Identify A, B, C, D and E.
A while substance A reacts with dilute `H_2SO_4` to produce a colourless gas B and a colourless solution C. The reaction between B and acidified `K_2Cr_2O_7` solution produces a green solution and slightly coloured precipitate D. The substance D burns in air to produce a gas E which reacts with B to yield D and a colourless liquid. Anhydrous copper sulphate is turned blue on adiition of this colourless liquid. Addition of aqueous `NH_3` or `NaOH` to C produces first a precipitate

Explanation

`ZnS(A)+dil.H_2SO_4rarrH_2S(B)+ZnSO_4(C)`
`3H_2S+Cr_2O_7^(2-)+8H^(o+)rarr2Cr^(3+)(Green)+3S(D)+7H_2O`
`S(D)+O_2rarrSO_2(E)`
`SO_2(E)+H_2S(B)toS(D)+H_2O` (Colourless liquid)
`Zn^(2+)+2OH^(ɵ)to2n(OH_2)darroverset(OH^(ɵ))toZn(OH)_4^(2-)`
(Clear solution) (C)