Question

Describe how washing soda is produced starting from sodium chloride (common salt). Write equations of all the reactions involved.

Answer 1

(i) A cold and concentrated solution of sodium chloride (called brine) is reacted with ammonia and carbon dioxide to obtain sodium hydrogencarbonate.

\[\ce{\underset{(Common salt)}{\underset{Sodium chloride}{NaCl}} + \underset{Ammonia}{NH3} + \underset{Water}{H2O} + \underset{dioxide}{\underset{carbon}{CO2}} -> \underset{carbonate}{\underset{Sodium hydrogen}{NaHCO3}} + \underset{chloride}{\underset{Ammonium}{NH4Cl}}}\]

Sodium hydrogencarbonate formed is only slightly soluble in water, so it precipitates out as a solid.

(ii) Sodium hydrogencarbonate is separated by filtration, dried and heated. On heating, sodium hydrogencarbonate decomposes to form sodium carbonate.

\[\ce{\underset{Sodium hydrogencarbonate}{2NaHCO3} ->[Heat] \underset{(Soda ash)}{\underset{Sodium carbonate}{Na2CO3}} + \underset{dioxide}{\underset{carbon}{CO2}} + \underset{Water}{H2O}}\]

The anhydrous sodium carbonate obtained here is called soda ash.

(iii) Anhydrous sodium carbonate (soda ash) is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation.

\[\ce{\underset{(Soda ash)}{\underset{Anhydrous sodium carbonate}{Na2CO3}} + \underset{Water}{10H2O} -> \underset{(Washing soda)}{\underset{Sodium carbonate decahydrate}{Na2CO3 * 10H2O}}}\]