Identify the correctly matched pairs:
$(i)$ $TiO_2$ - pigment industry
$(ii)$ $MnO_2$ - dry battery cells
$(iii)$ $Cu/Ni$ alloy - $UK$ 'copper' coins

On heating potassium permanganate,one of the following compounds is not obtained:

When a chromite ore $(A)$ is fused with sodium carbonate in free excess of air and the product is dissolved in water,a yellow solution of compound $(B)$ is obtained. After treatment of this yellow solution with sulphuric acid,compound $(C)$ can be crystallised from the solution. When compound $(C)$ is treated with $KCl$,orange crystals of compound $(D)$ crystallise out. Identify $A$ to $D$ and also explain the reactions.

The yellow colour solution of $Na_2CrO_4$ changes to orange-red on passing $CO_2$ gas due to the formation of

Correct statements from the following are:
$A$. Potassium dichromate is an oxidising agent and it oxidises $FeSO_4$ to $Fe_2(SO_4)_3$ in acidic medium.
$B$. Sodium dichromate can be used as primary standard in volumetric estimation.
$C$. $CrO_4^{2-}$ and $Cr_2O_7^{2-}$ are interconvertible in aqueous solution by varying the $pH$ of the solution.
$D$. $Cr-O-Cr$ bond angle in $Cr_2O_7^{2-}$ is $126^\circ$.
Choose the correct answer from the options given below:

The total number of $Mn=O$ bonds in $Mn_2O_7$ is