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.

Explanation

`K_(2)Cr_(2)O_(7)` is an orange compound. It is formed when `Na_(2)Cr_(2)O_(7)` reacts with KCl. In acidic medium, yellow coloured `CrO_(4)^(2-)` (chromate ion) changes into dichromate.
The given process is the preparation method of potassium dichromate from chromite ore.
A = Fe`Cr_(2)O_(4)`, B = `Na_(2)CrO_(4), C = Na_(2)Cr_(2)O_(7), D = K_(2)Cr_(2)O_(7)`.
(i) `4Fe underset([A])(Cr_(2)O_(4))+8Na_(2)CO_(3) + 7O_(2) rarr 8underset([B])(Na_(2)CrO_(4)) + 2Fe_(2)O_(3) + 8CO_(2)`
(ii) `2Na_(2)CrO_(4) + 2H^(+) rarr Na_(2)Cr_(2)O_(7) + 2Na^(+) + H_(2)O`
(iii) `Na_(2)underset([C])(Cr_(2)O_(7)) + 2KCl rarr K_(2)underset([D])(Cr_(2)O_(7)) + 2NaCl`