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ICSE Class 7 Chemistry Question 10 of 13

Physical and Chemical Changes — Question 12

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Question 12

Explain in brief the involvement of energy in - physical & chemical changes.

Answer

Physical changes use energy to change the state of matter.

Chemical changes release or absorb energy when changing a substance into a new substance.

  • Energy is released during decomposition of a substance, while
  • Energy is absorbed during formation of a new substance.

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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