In the electrolytic refining of copper metal,what is used as the cathode?

Which of the following metals exist in their native state in nature?
$(i)$ $Au$
$(ii)$ $Cu$
$(iii)$ $Ag$
$(iv)$ $Fe$

What is the chemical formula of oleum?

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?

Who discovered the method for the industrial production of ammonia?

Which one of the following properties is not generally exhibited by ionic compounds?