On heating to $400-500\,^oC$,relatively unstable hydrides and carbonates decompose. Which of the following will decompose when heated to $400-500\,^oC$ ?
$(I) \ LiH$ $ $ $(II) \ NaH$ $ $ $(III) \ Li_2CO_3$ $ $ $(IV) \ Na_2CO_3$

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.

The solubility of metal halides depends on their nature,lattice enthalpy and hydration enthalpy of the individual ions. Amongst fluorides of alkali metals,the lowest solubility of $LiF$ in water is due to

Given below are two statements,one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: The alkali metals and their salts impart characteristic colour to reducing flame.
Reason $R$: Alkali metals can be detected using flame tests.
In the light of the above statements,choose the most appropriate answer from the options given below.

Which of the following metals forms stable carbonates that do not decompose upon heating?

In the case of alkali metals,the covalent character decreases in the order