Write the preparation and properties of sodium hydroxide (caustic soda).
 

Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell. A brine solution, a highly concentrated water solution of common salt (sodium chloride) is electrolyzed using a mercury cathode and a carbon anode. Sodium metal discharged at the cathode combines with mercury to form sodium amalgam. Chlorine gas is evolved at the anode.

Cathode: ${\mathrm{Na}}^{+}+{\mathrm{e}}^{-}\stackrel{\mathrm{Hg}}{\to }\mathrm{Na}-\mathrm{amalgam}$

Anode: ${\mathrm{Cl}}^{-}\to \frac{1}{2}{\mathrm{Cl}}_2+{\mathrm{e}}^{-}$

The amalgam is treated with water to give sodium hydroxide and hydrogen gas.

$2\mathrm{Na}-\mathrm{amalgam}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}+2\mathrm{Hg}+{\mathrm{H}}_2$

Sodium hydroxide is a white, translucent solid. It melts at 591 K. It is readily soluble in water to give a strongly alkaline solution. Crystals of sodium hydroxide are deliquescent. The sodium hydroxide solution at the surface reacts with the ${\mathrm{CO}}_2$ present in the atmosphere to form ${\mathrm{Na}}_2{\mathrm{CO}}_3$