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ICSE Class 7 Chemistry Question 9 of 13

Physical and Chemical Changes — Question 11

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Question 11

Give reasons why - separation of mixtures e.g., iron from a mixture of iron and sulphur is a physical change, but heating a mixture of iron and sulphur is considered a chemical change.

Answer

As separation of iron and sulphur from its mixture is a

  • reversible process,
  • no new products are formed and
  • mixture has the same properties as iron and sulphur individually have. Hence, it is a physical change.

However, on heating of iron and sulphur,

  • a new product iron sulphide is formed
  • this new product [iron sulphide] has completely different properties from iron and sulphur.
  • it is an irreversible process. Hence, it is a chemical change.

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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