Which of the following reacts with water with the highest rate?

Consider the following statements.
$I$. $Cs^{+}$ ion is more highly hydrated than other alkali metal ions.
$II$. Among the alkali metals,only lithium forms a stable nitride by direct combination with nitrogen.
$III$. Among the alkali metals $Li, Na, K, Rb$,the metal $Rb$ has the highest melting point.
$IV$. Among the alkali metals $Li, Na, K, Rb$,only $Li$ forms peroxide when heated with oxygen.
Select the correct statement.

The metal that is extracted from sea water is

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

Which of the following chemicals,in addition to water,are used for the manufacture of $Na_2CO_3$ by the Solvay process?

Write the reaction of thermal decomposition of sodium azide.