungent smelling gas (smell of rotten eggs) H2S is evolved
S2- + 2H+ → H2S ↑

Nitrites (NO2-) : Adding to solid nitrite in cold yield pale blue liquid (due to presence of free nitrous acid HNO2 or its anhydrous N2O3) and the evolution of brown fumes of nitrogen dioxide , the latter being largely produced by combination of nitric oxide with oxygen of the air .

Acetate (CH3COO-) : Smell of vinegar due to the formation of the vapours of acetic acid .
CH3COO- + H+ → CH3COOH ↑

Thiosulphates (S2O3) : gives sulphur and sulphur dioxide .
S2O32- + 2H+ → S↓ + SO2 ↑ + H2O