Identify the substances that are oxidised and the substances that are reduced in the following reactions.
$(i)$ $4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$
$(ii)$ $CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l)$

(A) $(i)$ In the reaction $4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$,Sodium $(Na)$ is oxidised because it gains oxygen to form $Na_2O$,and Oxygen $(O_2)$ is reduced.
$(ii)$ In the reaction $CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l)$,Copper oxide $(CuO)$ is reduced to Copper $(Cu)$ because it loses oxygen,while Hydrogen $(H_2)$ is oxidised to Water $(H_2O)$ because it gains oxygen.