Choose the correct order of density of the alkali metals.

The chemical formula of feldspar is

When carbon monoxide is passed over solid caustic soda heated to $200^{\circ} C$,it forms

As compared to lithium,sodium reacts more quickly with water because:

Identify the correct statements from the following :
$I$. Tendency to form halide hydrates gradually increases from $Be$ to $Ba$ down the group.
$II$. Tendency to form stable superoxides increases from $Li$ to $Cs$ down the group.
$III$. Low solubility of $LiF$ is due to its high lattice energy.
$IV$. Solubility of carbonates of group-$2$ elements increases down the group.

The alkali metals form salt-like hydrides by direct synthesis at elevated temperatures. The thermal stability of these hydrides decreases in which of the following orders?