An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. In redox reaction the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.In the reaction : \\\\({ CuO\\quad +\\quad { H }_{ 2 } }\\quad \\longrightarrow \\quad Cu\\quad +\\quad { H }_{ 2 }O\\\\) n the reaction \\\\({ H }_{ 2 } \\\\) is oxidized since it has lost electrons and \\\\(Cu\\\\) is reduced since it has gained electrons.