Which of the following is the use of potassium permanganate $(KMnO_4)$?

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

Which among the following statements is incorrect for interstitial compounds?
$1$. They are very hard and rigid
$2$. They have higher melting point than pure metal
$3$. They do not show conductivity
$4$. They are chemically inert

The colorless species is

Provide the structures of the following ions:
$1.$ $CrO_4^{2-}$
$2.$ $Cr_2O_7^{2-}$

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.