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

The compounds A, B, C and D are given as under:
`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)`
The reactions are explained as under :
`underset((A))(4FeCr_(2)O_(4)+8Na_(2)CO_(3)+7O_(2)to underset((B))(8Na_(2)CrO_(4))+2Fe_(2)O_(3)+8CO_(2)`
`2NaCrO_(4)+2H^(+)to Na_(2)Cr_(2)O_(7)+2Na^(+)+H_(2)O`
`underset((C))(Na_(2)Cr_(2)O_(7))+2KCl to underset((D))(K_(2)Cr_(2)O_(7))+2NaCl`