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What are soaps ? Write the… (Class 10 Science 2025, Q34b)

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ScienceCBSE Class 10 · Board exam 2025, set 31/6/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 34(b)

Set in the CBSE Class 10 board exam of 2025, paper 31/6/1, section D, question 34(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(b) · 5 marks · Long answerCarbon and its Compounds

What are soaps ? Write the structure of a soap molecule. Explain the cleansing action of a soap. Why are soaps not considered suitable for washing clothes in a region where water is hard ? How is this problem overcomed ?

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/6/1.

Hints

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

Hint 1

Soaps are salts. Of which metals, and of which kind of acid?

Hint 2

Think of the two ends of a soap molecule: which end likes water and which likes oil? How do they arrange around oily dirt?

Hint 3

Which metal ions make water hard, and what do they form with soap?

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.

  1. Soaps are sodium or potassium salts of long-chain carboxylic (fatty) acids, e.g. sodium stearate .

    1 mark

  2. Structure of a soap molecule: a long hydrocarbon tail, –, attached to an ionic head, . The ionic head is hydrophilic (water-loving). The hydrocarbon tail is hydrophobic (water-repelling) and dissolves in oil.

    1 mark

  3. Cleansing action: most dirt is oily and does not dissolve in water. In soap solution, the hydrocarbon tails of soap molecules attach to the oily dirt, while the ionic heads face the water. The molecules form spherical clusters called micelles, with the oily dirt trapped in the centre. This forms an emulsion in water, and rinsing washes the micelles and the dirt away, leaving the clothes clean.

    1 mark

  4. Hard water contains calcium and magnesium salts. These react with soap to form an insoluble, curdy precipitate called scum. Much soap is wasted, little lather forms and the clothes are not cleaned properly.

    1 mark

  5. Solution: use detergents, which do not form insoluble precipitates with calcium and magnesium ions and work well in hard water. Alternatively, remove the hardness of the water before washing.

    1 mark

Answer: Soaps are Na/K salts of long-chain carboxylic acids with a hydrophilic ionic head and a hydrophobic tail; micelles trap oily dirt; hard water (Ca²⁺, Mg²⁺) forms scum with soap; use detergents or remove hardness.

Where the marks are

  • Definition of soap (1 mark)
  • Structure with hydrophilic head and hydrophobic tail (1 mark)
  • Cleansing action through micelle formation (1 mark)
  • Ca/Mg salts in hard water form scum with soap (1 mark)
  • Use of detergents or removal of hardness (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Carbon and its Compounds, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/6/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.