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(i) 'A' & 'B' are… (Class 10 Science SQP 2024-25, Q34B)

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ScienceCBSE Class 10 · Sample paper, session 2024-25 · 5 marks · Long answer

CBSE Class 10 Science sample paper 2024-25: question 34(B)

From the official CBSE sample question paper for 2024-25, 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

(i) 'A' & 'B' are sodium salts of long-chain carboxylic acid and long chain Sulphonic acid respectively. Which one of A or B will you prefer as a cleansing agent while using underground water (hand pump water)? Give the reason for your answer.
(ii) Elaborate on the process of cleansing action. Illustrate micelle with the help of labelled diagram.
(iii) Write the chemical equation of the preparation of soap from an ester . What is the name of this process?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Hand-pump water is hard. Which ions does hard water contain, and what do they do to ordinary soap?

Hint 2

A soap molecule has one end that likes water and one end that likes oil. Think about how many such molecules would arrange themselves around a droplet of grease.

Hint 3

An ester heated with NaOH splits into the sodium salt of the acid and an alcohol. Work out which acid and which alcohol make up CH₃COOCH₃.

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. (i) I would prefer B, the sodium salt of the long-chain sulphonic acid, which is a detergent.

    ½ mark

  2. Reason: Hand-pump (underground) water is hard because it contains and salts. With these ions, the soap A (a carboxylate) forms an insoluble scum, so the soap is wasted. The calcium and magnesium salts of sulphonic acids are soluble, so B forms no scum and still cleans well in hard water.

    ½ mark

  3. (ii) Structure of a soap molecule: A soap molecule has two ends that behave differently.

    • The ionic end () is hydrophilic and dissolves in water.
    • The long hydrocarbon chain is hydrophobic and dissolves in oil or grease.
    ½ mark

  4. Cleansing action: Most dirt is oily and does not dissolve in water. In soap solution, the hydrocarbon tails of many soap molecules dissolve in the oily dirt. The ionic heads stay outside, facing the water. In this way the molecules arrange themselves into spherical clusters called micelles, with the dirt trapped at the centre.

    1 mark

  5. Because the outer surfaces of the micelles are charged, the micelles stay suspended in water as an emulsion and do not come together. When the cloth is rinsed with water, the micelles carry the trapped dirt away, and the cloth becomes clean.

    ½ mark

  6. Labelled diagram of a micelle (described in words): Draw a circle for the oil/dirt droplet at the centre. Around it, draw many soap molecules arranged like spokes.

    • The zig-zag hydrocarbon tails (hydrophobic) point inwards into the oil droplet.
    • The round ionic heads (, hydrophilic) lie on the outer surface, facing the surrounding water.
    • A few ions are shown outside.

    Labels needed: oil droplet, hydrophobic tail, hydrophilic (ionic) head, , water.

    1 mark

  7. (iii) When the ester is heated with sodium hydroxide, it is hydrolysed to the sodium salt of the acid and the alcohol:

    The products are sodium ethanoate and methanol.

    ½ mark

  8. Making a soap by alkaline hydrolysis of an ester in this way is called saponification.

    ½ mark

Answer: (i) B (sulphonate/detergent): its Ca and Mg salts are soluble, so it forms no scum in hard water; (ii) the hydrophobic tails dissolve in oily dirt and the ionic heads face water, forming micelles that are rinsed away (labelled micelle diagram); (iii) CH₃COOCH₃ + NaOH → CH₃COONa + CH₃OH, saponification

Where the marks are

  • Chooses B and explains that Ca/Mg salts of sulphonic acids are soluble while carboxylates form scum in hard water (1 mark)
  • Explains hydrophilic head / hydrophobic tail, micelle formation and removal of dirt by rinsing (2 marks)
  • Correctly labelled micelle diagram (1 mark)
  • Balanced equation for alkaline hydrolysis of the ester (0.5 mark)
  • Names the process as saponification (0.5 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 paperSample paper 2024-25: 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.