Question

Calcium burns in nitrogen to produce a white powder which dissolves in sufficient water to produce a gas (A) and an alkaline solution . The solution on exposure to air produces a thin solid layer of (B) on the surface . Identify the compounds A and B .

Answer 1

Ca burns in air to form CaO and `Ca_(3)N_(2)`.
`2 Ca + O_(2) overset(Delta)(to) 2 CaO , 3 Ca + N_(2) overset(Delta)(to) Ca_(3)N_(2)`
Calcium nitride on hydrolysis with `H_(2)O` gives ammonia (A)
`Ca_(3) N_(2) + 6 H_(2)O to 3 Ca(OH)_(2) + underset(A) (2 N)H_(3)`
The alkaline solution of `Ca(OH)_(2)` thus formed reacts with `CO_(2)` present in the air to form `CaCO_(3)` which being insoluble forms a thin solid layer on the surface .
`Ca(OH)_(2) + CO_(2) to underset(B)(CaC)O_(3) + H_(2)O`
Thus , A = `NH_(3)` and `B = CaCO_(3)` (white powder)