CO32− : On action of dil sulphuric acid, CO2 gas is released which turns lime water milky. S2− : On action of dil sulphuric acid, H2S gas is released which turns lead acetate paper black. SO32− : On action of dil H2SO4,SO2 gas is evolved which turns acidified potassium dichromate solution green. NO2−: On action of dil H2SO4,NO2 gas is evolved which turns KI solution containg starch blue.