What is the gas produced in the reaction: $KO_2 + CO_2 \to ?$

Compound $A$ is prepared by the electrolysis of an aqueous solution of $B$ using a Castner-Kellner cell with mercury as the cathode and carbon as the anode. $A$ and $B$ respectively are:

Which of the following ions forms a highly soluble hydroxide in water?

Based on lattice energy and other considerations,which one of the following alkali metal chlorides is expected to have the highest melting point?

Write a note on the physical properties of alkali metal elements (group $1$).

Which of the following metals forms an amphoteric oxide?