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ICSE Class 7 Chemistry Question 8 of 13

Physical and Chemical Changes — Question 10

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Question 10

Photosynthesis is a natural process by which green plants manufacture food in the presence of sunlight.

(a) Are any new products formed during the above process?

(b) Can the change be reversed or is irreversible?

(c) State the conclusions which can be drawn, to represent photosynthesis as a physical or a chemical change.

Answer

(a) Glucose and oxygen are formed during photosynthesis. Hence, new products are formed.

(b) Carbon dioxide and water cannot be obtained back, from glucose, hence, the change is irreversible.

(c) Photosynthesis is a chemical change as it is irreversible and leads to the formation of new products.

ICSE Class VII Chemistry Chapter 2 12 Marks

Physical and Chemical Changes — Quick Study Guide

Changes around us are classified as physical (no new substance formed) or chemical (new substance formed). Understanding the difference is fundamental to chemistry.

Key Definitions

  • Physical change: No new substance formed, usually reversible (e.g., melting ice, dissolving sugar)
  • Chemical change: New substance formed, usually irreversible (e.g., burning, rusting, cooking)
  • Exothermic: Reaction that releases heat (e.g., burning fuel)
  • Endothermic: Reaction that absorbs heat (e.g., photosynthesis)

Six Signs of Chemical Change

Colour change, gas evolution, temperature change, precipitate formation, smell change, light/sound emission.

Exam Tips

  • Burning a candle = BOTH physical and chemical change
  • Dissolving salt/sugar in water = physical change (recoverable)
  • Not all physical changes are reversible (tearing paper)
  • Learn the comparison table (6 differences) thoroughly

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