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ICSE Class 7 Chemistry Question 6 of 13

Physical and Chemical Changes — Question 3

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Question 3

Give three reasons why melting of wax, is considered a physical change while burning of a candle, a chemical change.

Answer

Melting of wax is considered a physical change, whereas burning of candle is a chemical change because of the following reasons:

(a) Melting of wax is a temporary and reversible change whereas, burning a candle is a permanent and irreversible change.

(b) During melting of wax, heat energy is absorbed and on cooling it is evolved forming the original substance i.e., wax. Hence, no marked evolution or absorption of heat takes place. So, it is a physical change.
However, during the burning of candle, the wax decomposes. The products formed cannot combine back to give wax. Hence, it is a chemical change.

(c) During melting of wax no new substance is formed. The original substance is wax and the final product is also wax. Hence, it is a physical change.
During burning of candle new substances are formed. The original substance is the burning candle. The final products are carbon dioxide and water vapour. 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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