(i) 4Na(s)+O2(g)→2Na2O(s)
In this reaction, Na is oxidized because it combines with O2 to form Na2O. O2 is reduced because it is converted into Na2O
.(ii) CuO(s)+H2(g)→Cu(s) + H2O(l)
In this reaction, CuO is reduced because it loses oxygen. H2 is oxidized because it combines with oxygen of CuO to form water