$A$ tetra-atomic molecule $(A)$ on reaction with excess oxygen produces substance $(B)$. $(B)$ is a dehydrating agent. The substances $(A)$ and $(B)$ respectively will be:

$FeSO_4$ gives a brown ring with ........

Heating of $(NH_4)_2Cr_2O_7$ releases a gas. Which of the following also produces the same gas?

Which of the following represents the correct order of the inert pair effect?

Correct statements from the following are: $A$. Nitrogen in oxidation states from $+1$ to $+4$ disproportionates in acid medium. $B$. Nitrogen has the ability to form $d\text{--}p$ multiple bonds with itself and other elements with small size and high electronegativity. $C$. $N$-$N$ single bond is stronger than $P$-$P$ single bond. $D$. Nitrogen has highest density in its group due to small size. $E$. The maximum covalency of nitrogen is four since it has only four valence orbitals for bonding.

Which blue-colored liquid is obtained upon reacting equimolar amounts of two gases at $-30\,^{\circ}C$?