Manjeet wants to wash clothes, but the water available is hard. He compares the action of soap and detergent in hard water, to decide which one to use for washing his clothes. He observes: Soap forms scum in hard water. Detergent produces lather easily in hard water. Which of the following inferences is correct? (A) Ca²+ and Mg²+ ions react with soap, forming insoluble compounds, whereas detergents do not form such insoluble salts. (B) Soap is acidic, and detergents are basic. (C) Soap react with Na^+ ions in hard water to form scum while detergents do not react with Na^+ ions. (D) Detergents form lather easily because they are ionic compounds and react with Ca²+ and Mg²+ ions while soaps do not react with hard water.

ScienceCBSE Class 10 · Sample paper, session 2026-27 · 1 mark · MCQ

CBSE Class 10 Science sample paper 2026-27: question 19

From the official CBSE sample question paper for 2026-27, section B, question 19, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q19 · 1 mark · MCQCompetency-basedCarbon and its Compounds

Manjeet wants to wash clothes, but the water available is hard. He compares the action of soap and detergent in hard water, to decide which one to use for washing his clothes. He observes: Soap forms scum in hard water. Detergent produces lather easily in hard water. Which of the following inferences is correct?

(A)Ca $^{2 +}$ and Mg $^{2 +}$ ions react with soap, forming insoluble compounds, whereas detergents do not form such insoluble salts.
(B)Soap is acidic, and detergents are basic.
(C)Soap react with Na $^{+}$ ions in hard water to form scum while detergents do not react with Na $^{+}$ ions.
(D)Detergents form lather easily because they are ionic compounds and react with Ca $^{2 +}$ and Mg $^{2 +}$ ions while soaps do not react with hard water.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

Open one hint at a time and try again before the next.

Hint 1

Which ions cause hardness of water?

Hint 2

What is scum chemically, and does a detergent form it?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

Hard water contains $C a^{2 +}$ and $M g^{2 +}$ ions. These react with soap (sodium salts of long-chain carboxylic acids) to form insoluble calcium and magnesium salts, called scum, so lather does not form easily.
Detergents (sulphonate or ammonium salts) do not form insoluble salts with $C a^{2 +}$ and $M g^{2 +}$ , so they lather well in hard water.
So the correct inference is option (A). Options C and D are wrong because $N a^{+}$ does not cause hardness and detergents do not react with the hardness ions. Option B is wrong because soap solutions are basic.
1 mark

Answer: Option (A)

Where the marks are

Selects (A): Ca²⁺/Mg²⁺ form insoluble scum with soap but not with detergents (1 mark)

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Chapter: Carbon and its Compounds (notes, formulas, where marks are lost and every past question)

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CBSE Class 10 Science sample paper 2026-27: question 19

Hints

Step-by-step solution

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