What type of oxides are formed when non-metal elements combine with dioxygen gas?

$2 \, mL$ each of concentrated $HCl$,$HNO_3$,and a mixture of concentrated $HCl$ and concentrated $HNO_3$ in the ratio of $3:1$ were taken in test tubes labeled as $A$,$B$,and $C$. $A$ small piece of metal was put in each test tube. No change occurred in test tubes $A$ and $B$,but the metal dissolved in test tube $C$. The metal could be:

Name the metal which:
$(a)$ becomes blackish after a certain time.
$(b)$ develops a green coating on exposure to air.
$(c)$ does not corrode at all.
$(d)$ is used in making steel.
$(e)$ is non-lustrous.

Account for the following:
$(a)$ Aluminium is more reactive than iron,but its corrosion is less than iron.
$(b)$ Hydrogen gas is not evolved when zinc metal reacts with dilute $HNO_3$.
$(c)$ Carbon is not used for reducing aluminium from aluminium oxide.

Write chemical equations for the reactions taking place when:
$(i)$ iron is strongly heated in air.
$(ii)$ lead carbonate is calcined.
$(iii)$ chromium oxide $(Cr_{2}O_{3})$ is heated with aluminium powder.

$(a)$ Carbon cannot be used as a reducing agent to obtain $Mg$ from $MgO$. Why?
$(b)$ How is sodium obtained from molten sodium chloride? Give the equations of the reactions.
$(c)$ How is copper obtained from its sulphide ore? Give the equations of the reactions.