Changes Around Us: Physical and Chemical – Class 7 Science notes, important questions and MCQs

Chapter overview

Changes are physical (no new substance forms, often reversible) or chemical (new substances form, usually irreversible).

Physical changes

• Change in shape, size or state: melting ice, tearing paper, dissolving sugar, boiling water.
• No new substance is formed; often reversible.
• Crystallisation gives pure crystals of a substance from its solution.

Chemical changes

• New substance forms: burning wood, rusting, cooking food, curdling of milk, ripening fruit.
• Signs: heat or light given out, gas formed, colour change, smell change.
• Baking soda + vinegar → carbon dioxide gas (bubbles).

Key terms

Physical change

change in which no new substance forms

Chemical change

change in which new substances form

Crystallisation

obtaining pure crystals from a solution

Formulas

• Iron + oxygen + water → rust (iron oxide)
• Vinegar + baking soda → carbon dioxide + other substances

Important questions with answers

Quick revision

• Physical: no new substance.
• Chemical: new substance, often irreversible.
• Rusting is a chemical change.

Exam tips

• Burning a candle shows both: wax melting (physical) and wax burning (chemical).
• Galvanising (zinc coating) prevents rusting.

Practice MCQs

1. Melting of ice is a

   1. chemical change
   2. neither
   3. both
   4. physical change
   Show answer

   Answer: (d) physical change. Only state changes.

2. Burning of paper is a

   1. physical change
   2. no change
   3. reversible change
   4. chemical change
   Show answer

   Answer: (d) chemical change. Ash and gases form.

3. Which is a chemical change?

   1. cutting vegetables
   2. dissolving salt
   3. curdling of milk
   4. folding a cloth
   Show answer

   Answer: (c) curdling of milk. New substance (curd).

4. Bubbles seen when baking soda is added to vinegar are of

   1. oxygen
   2. hydrogen
   3. carbon dioxide
   4. nitrogen
   Show answer

   Answer: (c) carbon dioxide. CO₂.

5. Which is a chemical change

   1. melting of wax
   2. breaking glass
   3. dissolving salt
   4. rusting of iron
   Show answer

   Answer: (d) rusting of iron. New substance forms.

6. A burning candle shows

   1. only physical change
   2. both physical and chemical changes
   3. only chemical change
   4. no change
   Show answer

   Answer: (b) both physical and chemical changes. Melting and burning.

7. Pure copper sulphate crystals can be obtained from its solution by

   1. evaporation to dryness only
   2. crystallisation
   3. filtration only
   4. boiling the crystals
   Show answer

   Answer: (b) crystallisation. Crystallisation.

8. Burning a candle involves

   1. only physical change
   2. only chemical change
   3. no change
   4. both physical and chemical changes
   Show answer

   Answer: (d) both physical and chemical changes. Wax melts (physical) and burns (chemical).