The melting point of a mixture of $Na_2CO_3 + K_2CO_3$ is:

(Milky Cloud) $C \xleftarrow{CO_2} A + Na_2CO_3 \rightarrow B + C$
The chemical formulae of $A$ and $B$ are

The atomic radii of the alkali metals follow the order

Match the following alloys in List-$I$ with their uses in List-$II$:

List-$I$ (Alloy)	List-$II$ (Use)
$A$. $Li-Pb$	$I$. In aircraft construction
$B$. $Be-Cu$	$II$. To make bearings for motor engines
$C$. $Mg-Al$	$III$. To make tetraethyl lead
$D$. $Na-Pb$	$IV$. To make high strength springs

$A$ metal $M$ reacts with $N_2$ to form a compound $A$ $(M_3N)$. Heating $A$ at a high temperature gives back $M$,and passing $A$ through $H_2O$ results in the evolution of gas $B$. Gas $B$ turns $CuSO_4$ solution blue. What are $A$ and $B$ respectively?

When sodium $(Na)$ metal is dissolved in liquid ammonia $(NH_3)$,it imparts a blue colour to the solution. This blue coloration is due to: