Q1. [3] medium thorough-understanding
Starting from common salt (NaCl), outline the sequence of steps by which washing soda (Na₂CO₃·10H₂O) is obtained industrially. Why is the recrystallisation step essential, and what role does the water of crystallisation play in the properties of washing soda?
Generated by claude-sonnet-4-6 · 2026-06-26 01:05 · grounding rag
Model Answer
Steps to obtain washing soda from NaCl:
- Electrolyse brine (NaCl solution) → NaOH (chlor-alkali process).
- React NaCl + H₂O + CO₂ + NH₃ → NaHCO₃ (baking soda).
- Heat NaHCO₃: $2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2$
- Recrystallise Na₂CO₃: $\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3\cdot10\text{H}_2\text{O}$ (washing soda).
Recrystallisation is essential to incorporate the water of crystallisation and obtain the pure crystalline form of washing soda.
Water of crystallisation is the fixed number (10) of water molecules present per formula unit. It gives washing soda its crystalline structure; the compound is not wet, but the water is chemically bound within the crystal lattice.
Source: Chapter 2, Section 2.4.3 & 2.4.4
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Explanation
- Examiners expect the sequence to be clear: NaCl → baking soda → Na₂CO₃ (by heating) → washing soda (by recrystallisation). Missing any step loses marks.
- The equation for recrystallisation and for heating NaHCO₃ are commonly asked — write them.
- For water of crystallisation, the key point is that it is chemically fixed, not free water, so the salt is not "wet" — this directly addresses the textbook question "Does 10H₂O make Na₂CO₃ wet?"
- Avoid padding; three tight points covering steps, recrystallisation purpose, and water of crystallisation role are sufficient for 3 marks.