Which of the following substances releases $SO_2$ gas upon reaction with dilute $H_2SO_4$?

For the identification of which ion is $Na_2CO_3$ $NOT$ useful?

Which of the following compounds will give a precipitate with $Pb(NO_3)_2$ but will not give a precipitate with $Ba(NO_3)_2$?

When a few drops of $KMnO_4$ are added to an acidic solution of an anion,which of the following,if present,will $NOT$ decolorize $KMnO_4$?

When a solution of a mixture having two inorganic salts was treated with freshly prepared ferrous sulphate in an acidic medium,a dark brown ring was formed,whereas on treatment with neutral $FeCl_3$,it gave a deep red colour which disappeared on boiling and a brown-red precipitate was formed. The mixture contains:

Anion which can react with $dil. H_2SO_4$ is