Which of the following compounds liberates oxygen on heating?

The hydration energy of $Mg^{2+}$ is

The stability of the following alkali metal chlorides follows the order:

Compound $A$ reacts with $NH_4Cl$ and forms a compound $B$. Compound $B$ reacts with $H_2O$ and excess of $CO_2$ to form compound $C$,which on reaction with saturated $NaCl$ solution forms sodium hydrogen carbonate. Compounds $A, B$ and $C$ are respectively:

Alkali metals possess metallic lustre when freshly cut because

Which of the following alkali metal ions has the lowest ionic mobility in an aqueous solution?