Question 163 marksShort AnswerSection B

List the important products of the Chlor-alkali process. Write one important use of each.

Acids, Bases and Salts
Washing Soda

Official answer

Washing soda is prepared by the recrystallization of sodium carbonate.


Chemical Equation:

Na2CO3(s)+10H2O(l)→Na2CO3⋅10H2O(s)Na_2CO_3(s) + 10H_2O(l) \rightarrow Na_2CO_3 \cdot 10H_2O(s)


Type of Salt:

It is a basic salt.


Type of Hardness of Water Removed:

It is used to remove permanent hardness of water.

Answer keywords:RecrystallizationSodium carbonate decahydrateBasic saltPermanent hardnessWater of crystallization

Marking scheme

  1. 1.Stating that washing soda is prepared by the recrystallization of sodium carbonate. (0.5 marks)
  2. 2.Writing the balanced chemical equation: Na2CO3+10H2O→Na2CO3⋅10H2ONa_2CO_3 + 10H_2O \rightarrow Na_2CO_3 \cdot 10H_2O (1 mark)
  3. 3.Identifying the salt as a basic salt. (0.5 marks)
  4. 4.Identifying that it removes permanent hardness of water. (1 mark)

Hint

Recall that washing soda is sodium carbonate decahydrate (Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O) and is a basic salt used to treat hard water.

Quick oral answer

Q: What is the chemical formula of washing soda? A: The chemical formula of washing soda is Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O. Q: Why does washing soda turn red litmus blue? A: Because it is a basic salt formed from a strong base (NaOHNaOH) and a weak acid (H2CO3H_2CO_3), and its aqueous solution is alkaline.

Common mistakes

  1. 1.Writing the chemical formula of washing soda as Na2CO3Na_2CO_3 instead of Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O.
  2. 2.Confusing permanent hardness with temporary hardness.
  3. 3.Classifying washing soda as an acidic or neutral salt.

Analysis and explanation

Washing soda (Na2CO3⋅10H2ONa_2CO_3 \cdot 10H_2O) is a sodium salt containing ten molecules of water of crystallization. When anhydrous sodium carbonate (soda ash) is treated with water, it undergoes recrystallization, which is a highly controlled hydration process. Because it is formed from a strong alkali (NaOHNaOH) and a weak acid (H2CO3H_2CO_3), its aqueous solution undergoes hydrolysis to yield hydroxide ions (OH−OH^-), making it basic. It is highly effective in removing permanent hardness of water caused by dissolved chlorides and sulfates of calcium and magnesium by precipitating them as insoluble carbonates.

Interesting facts

Anhydrous sodium carbonate (Na2CO3Na_2CO_3) is commonly known as 'soda ash'.

Washing soda is widely used in the glass, soap, and paper industries, as well as a cleaning agent for domestic purposes.

Question details

Q16 of 39 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2020
Section
Section B
Marks
3
Type
Short Answer
Chapter
Acids, Bases and Salts
Topic
Washing Soda
Bloom's level
Understanding
Syllabus topic
Salts: Sodium hydroxide, Bleaching powder, Baking soda, Washing soda, Plaster of Paris.

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Washing Soda" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.