What is the product formed when sodium is heated in a current of dry ammonia?

$A$ metal $M$ reacts with nitrogen gas to afford $M_3N$. $M_3N$ on heating at high temperature gives back $M$ and on reaction with water produces a gas $B$. Gas $B$ reacts with aqueous solution of $CuSO_4$ to form a deep blue compound. $M$ and $B$ respectively are

The solubility of $Na_2SO_4$,$BeSO_4$,$MgSO_4$ and $BaSO_4$ in water follows the order:

The metal that produces $H_{2}$ with both dil. $HCl$ and $NaOH_{(aq)}$ is

$Na_2CO_3 \xrightarrow{\Delta} A$,'$A$' is

Give two uses of caustic soda.