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

(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 $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions, which are responsible for making water hard.

Hard waterCalcium ionsMagnesium ionsCalcium SulphateCalcium ChlorideMagnesium Chloride

Marking Scheme

1Identification of calcium and magnesium salts as the cause of water hardness (0.5 marks)
2Selection 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 ( $\text{Ca}^{2+}$ ) and magnesium ( $\text{Mg}^{2+}$ ). 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.

Analysis & Explanation

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

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

Why other options are incorrect:

(2) Sodium Sulphate ( $\text{Na}_2\text{SO}_4$ ), (4) Potassium Sulphate ( $\text{K}_2\text{SO}_4$ ), and (5) Sodium Hydrogen Carbonate ( $\text{NaHCO}_3$ ) release sodium ( $\text{Na}^+$ ) and potassium ( $\text{K}^+$ ) 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.

Common Mistakes

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

Interesting Facts

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

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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.