Explain the following:
$(a)$ The reactivity of $Al$ decreases if it is dipped in $HNO_3$.
$(b)$ Carbon cannot reduce the oxides of $Na$ or $Mg$.

(N/A) When $Al$ is dipped in $HNO_3$,a thin,protective,and non-reactive layer of aluminum oxide $(Al_2O_3)$ forms on its surface. This layer prevents further reaction of the metal with the acid,thereby decreasing its reactivity.
$(b)$ $Na$ and $Mg$ are highly reactive metals that have a much stronger affinity for oxygen than carbon does. Therefore,carbon cannot displace oxygen from $Na_2O$ or $MgO$ to reduce them to their respective metals.