Question

(a) Describe how washing soda is produced starting from sodium chloride (common salt). Write equations of all the reactions involved.
(b) State whether an aqueous solution of washing soda is acidic or alkaline? Give reason for your answer.
(c) What is meant by saying that washing soda has detergent properties?
(d) Give two important uses of washing soda (or sodium carbonate).

Answer 1

(a) Production of washing soda: Washing soda is produced from sodium chloride (or common salt) in the following three steps:

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

$\underset{Sodiumchloride\left(commonsalt\right)}{NaCl}+\underset{Ammonia}{N{H}_3}+\underset{Water}{H_{2}O}+\underset{Carbondioxide}{C{O}_2}\to \underset{Sodiumhydrogencarbonate}{NaHC{O}_3}+\underset{Ammoniumchloride}{N{H}_{4}Cl}$

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

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

$\underset{Sodiumhydrogencarbonate}{2NaHC{O}_3}\stackrel{Heat}{\to }\underset{Sodiumcrbonate\left(sodaash\right)}{N{a}_{2}C{O}_3}+\underset{carbondioxide}{C{O}_2}+\underset{water}{H_{2}O}$

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 :

$\underset{Anhydroussodiumcarbonate\left(Sodaash\right)}{N{a}_{2}C{O}_3}+\underset{water}{10H_{2}O}⟶\underset{Sodiumcarbonatedecahydrate\left(washingsoda\right)}{N{a}_{2}C{O}_3.10H_{2}O}$

(b) An aqueous solution of washing soda is alkaline because it is made of a strong base and a weak acid.

(c) Washing soda has detergent properties because it can remove dirt and grease from dirty clothes.

(d) Two important uses of washing soda are-

(i) It is used as a cleansing agent for domestic purposes.

(ii) It is used for removing permanent hardness of water.