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Answer the following. (Class 10 Science 2023, Q34b)

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ScienceCBSE Class 10 · Board exam 2023, set 31/1/1 · 5 marks · Long answer

CBSE Class 10 Science 2023: question 34(b)

Set in the CBSE Class 10 board exam of 2023, paper 31/1/1, 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 answerCompetency-basedCarbon and its Compounds

Answer the following.

  1. (i)

    With the help of diagram, show the formation of micelles, when soap is applied on oily dirt.

    0 marks
  2. (ii)

    Take two test tubes X and Y with 10 mL of hard water in each.
    In test tube 'X', add few drops of soap solution and in test tube 'Y' add a few drops of detergent solution. Shake both the test tubes for the same period.
    (1) In which test tube the formation of foam will be more ? Why ?
    (2) In which test tube is a curdy solid formed ? Why ?

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/1/1.

Hints

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

Hint 1

Think about the two ends of a soap molecule: which end likes water and which likes oil?

Hint 2

Arrange the molecules around an oil drop so that each end is in the medium it likes.

Hint 3

Hard water contains Ca²⁺ and Mg²⁺ ions. Which cleansing agent forms an insoluble precipitate with them?

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) A soap molecule is a sodium salt of a long-chain carboxylic acid. It has an ionic (hydrophilic) end, , which dissolves in water, and a long hydrocarbon (hydrophobic) tail, which dissolves in oil.

    ½ mark

  2. When soap is applied on oily dirt, the hydrocarbon tails dissolve in the oil droplet. The ionic heads stay outside, facing the water.

    ½ mark

  3. Diagram (described): draw an oil droplet at the centre. Draw many soap molecules arranged radially around it, with zig-zag tails pointing inwards into the droplet and circular heads on the outside. Mark ions around the outside. Label: 'oil droplet', 'hydrophobic tail', 'ionic/hydrophilic head', 'micelle'. This spherical cluster is a micelle.

    ½ mark

  4. The micelles stay in solution as a colloid and do not come together, because their charged surfaces repel each other. When the cloth is rinsed with water, the oily dirt trapped inside the micelles is washed away.

    ½ mark

  5. (ii)(1) More foam forms in test tube Y (detergent).

    ½ mark

  6. Detergents do not form insoluble salts with the calcium and magnesium ions of hard water. So they stay effective in hard water and lather well.

    1 mark

  7. (ii)(2) A curdy solid forms in test tube X (soap).

    ½ mark

  8. Soap reacts with the calcium and magnesium salts in hard water and forms an insoluble precipitate called scum (curdy white solid). So little lather forms.

    1 mark

Answer: (i) Micelle: hydrocarbon tails dissolve in the oil droplet at the centre and ionic heads face the water outside (diagram); (ii)(1) Test tube Y, because detergents work in hard water; (2) Test tube X, because soap forms insoluble scum with Ca²⁺/Mg²⁺ salts

Where the marks are

  • Correct labelled diagram of a micelle around an oil droplet, showing hydrophobic tails inward and ionic heads outward, with explanation (2 marks)
  • Test tube Y (½ mark)
  • Reason: detergents are effective in hard water (1 mark)
  • Test tube X (½ mark)
  • Reason: soap forms insoluble scum with calcium/magnesium salts (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 paper2023 board paper, set 31/1/1: 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.