When a chromite ore (A) is fused with sodium carbonate in free excess of air and the product is dissolved in water, yellow solution of a 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

`A = FeCr_(2)O_(4) , B = Na_(2)CrO_(4) , C = Na_(2)Cr_(2)O_(7), 2H_(2)O , D = K_(2)Cr_(2)O_(7)`,
Reactions :
`(i)underset((A))(4FeCr_(2)O_(4)+8Na_(2)CO_(3)+7O_(2))rarr underset((B))(8Na_(2)CrO_(4)+2Fe_(2)O_(3)+8CO_(2))`
(ii) `2NaCrO_(4)+H_(2)SO_(4) rarr Na_(2)Cr_(2)O_(7)(C )+Na_(2)SO_(4)+H_(2)O`
(iii) `Na_(2)Cr_(2)O_(7)(C )+2KCl rarr K_(2)Cr_(2)O_(7)(D)+2NaCl`