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

1. Preparation of washing soda:

1. A cold and concentrated solution of sodium chloride is reacted with ammonia (NH₃) and carbon dioxide to form sodium hydrogen carbonate (NaHCO₃) and ammonium chloride (NH₄Cl). Sodium hydrogen carbonate precipitates as a solid as it is slightly soluble in water.

NaCl + NH₃ + H₂O + CO₂ → NaHCO₃ + NH₄Cl

2. The sodium hydrogen carbonate precipitated is separated by filtration, and then dried and heated. On heating, it decomposes to form sodium carbonate (Na₂CO₃). The anhydrous sodium carbonate formed here is called soda ash.

2NaHCO₃ → Na₂CO₃ + CO₂+ H₂O

3. The soda ash is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation.

Na₂CO₃ + 10H₂O → Na₂CO₃.10H₂O

2. An aqueous solution of washing soda is alkaline, because sodium carbonate is a salt prepared from the reaction between a weak acid, carbonic acid (H₂CO₃), and a strong base, sodium hydroxide (NaOH). When sodium carbonate is dissolved in water, it gets hydrolysed to some extent and forms sodium hydroxide and carbonic acid.

Na₂CO₃(s) + 2H₂O(l) → 2NaOH(aq) + H₂CO₃(aq)
Sodium hydroxide, being a strong base, is fully ionised to give a large amount of hydroxide ions (OH^-), and carbonic acid, being a weak acid, is slightly ionised to form a small amount of hydrogen (H⁺) ions. Therefore, an aqueous solution contains more hydroxide ions than hydrogen ions, which makes the solution basic.

3. By saying that washing soda has detergent properties, or cleansing properties, one means that it can remove dirt and grease from dirty clothes, etc. It attacks dirt and grease to form water-soluble products, which are washed away on rinsing with water.

4. Uses of washing soda:

1. It is used for removing permanent hardness of water.
2. It is used in the manufacture of glass, soap, paper and sodium compounds, such as borax.