Question

a brown substance x on heating in air forms a substance y . when hydrogen gas is passed over heated y. it again changes back into x. name the substance x and y

Answer 1

the substance is copper.
On heating, it becomes CuO copper oxide.
When H2 is passed, it becomes,
CuO + H2 gives Cu + H2O.

Answer 2

====,X is Copper=====
when x is in air it forms copper oxide CuO
so====== y is CuO=======
because after passing H2 from it....following rxn takes place
CuO + H2 -------> Cu + H2O......