Chapter 2 · Question 14
How is washing soda obtained from sodium chloride? What is water of crystallisation? Give the formulae of washing soda and gypsum with their respective water of crystallisation.
Q14
How is washing soda obtained from sodium chloride? What is water of crystallisation? Give the formulae of washing soda and gypsum with their respective water of crystallisation.
Answer Revealed
Direct Answer:
Washing soda () is obtained from sodium chloride through a sequence: first, baking soda () is produced from NaCl; baking soda on heating gives sodium carbonate (); recrystallisation of sodium carbonate from water yields washing soda: . Water of crystallisation is the fixed number of water molecules chemically attached to one formula unit of a salt. Washing soda has 10 water molecules () and gypsum has 2 water molecules ().
Simple Explanation
Washing soda is obtained from common salt through a short chain of reactions. Common salt (NaCl) is first used to make baking soda (), which on heating gives sodium carbonate (). Sodium carbonate is then recrystallised from water to get washing soda. Each formula unit has 10 fixed water molecules, so its formula is . Those attached water molecules are called water of crystallisation. Gypsum () has 2 water molecules. When blue copper sulphate crystals are heated, they turn white after losing their water of crystallisation, and the blue colour returns when water is added again.
Exam-Ready Structure
Washing soda and water of crystallisation are interlinked concepts. Washing soda's preparation illustrates the industrial value of common salt as a starting material, while water of crystallisation explains the hydrated nature of many salt crystals. 1. Preparation of washing soda: (a) Common salt (NaCl) is the raw material. (b) Step 1: NaCl is converted to sodium hydrogencarbonate (, baking soda) through the reaction . (c) Step 2: Baking soda on heating decomposes to sodium carbonate: . (d) Step 3: Sodium carbonate is dissolved in water and recrystallised to obtain washing soda: . 2. Water of crystallisation: (a) Definition: It is the fixed number of water molecules present in one formula unit of a salt. (b) These water molecules are chemically associated with the salt and are part of its crystal structure. (c) When a hydrated salt is heated, the water of crystallisation is driven off as water vapour. Activity 2.15 demonstrates this: heating blue copper sulphate crystals () in a dry boiling tube turns them into a white powder (anhydrous ), and water droplets appear on the inner walls of the tube. Adding a few drops of water to the white powder restores the blue colour. 3. Examples: (a) Washing soda: (sodium carbonate decahydrate) — 10 molecules of water of crystallisation. (b) Gypsum: (calcium sulphate dihydrate) — 2 molecules of water of crystallisation. 4. Uses of washing soda: in glass, soap, and paper industries; manufacturing sodium compounds like borax; domestic cleaning; removing permanent hardness of water.
Key Points
- Washing soda preparation: NaCl
- Water of crystallisation: fixed number of water molecules per formula unit of a salt
- (blue) (white) + ; adding water restores blue colour
- Washing soda: (10 water molecules); gypsum: (2 water molecules)
- Uses of washing soda: glass/soap/paper industries, domestic cleaning, softening hard water
Common Mistakes
- Confusing washing soda () with baking soda () — washing soda has more sodium and no hydrogen in the anion
- Writing the formula of washing soda without water of crystallisation — the correct formula includes the
Related Questions
Q13
How is baking soda prepared? Write its chemical equation for heating during cooking. Explain the difference between baking soda and baking powder, and how baking soda is useful as an antacid.
Q15