What is the chemical formula of oleum?

What will happen when:
$(i)$ Magnesium is treated with very dilute nitric acid?
$(ii)$ Steam is passed over red hot iron?

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.

Which substance is used for the chemical reduction of metal oxides like chromium and manganese to obtain the metal?

$2 \, mL$ 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:

An alloy is