A metal M reacts with ${\mathrm{N}}_2$ to give a compound ‘A’ $\left({\mathrm{M}}_3\mathrm{N}\right).$ ‘A’ on heating at high temperature gives back ‘M’ and ‘A’ on reacting with ${\mathrm{H}}_2\mathrm{O}$ gives a gas ‘B’. ‘B’ turns ${\mathrm{CuSO}}_4$ solution blue on passing through it. A and B can be

Lithium forms nitride on heating with nitrogen;Lithium nitride gives ammonia when heated with ${\mathrm{H}}_2\mathrm{O}$. Ammonia gas forms tetrammine copper complex with ${\mathrm{CuSO}}_4$ solution.

$$\begin{gathered} 6\mathrm{Li} + {\mathrm{N}}_2 \stackrel{ }{\to } 2{\mathrm{Li}}_3\mathrm{N} \\ {\mathrm{Li}}_3\mathrm{N} + 3{\mathrm{H}}_2\mathrm{O} \stackrel{ }{\to } 3\mathrm{LiOH} + {\mathrm{NH}}_3 \\ {\mathrm{CuSO}}_4 + 4{\mathrm{NH}}_3 \stackrel{ }{\to } \left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]{\mathrm{SO}}_4 \end{gathered}$$