Physical and Chemical Changes — Question 9
Back to all questionsQuestion 9
State why addition of sodium chloride to water is considered a physical change, while addition of dilute sulphuric acid to zinc is considered a chemical change.
Addition of sodium chloride (salt) to water is considered a physical change because:
(a) It is a temporary and reversible change. On adding, the salt dissolves in water and forms a salt solution. On evaporation of the salt solution the original salt remains behind. Hence, it is a reversible change.
(b) No new substances are formed. Original and the final product is the same [NaCl].
Addition of dilute sulphuric acid to zinc is a chemical change because :
Zinc + dil. Sulphuric Acid ⟶ Zinc Sulphate + Hydrogen gas
(a) It is a permanent and irreversible change.
(b) New substances zinc sulphate and hydrogen gas are formed.
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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