Question

Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)

Answer 1

in option 1 oxidized is 4Na and reduced is O2
in option 2 oxidized is   H2     and reduced is Cuo

Answer 2

The substance that added with oxygen is said to be oxidized whereas removal of oxygen is called as reduction and the substance is said to be reduced.

So, we need to check the addition of oxygen or removal of oxygen to check the oxidation and reduction.  

(i) Na changes to $Na_2O$. Hence, oxygen added so here Na oxidized and $O_2$ reduced.  

(ii) CuO changes to Cu hence oxygen is removed from CuO. Hence, CuO is reduced and $H_2$ is oxidized.