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'.
(i) Name the substances X and Y.
(ii) Name the chemical processes occurring during both the changes.
(iii) Write the chemical equations.

Explanation

(i) The substance X is copper and Y is copper (II) oxide or CuO.
(ii) The process of change of X into Y is oxidation. The process of change of Y into X is reduction.
(iii) The chemical equations are :
`{:(2Cu(s) + O_(2)(g) overset("heat")to, 2CuO(s) " (Oxidation)"),((X), " "(Y)):}`
`{:(CuO(s) + H_(2)(g) overset("heat")to, Cu(s) + H_(2)O(l) " (Reduction)"),((Y), ""(X)):}`