A metal M reacts with N2 to give a compound ‘A’ M3N. ‘A’ on heating at high temperature gives back ‘M’ and ‘A’ on reacting with H2O gives a gas ‘B’. ‘B’ turns CuSO4 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 H2O. Ammonia gas forms tetrammine copper complex with CuSO4 solution.6Li  +  N2  →   2Li3N Li3N + 3H2O →  3LiOH  +  NH3 CuSO4 + 4NH3 →   CuNH34SO4