An orange solid `(A)` on heating gave a green residue `(B)`, colourless gas `( C)` and water vapour. The dry gas `( C)` on passing over heated magnesium gave a white solid `(D).(D)` on reaction with water have a gas `( E)` which formed dense white fumes with `HCl`. Indentify `(A)` to `( E)` and give the reactions.

Explanation

`underset ("Orange solid") ((A)) overset (Delta) rarr underset ("Green resi due")((B)) + underset ("Colourless gas")((C)) + H_2 O`
`(C) + Mg rarr underset ("White gas solid") ((D)) overset (H_2 O)rarr (E) overset (HCl)rarr` Dense white fumes.
Since `( E)` forms dense white fumes with `HCl, (E)` must be ammonia, `NH_3`.
`NH_3 + HCl rarr NH_4 Cl` (Dense white forms)
`NH_3` is formed by hydrolysis of solid `(D)` hence, `(D)` is `Mg_3 N_2`, magnesium nitride.
`underset((D)) (Mg_3 N_2) + 6 H_2 O rarr 3 Mg (OH)_2 + underset ((E)) (2 NH_3)`
Formation of `(D)` indicates that gas `( C)` is `N_2`.
Thus `(A)` is ammonium dichromate.
`underset((A))underset("Orange solid")((NH_(4))_(2)Cr_(2)O_(7))overset(Delta)tounderset((B))underset("Green solid")(Cr_(2)O_(3))+underset((C))underset("Colourless gas")(N_(2))+4H_(2)O`
Hence `(A) is (NH_4)_Cr_2 O_7, (B) is Vr_2 O_3, ( C) is N_2, (D) is Mg_3 N_2 and ( E) is NH_3`.