Which one of the alkali metal can directly react with N₂ of air?

Li⁺ has the highest hydration enthalpy.

Hence it is most hydrated

Therefore, Correct order of hydrated radii is Cs⁺ < Rb⁺ < K⁺ < Na⁺ < Li⁺

Sodium carbonate is a white crystalline solid which exists as a decahydrate,  This is also called washing soda.

Suspension of slaked lime in water is known as milk of lime

Crude sodium chloride obtained by crystallisation of brine solution contains CaSO₄, Na₂SO₄ and MgCl₂

Beryllium salts are easily hydrolysed

Gypsum : CaSO₄ * 2H₂O

$N{H}_3$ is recovered when solution containing $N{H}_{4}Cl$ is treated with $Ca{\left(OH\right)}_2$   (slaked lime).

$2N{H}_{4}Cl+Ca{\left(OH\right)}_2\to 2N{H}_3+CaC{l}_2+H_{2}O$ .