$A$ copper coin is completely covered with a gold film and is placed in dilute $HNO_3$. This will result in the formation of:

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$Pb_3O_4 + 4HNO_3 \rightarrow 2Pb(NO_3)_2 + PbO_2 + 2H_2O$

In which of the following reactions is there no change in the valency (oxidation state) of the elements?

Identify whether the following reactions act as oxidation or reduction processes:
$(i)$ $FeSO_4 + Mg \to MgSO_4 + Fe$
$(ii)$ $Cu + 4HNO_3 \to Cu(NO_3)_2 + 2NO_2 + 2H_2O$
$(iii)$ $H_2S + Cl_2 \to S + 2HCl$

By passing $H_2S$ gas in acidified $KMnO_4$ solution,we get

The colour of an iodine solution is discharged by shaking it with an aqueous solution of: