Mercury is transported in containers made of

The number of alkali metal$(s)$,from $Li$,$K$,$Cs$,$Rb$ having ionization enthalpy greater than $400 \ kJ \ mol^{-1}$ and forming stable super oxide is $.......$

The strongest reducing agent among the alkali metals is

Identify the correct statements about alkali metals.
$A$. The order of standard reduction potential $(M^{+} \mid M)$ for alkali metal ions is $Na > Rb > Li$.
$B$. $CsI$ is highly soluble in water.
$C$. Lithium carbonate is highly stable to heat.
$D$. Potassium dissolved in concentrated liquid ammonia is blue in colour and paramagnetic.
$E$. All the alkali metal hydrides are ionic solids.
Choose the correct answer from the options given below.

State the reasons for the following:
$(a)$ Why is a solution of $Na_{2}CO_{3}$ alkaline?
$(b)$ Why are alkali metals prepared by the electrolysis of their fused chlorides?
$(c)$ Why is sodium found to be more useful than potassium in biological systems?

When sodium is dissolved in liquid $NH_3$,the solution becomes blue due to the presence of: