Worksheet Solutions: Physical and Chemical Changes | Chemistry Class 7 ICSE PDF Download

Part A: Multiple Choice Questions 

Q1. Which of the following is a reversible change?
a) Rusting of iron
b) Burning of paper
c) Melting of butter
d) Cooking of rice

Answer: c) Melting of butter
Reasoning: In melting, butter changes from solid to liquid but remains the same substance. On cooling, it can solidify again. Since no new substance is formed and the process can be reversed, it is a reversible physical change.

Q2. Which of these is a chemical change?
a) Folding a sheet of paper
b) Dissolving salt in water
c) Freezing water
d) Curdling of milk

Answer: d) Curdling of milk
Reasoning: When milk curdles, it forms curd — a new substance with different taste, smell, and properties. It cannot be changed back into milk, proving it is an irreversible chemical change.

Q3. Which change is exothermic (releases heat)?
a) Melting of ice
b) Evaporation of water
c) Burning of a candle
d) Boiling of water

Answer: c) Burning of a candle
Reasoning: During burning, substances react with oxygen to form new products like carbon dioxide and water vapor. The process gives out heat and light, so it is both a chemical change and an exothermic change.

Q4. When a solid directly changes into gas, the process is called:
a) Condensation
b) Boiling
c) Evaporation
d) Sublimation

Answer: d) Sublimation
Reasoning: In sublimation, a solid like naphthalene or iodine directly changes into vapor without becoming liquid in between. Since no new substance forms and the vapor can solidify back, it is a reversible physical change.

Q5. Which of these is a man-made chemical change?
a) Growth of a plant
b) Change of seasons
c) Making of chapatti
d) Formation of clouds

Answer: c) Making of chapatti
Reasoning: A chapatti is made by heating dough on a pan. Dough changes into a new substance (chapatti) that cannot turn back into dough. Since humans cause it and it is irreversible, it is a man-made chemical change.

Part B: Short Answer Questions (with Reasoning)

Q6. Why is the melting of ice considered a physical change?
Answer:
When ice melts, it changes into water. Both ice and water are chemically the same (H₂O). Since no new substance forms and the process can be reversed by freezing, it is a physical change.

Q7. Differentiate between desirable and undesirable changes with reasoning and examples.
Answer:

• Desirable changes are useful to us. Example: Cooking of food is desirable because it makes food edible.

• Undesirable changes are harmful. Example: Rusting of iron is undesirable because it damages iron objects.
  Thus, whether a change is desirable or not depends on its usefulness to us.

Q8. Why is rusting of iron a chemical change?
Answer:
Rusting occurs when iron reacts with oxygen and moisture to form a brown flaky substance called rust. Rust is a new substance with properties different from iron. Since the process is irreversible and permanent, rusting is a chemical change.

Q9. How does temperature affect the rate of evaporation? Explain with reasoning.
Answer:
Higher temperature increases the energy of water molecules, making them move faster. As a result, they escape into the air more quickly. Hence, evaporation is faster at higher temperatures (e.g., wet clothes dry faster on a sunny day).

Q10. Classify the following as slow or fast changes with reasoning:

• Bursting of a cracker

• Growth of a plant

Answer:

• Bursting of a cracker: Fast change, because it happens within seconds.

• Growth of a plant: Slow change, because it takes months or years.
  The classification depends on time taken for the change to occur.

Part C: Long Answer Questions (with Stepwise Reasoning)

Q11. Explain with steps why burning of paper is a chemical change.
Answer (Stepwise Reasoning):

1. Paper is held near a flame.

2. It burns and produces ash, smoke, and gases.

3. These are new substances with properties completely different from paper.

4. The process cannot be reversed (ash cannot become paper again).
   Conclusion: Burning is a chemical, irreversible, and exothermic change because new substances are formed and heat is released.

Q12. Compare physical and chemical changes with proper reasoning.
Answer (Stepwise Reasoning):

1. New substance:

   • Physical change: No new substance forms (melting of ice → water).

   • Chemical change: New substances form (curdling of milk → curd).

2. Reversibility:

   • Physical change: Usually reversible.

   • Chemical change: Mostly irreversible.

3. Energy change:

   • Physical change: May involve little or no energy change.

   • Chemical change: Always involves energy change (heat, light, sound).

4. Properties:

   • Physical change: Only appearance or state changes.

   • Chemical change: Composition and properties change.

Conclusion: Physical changes are temporary and reversible, while chemical changes are permanent and form new substances.

Q13. Ramesh observed these in his kitchen:

• Ice cubes melting in a glass.

• His mother baking a chapatti.

• Water boiling in a kettle.
  Classify each as physical or chemical change and give reasoning.

Answer (Stepwise Reasoning):

1. Ice melting → Physical change.

   • Ice (solid) turns into water (liquid). Same substance (H₂O), reversible.

2. Baking chapatti → Chemical change.

   • Dough changes into chapatti, a new substance with different properties, irreversible.

3. Boiling water → Physical change.

   • Water changes into steam. No new substance forms, reversible by condensation.