A  metal X  on heating in nitrogen gas gives Y.Y  on treatment with $H_{2}O$   gives a colorless   gas  which when passed through  $CuS{O}_4$  solution gives a blue colour .Y is  

Y is $M{g}_3{N}_2.$   
Magnesium react with nitrogen to from magnesium nitride. Magnesium nitride reacts with water to from ammonia and magnesium hydroxide. When ammonia gas is passed through $CuS{O}_4$  Solution, it forms a blue precipitate.