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Design an experiment to… (Class 10 Science 2024, Q35a)

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ScienceCBSE Class 10 · Board exam 2024, set 31/5/1 · 5 marks · Long answer

CBSE Class 10 Science 2024: question 35(a)

Set in the CBSE Class 10 board exam of 2024, paper 31/5/1, section D, question 35(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q35(a) · 5 marks · Long answerLife Processes

Design an experiment to demonstrate that carbon dioxide is essential for photosynthesis. Write the observation and conclusion of the experiment.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/5/1.

Hints

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

Hint 1

You need two identical set-ups that differ only in whether is available. What chemical absorbs ?

Hint 2

First remove the starch already present in the leaves. After exposure to light, which test shows whether new starch has been made?

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. Take two healthy potted plants, A and B, of nearly the same size.

    ½ mark

  2. Keep both in darkness for about three days so that their leaves are destarched.

    ½ mark

  3. Place a watch glass containing potassium hydroxide (KOH) beside plant A only; plant B gets no KOH. KOH absorbs carbon dioxide.

    ½ mark

  4. Cover each plant with a separate bell jar on a glass plate, and seal the bottom of the jars with Vaseline so that the set-up is airtight.

    ½ mark

  5. Keep both plants in sunlight for about two hours.

    ½ mark

  6. Pluck a leaf from each plant, boil it in water and then in alcohol to remove chlorophyll, wash it and test it with iodine solution for starch.

    ½ mark

  7. Observation: the leaf of plant A (with KOH) does not turn blue-black. The leaf of plant B turns blue-black, showing that starch is present.

    1 mark

  8. Conclusion: KOH absorbed the in jar A, so no photosynthesis (no starch) took place in plant A. Hence carbon dioxide is essential for photosynthesis.

    1 mark

Answer: Destarched plants A (with KOH) and B (without) are kept under sealed bell jars in sunlight and their leaves tested with iodine. Leaf A shows no blue-black colour and leaf B turns blue-black, so is essential for photosynthesis.

Where the marks are

  • Takes two similar potted plants (½ mark)
  • Destarches them by keeping in the dark (½ mark)
  • Places KOH beside only one plant (½ mark)
  • Covers with bell jars sealed with Vaseline (½ mark)
  • Keeps the plants in sunlight (½ mark)
  • Tests a leaf from each with iodine (½ mark)
  • Gives the correct observation for both leaves (1 mark)
  • Concludes that is essential, since KOH absorbed it (1 mark)

Revise the chapter

  • Chapter: Life Processes (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Life Processes, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 31/5/1: 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.