Two species (A) and (B) exists in equilibrium at pH of about 4 and can be interconverted by changing the pH. Acidified solution of (B) is orange, and on adding `H_(2)O_(2)` it forms deep blue colour due to the formation of compound (C ), this blue colour fades away gradually. Further acidified solution (B) on reaction with NaCl gives organge red fumes due to the formation of (D ) . Identify (A), (B) , (C) & (D).

Explanation

`2CrO_(2)+2H^(+)underset("alkali")overset("acid")(hArr)underset("Chormate (yellow)")(2HCrO_(4)^(-))underset("alkali")overset("acid")(hArr)underset("Dichromate ( oragne)(B)")(Cr_(2)O_(7)^(2-)+2H_(2)O)`
`underset((B))(Cr_(2)O_(7)^(2-))+2H^(+)+4H_(2)O_(2)rarr underset((C ) "blue colour")(2CrO_(5)+5H_(2)O)`
Blue colour fades away gradually due to decomposition of `CrO_(5)` into `Cr^(3+)` ions and oxygen. Acidified solution of ` : B+NaClrarr CrO_(2)Cl_(2)` (Chromyl chloride ). (D) Orange red fumes.