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Physical and Chemical Changes — Question 5

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Question 5

A burning candle shows both physical and chemical changes. Explain.

Answer

When a candle is lighted it, the carbon present in wax oxidises and produces carbon dioxide and water vapours as the new substances and along with it there is evolution of heat and light. This shows a chemical change. The candle also becomes smaller and smaller. When a candle burns wax melts and turns into liquid state that falls on floor and again it solidifies which shows a physical change. So burning of a candle is both a physical and 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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