Pure concentrated $HNO_3$ makes iron passive as the surface is covered with a protective layer of:

Which of the following compounds is a tribasic acid?

Which of the following is an ionic compound?

How many allotropic forms of black phosphorus exist?

Which reaction$(s)$ among the following produce dinitrogen?
$1. NH_4Cl(aq) + NaNO_2(aq) \longrightarrow$
$2. NH_2CONH_2 + 2H_2O \longrightarrow$
$3. Ba(N_3)_2 \longrightarrow$
$4. NH_4Cl + Ca(OH)_2 \longrightarrow$

Select the correct statement.