Lithium shows diagonal relationship with element $X$ and aluminum with $Y$. $X$ and $Y$ respectively are

State as to why:
$(a)$ a solution of $Na_2CO_3$ is alkaline?
$(b)$ alkali metals are prepared by electrolysis of their fused chlorides?
$(c)$ sodium is found to be more useful than potassium?

Select the correct statement from the following.

Hydrogen gas is evolved by the action of cold dilute $HNO_3$ on which of the following metals?

Which metal nitrate liberates $NO_{2}$ gas upon heating?

In the Solvay process,can we obtain sodium carbonate directly by treating the solution containing $(NH_4)_2CO_3$ with sodium chloride? Explain.