Science (086) — CBSE Class 10 board question

Steps for experimental verification that sunlight is essential for photosynthesis:

1. Take a healthy potted plant and keep it in a dark room for three days so that all existing starch is used up (destarch the plant).
2. Cover a portion of a leaf on both sides with black paper strips to block sunlight, while leaving the rest of the leaf exposed.
3. Place the plant in bright sunlight for about six hours.
4. Pluck the leaf; remove the black paper.
5. Boil the leaf in water, then in alcohol (using a water-bath) to remove chlorophyll.
6. Dip the leaf in dilute iodine solution.
7. Observation: The exposed (sunlit) area turns blue-black (starch present); the covered area remains brown (no starch).
8. Conclusion: Sunlight is essential for photosynthesis, as starch is produced only in areas that received light.

Source: Life Processes, Section 5.2.1 (Activity based on Activities 5.1 & 5.2)

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• Examiners expect the answer as sequential steps — destarching → partial covering → sunlight exposure → destaining in alcohol → iodine test → observation → conclusion.
• Key terms to include: destarch, black paper, alcohol (chlorophyll removal), iodine test, blue-black colour.
• The contrast between covered and uncovered portions is the core proof — always state both results.
• "Water-bath" for alcohol heating is a safety detail that can earn an extra half-mark.