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Sara took 2 mL of… (Class 10 Science SQP 2025-26, Q28)

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ScienceCBSE Class 10 · Sample paper, session 2025-26 · 4 marks · Case-based

CBSE Class 10 Science sample paper 2025-26: question 28

From the official CBSE sample question paper for 2025-26, section B, question 28, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q28 · 4 marks · Case-basedCompetency-basedAcids, Bases and Salts

Sara took 2 mL of dilute NaOH solution in a test tube and added two drops of phenolphthalein solution to it. The solution turned pink in colour. She added dilute HSO to the above solution drop by drop until the solution in the test tube became colourless. 40 drops of dilute HSO were used for the change in colour from pink to colourless. When Sara added a drop of NaOH to the solution, the colour changed back to pink again. Sara now tried the activity with different volumes of NaOH and recorded her observation in the table given below:

S. No.Volume of dil. NaOH taken (mL)Drops of dil. HSO used
1220
2330
3440

Answer the following questions based on the above information:

  1. A

    If Sara used concentrated HSO in place of dilute HSO, how many drops will be required for the change in colour to be observed?
    (a) 40
    (b) < 40
    (c) > 40
    Justify your answer.

    1 mark
  2. B

    Sara measured 20 drops of dil. HSO and found its volume to be 1 mL. If Sara observed a change in colour of NaOH solution by using 3 mL of HSO, how many mL of NaOH did she add to the test tube initially?

    1 mark

  3. OR

    B (OR)

    Sara takes 10 drops of dilute HSO in the test tube and adds two drops of phenolphthalein solution to it. Then she adds NaOH dropwise. Sara observes a change in colour after adding 20 drops of NaOH. What change in colour would she observe and why?

    1 mark
  4. C

    Write a balanced chemical equation for the reaction taking place in the above experiment. Which of the following is true and why? The reaction is a
    (a) neutralisation and double displacement reaction
    (b) neutralisation and precipitation reaction
    (c) precipitation and double displacement reaction
    (d) neutralisation, double displacement as well as precipitation reaction.

    2 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Look at the table: how many drops of acid are needed per mL of NaOH?

Hint 2

Convert 3 mL of acid into drops using 20 drops = 1 mL.

Hint 3

Is any product of the reaction insoluble?

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. A. Option (b) < 40. Concentrated supplies more ions per drop than the dilute acid, so fewer drops neutralise the same amount of NaOH.

    1 mark

  2. B. From the table, 2 mL NaOH needs 20 drops = 1 mL of acid. So 1 mL of acid neutralises 2 mL of NaOH. Then 3 mL of acid (60 drops) neutralises 6 mL of NaOH.

    1 mark

  3. B (OR) (alternative to B, 1 mark): The colour changes from colourless to pink. Phenolphthalein is colourless in the acid. Once all the acid is neutralised, the next drop of NaOH makes the solution basic and the indicator turns pink.

  4. C.

    1 mark

  5. Option (a) neutralisation and double displacement is true. A base reacts with an acid to give salt and water (neutralisation), and the ions are exchanged: pairs with and with (double displacement). It is not a precipitation reaction because is soluble in water.

    1 mark

Answer: A. (b) < 40; B. 6 mL NaOH (OR: colourless to pink); C. , option (a)

Where the marks are

  • (b) < 40 with reason: more H⁺ per drop (1 mark)
  • 6 mL NaOH using 1 mL acid : 2 mL NaOH, or colourless to pink with reason (1 mark)
  • Balanced equation (1 mark)
  • Option (a) with justification, including Na₂SO₄ being soluble (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: 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.