(I) Substance X is 'Copper' and substance Y is 'Copper oxide'.
(ii) When copper is heated in air, oxidation takes place. When hydrogen gas is passed over heated copper oxide, reduction takes place.
(iii) The chemical equations involved are:-
2Cu (s) + O2 (g) ----> 2CuO
CuO + H2 (g) ----> Cu + H2O