Chemical Reaction Equilibrium

To solve the problem, we need to identify compounds (A), (B), (C), and (D) in the given reaction sequence starting from the fusion of with .

1. Reaction of with in Air:
contains Cr in the +3 state. When fused with in the presence of air (oxygen), Cr³⁺ is oxidized to Cr⁶⁺, forming sodium chromate ( ), a yellow solution. The reaction is:

.
Compound (A) is .

2. Acidification of Compound (A):
Acidifying converts chromate ( ) to dichromate ( ) via:

.
Compound (B) is .

3. Reaction of Compound (B) with KCl:
reacts with KCl to form potassium dichromate ( ), an orange-colored compound:

.
Compound (C) is .

4. Oxidation of by Acidified Compound (C):
Acidified acts as an oxidizing agent, converting (where S is +4) to (where S is +6):

.
Compound (D) is .

Final Answer:
(A) is , (B) is , (C) is , and (D) is .