Science — CBSE Class 10 board question

Aim: To demonstrate that CO₂ is essential for photosynthesis.

Materials Required: Two healthy potted plants, two bell jars, two glass plates, watch-glass, potassium hydroxide (KOH), vaseline, iodine solution, alcohol.

Procedure:

1. Keep both potted plants in a dark room for three days to destarch them.
2. Place each plant on a separate glass plate. Place a watch-glass containing KOH beside Plant A (KOH absorbs CO₂). Leave no KOH beside Plant B.
3. Cover both plants with separate bell jars. Seal the base with vaseline to make the setup airtight.
4. Keep both plants in sunlight for two hours.
5. Pluck a leaf from each plant; boil in water, then in alcohol to remove chlorophyll. Test with iodine solution.

Observation:

• Leaf from Plant A (with KOH) — no blue-black colour with iodine (no starch formed).
• Leaf from Plant B (without KOH) — turns blue-black (starch present).

Conclusion: Plant A, deprived of CO₂, could not perform photosynthesis and hence produced no starch. This proves that carbon dioxide is essential for photosynthesis.

Source: Life Processes, Activity 5.2

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• Examiners expect a named setup (bell jar + KOH), a clear control (Plant B without KOH), and the starch test as the detection method.
• State observations for both plants separately — this earns dedicated marks.
• The conclusion must explicitly link absence of CO₂ → no starch → CO₂ essential for photosynthesis.
• KOH's role (absorbing CO₂) should be mentioned; it is a key detail that differentiates Plant A from Plant B.