Which of the following substances releases nitrogen gas upon thermal decomposition?

It is because of the inability of $ns^{2}$ electrons of the valence shell to participate in bonding that:

Which of the following compounds releases nitrogen dioxide $(NO_2)$ upon heating?

$A$ nitrogen oxide that forms "in situ" when dilute $FeSO_4$ is treated with an aqueous solution of nitrate ion and then carefully adding conc. $H_2SO_4$ along the sides of the test tube, is

Pure $HNO_3$ is colorless. However,it turns slightly brown when exposed to light due to its decomposition into $.....$.

Select the correct statement.