Which of the following does not form double salts?

The commercial production of sodium carbonate is done by

Characteristic feature of alkali metals is

Heating a metal $X$ in nitrogen gas gives $Y$. The reaction of $Y$ with $H_2O$ produces a colorless gas. When this gas is passed through $CuSO_4$ solution,it produces a blue color. What is $Y$?

The atomic radii of alkali metals $(M)$ lie in the order $Li < Na < K < Rb$,but the radii of $M^{+}$ ions in aqueous solution lie in the reverse order $Li^{+} > Na^{+} > K^{+} > Rb^{+}$. What is the reason for this reverse order (on going from $Li$ to $Rb$)?

Which of the following products is formed when $Zn$ reacts with $NaOH$?