An element reacts with oxygen to give a compound with a high melting point. This compound is also soluble in water. The element is likely to be:

Which non-metal element possesses luster?

Galvanisation is a method of protecting:

Give two examples each of metals that are good conductors and poor conductors of heat,respectively.

Give reasons for the following:
$(i)$ Carbon can reduce zinc oxide to zinc but not calcium oxide to calcium.
$(ii)$ Copper and aluminium wires are usually used for electricity transmission.

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?