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Acids, Bases and Salts — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceAcids, Bases and Salts5 Marks
Question
Describe how washing soda is produced starting from sodium chloride (common salt). Write equations of all the reactions involved.
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Answer

  1. Production of washing soda: Washing soda is produced from sodium chloride (or common salt) in the following three steps:
  1. A cold and concentrated solution of sodium chloride (called brine) is reacted with ammonia and carbon dioxide to obtain sodium hydrogen carbonate:
$\ \ \ \ \ \ \text{N}\bar{\text{a}}\text{Cl}\ \ +\ \ \text{NH}_3\ \ +\ \ \text{H}_2\text{O}\ \ +\ \ \text{CO}_2\rightarrow\text{NaHCO}_3\ \ +\ \ \text{NH}_4\text{Cl}\\^{\text{Sodium chloride}}\ ^{\text{Ammonia}}\ \ \ \ \ \ ^{\text{Water}}\ \ \ \ \ \ \ \ ^\text{Carbon}\ \ \ ^{\text{Sodium hydrogen}}\ \ \ \ \ \ ^{\text{Ammonium}}\\\ ^{\text{(Common Salt)}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^{\text{dioxde}}\ \ \ \ \ \ ^{\text{Carbonate}}\ \ \ \ \ \ \ \ \ \ \ \ ^{\text{Chloride}}$

Sodium hydrogen carbonate formed is only slightly soluble in water, so it precipitates out as a solid.
  1. Sodium hydrogen carbonate is separated by filtration, dried and heated. On heating, sodium hydrogen carbonate decomposes to form sodium carbonate:
$\ \ 2\text{NaCHO}_3\ \ \xrightarrow{\text{Heat}}\ \ \text{Na}_2\text{CO}_3\ \ +\ \ \text{CO}_2\ \ +\ \ \text{H}_2\text{O}\\^{\text{Sodium hydrogen}}\ \ \ \ \ \ \ \ ^{\text{Sodium carbonate}}\ \ \ ^{\text{Carbonate}}\ \ \ \ \ \ ^{\text{Water}}\\\ \ \ \ \ ^\text{Carbonate}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^{\text{(Sodaash)}}\ \ \ \ \ \ \ \ \ \ ^{\text{dioxide}}$

The anhydrous sodium carbonate obtained here is called soda ash.
  1. Anhydrous sodium carbonate (soda ash) is dissolved in water and recrystallised to get washing soda crystals containing 10 molecules of water of crystallisation:
$\ \ \ \ \ \ \ \ \ \ \text{Na}_2\text{CO}_3\ \ \ \ \ \ \ \ \ +1\text{CH}_2\text{O}\rightarrow\ \ \ \text{Na}_2\text{CO}_3,1\text{CH}_2\text{O}\\^{\text{Anhydroussdiuumcarbonate}}\ \ \ \ \ \ ^{\text{Water}}\ \ \ \ \ \ \ \ ^{\text{Sodiumcarbonatedecahydrate}}\\\ \ \ \ \ \ \ \ \ \ \ \ \ ^{\text{(Sodaas)}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^{\text{(Washingsoda)}}$
  1. An aqueous solution of washing soda is alkaline because it turns red litmus to blue.
  2. Washing soda has detergent properties because it can remove dirt and grease from dirty clothes.
  1.  
  1. It is used as cleansing agent for domestic purposes.
  2. It is used for removing permanent hardness of water.

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