Question 291 markMCQSection B

Hard water required for an experiment is not available in a school laboratory. However, following salts are available in the laboratory. Select the salts which may be dissolved in water to make it hard for the experiment. (1) Calcium Sulphate (2) Sodium Sulphate (3) Calcium Chloride (4) Potassium Sulphate (5) Sodium Hydrogen Carbonate (6) Magnesium Chloride

Carbon and its Compounds
Hard Water and Soft Water

Options

(A)1, 2 and 4
(B)1, 3 and 6
(C)3, 5 and 6
(D)2, 4 and 5

Official answer

The correct option is B (1, 3 and 6). Dissolving Calcium Sulphate, Calcium Chloride, and Magnesium Chloride in water introduces Ca2+\text{Ca}^{2+} and Mg2+\text{Mg}^{2+} ions, which are responsible for making water hard.

Answer keywords:Hard waterCalcium ionsMagnesium ionsCalcium SulphateCalcium ChlorideMagnesium Chloride

Marking scheme

  1. 1.Identification of calcium and magnesium salts as the cause of water hardness (0.5 marks)
  2. 2.Selection of correct option B containing salts 1, 3, and 6 (0.5 marks)

Hint

Recall that water hardness is specifically caused by the dissolved salts of calcium (Ca2+\text{Ca}^{2+}) and magnesium (Mg2+\text{Mg}^{2+}). Look for the salts containing these cations.

Quick oral answer

Q: Why does soap not lather easily in hard water? A: Soap reacts with the calcium and magnesium ions present in hard water to form an insoluble, sticky precipitate called scum, which wastes soap and prevents lather formation.

Common mistakes

  1. 1.Students often confuse sodium or potassium salts with calcium or magnesium salts, thinking any sulphate or chloride causes hardness.
  2. 2.Confusing sodium hydrogen carbonate (which is baking soda) with calcium hydrogen carbonate (which causes temporary hardness).

Analysis and explanation

Hardness of water is caused by the presence of soluble hydrogen carbonates, chlorides, and sulphates of calcium and magnesium.

  • (1) Calcium Sulphate (CaSO4\text{CaSO}_4): Dissolves to release Ca2+\text{Ca}^{2+} ions, causing permanent hardness.
  • (3) Calcium Chloride (CaCl2\text{CaCl}_2): Dissolves to release Ca2+\text{Ca}^{2+} ions, causing permanent hardness.
  • (6) Magnesium Chloride (MgCl2\text{MgCl}_2): Dissolves to release Mg2+\text{Mg}^{2+} ions, causing permanent hardness.

Why other options are incorrect:

  • (2) Sodium Sulphate (Na2SO4\text{Na}_2\text{SO}_4), (4) Potassium Sulphate (K2SO4\text{K}_2\text{SO}_4), and (5) Sodium Hydrogen Carbonate (NaHCO3\text{NaHCO}_3) release sodium (Na+\text{Na}^+) and potassium (K+\text{K}^+) ions. These ions do not react with soap to form scum and do not cause hardness in water. Therefore, options A, C, and D are incorrect.

Interesting facts

Hard water is not harmful to health, but it causes scale buildup in pipes and boilers, reducing heating efficiency and clogging plumbing.

Question details

Q29 of 40 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2013
Section
Section B
Marks
1
Type
MCQ
Chapter
Carbon and its Compounds
Topic
Hard Water and Soft Water
Bloom's level
Understanding
Syllabus topic
Soaps and detergents; Cleansing action of soaps; Hard and soft water

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2013?

This question carries 1 mark in the CBSE Class 10 Science 2013 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Hard Water and Soft Water" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2013 paper?

This is a MCQ question from Section B in the CBSE Class 10 Science 2013 paper.