Which of the following acids is called 'the king of chemicals'?

What is the chemical formula for potassium dichromate?

In the $Hall-Heroult$ process,which substances are added to alumina to lower its melting point?

In which state does the element phosphorus exist at room temperature?

A non-metal $A$, which is the largest constituent of air, when heated with $H_2$ in a $1 : 3$ ratio in the presence of a catalyst $(Fe)$, gives a gas $B$. On heating with $O_2$, it gives an oxide $C$. If this oxide is passed into water in the presence of air, it gives an acid $D$, which acts as a strong oxidising agent.
$(a)$ Identify $A$, $B$, $C$, and $D$.
$(b)$ To which group of the periodic table does this non-metal belong?

What is the chemical formula of sodium sulfite?