A pale yellow substance `(A)` when heated with `conc. HNO_3` evolves a brown coloured gas `(B)`. The substance `(A)` also dissolves in sodium sulphite solution on heating. A clear solution `(c)` is formed which on acidification gives a turbid solution and a pungent smelled gas `(D)` which is formed by the substance `(A)` in air. The solution `(c)` decolourises iodine solution, Identify `(A)` to `(D)`.

Explanation

(c) soln. + `I_2 soln. rarr` Decolourisation of `I_2` soln
Pale yellow substance (A) is sulphur
`S + 6 HNO_3("conc.") overset (Delta) rarr underset((c)) (Na_2 S_2 O_3)`
`Na_2 S_2 O_3`, on acidification, gives a turbid solution and a pungent smelling gas due to formation of `S` and `SO_2` respectively.
`Na_2 S_2 O_3 + H_2 SO_4 rarr NaSO_4 + underset ((D)) SO_2 + S darr + H_2 O`
`underset ((A)) S + underset (("air")) O_2 rarr (D)SO_2`
`2Na_2 S_2 O_3 + I_2 rarr Na_2 S_4 O_6 + underset ("Colourless solution") 2 NaI`
Since `I_2` is reduced to `I^(Ө)`, a colourless solution is formed.