alkali metal (A) on flame test gives a crimson red colour to the Busen flame. (A) on heating in air gives compound (C ) and gas (D). Gas (D) with Nessler's regent gives a brown precipitate Identify (A), (B), (C ) and (D).

Explanation

Flame colouration of (A) shows it to be lithium.
`underset((A))(Li)+underset((Air))(O_(2))overset(Delta)rarr Li_(2)O`
`6Li+underset((Air))(N_(2)) overset(Delta)rarr2Li_(3)N`
`Li_(2)O+H_(2)Orarr 2LiOH`
`underset((B))(Li_(3))N + H_(2)O rarr underset((C ))(LiOH) + underset((D))(NH_(3))`
On hydrolysis of compound (B), the gas produced (D) gives a brown ppt. with Nessler's reagent, it shows the gas to be `NH_(3)`. which is produced only on the hydrolysis of `Li_(3)N` and not `Li_(2)O`. Hence, (B) is `Li_(3)N`. Hence, `(A) is Li, (B) is Li_(3)N, (C ) is LiOH and (D) is NH_(3)`.