Which compound has the lowest thermal stability?

Which of the following is Indian saltpetre?

$Na_2CO_3$ can be manufactured by Solvay's process,but $K_2CO_3$ cannot be prepared by this method because:

Comment on each of the following observations:
$(a)$ The mobilities of the alkali metal ions in aqueous solution are
$Li^{+} < Na^{+} < K^{+} < Rb^{+} < Cs^{+}$
$(b)$ Lithium is the only alkali metal to form a nitride directly.
$(c)$ $E^{\circ}$ for $M^{2+}_{(aq)} + 2e^{-} \longrightarrow M_{(s)}$ (where $M = Ca, Sr$ or $Ba$) is nearly constant.

Select the correct statement.

Which of the following is the weakest reducing agent?