Read the following passage and answer the questions that follow.
During a science fair, Priya set up two identical healthy potted plants of the same species and size. She placed both in a dark room for three days. She then placed Plant A under a bell jar sealed airtight to a glass plate, with a watch-glass of potassium hydroxide inside the jar. Plant B was placed under a similar sealed bell jar but without any potassium hydroxide. Both plants were kept in bright sunlight for two hours. Priya then tested a leaf from each plant for the presence of starch.
(i) What was the purpose of keeping the plants in a dark room for three days before the experiment? (1 mark)
(ii) What result did Priya observe when she tested the leaf of Plant A with iodine solution? Give a reason. (1 mark)
(iii) What role does potassium hydroxide play in this experiment? (1 mark)
(iv) What conclusion can Priya draw from comparing the results of both plants? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:00 · grounding rag
Model Answer
(i) The plants were kept in the dark room for three days to destarch them — i.e., to ensure all previously stored starch in the leaves was used up, so that any starch found later is produced only during the experiment.
(ii) The leaf of Plant A showed no colour change (remained brown/orange) with iodine solution, indicating absence of starch. This is because potassium hydroxide absorbs the CO₂ inside the jar, so Plant A had no CO₂ available for photosynthesis and could not produce starch.
(iii) Potassium hydroxide (KOH) absorbs carbon dioxide from the air inside the sealed bell jar, making CO₂ unavailable to Plant A for photosynthesis.
(iv) Priya can conclude that carbon dioxide is necessary for photosynthesis. Plant B (with CO₂) produced starch and turned blue-black with iodine, while Plant A (without CO₂) did not.
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Explanation
- This is a classic "necessity of CO₂ for photosynthesis" experiment from Life Processes (Chapter 6, CBSE Class 10).
- The source passages provided are from Chemistry and Respiration; however, this experiment is standard NCERT biology content — answer is based on established board exam expectations.
- Key terms examiners look for: destarching, KOH absorbs CO₂, iodine test, blue-black = starch present, and the conclusion linking CO₂ to photosynthesis.
- Each sub-question is 1 mark — keep each answer to 1–2 lines.