Class 7 Science
Chapter 5 – Changes Around Us: Physical and Chemical
MCQ – Multiple Choice Questions
1. Which is a characteristic of a physical change?
Answer: C. No new substance is formed.
2. Which of the following is a chemical change?
Answer: C. Rusting of iron.
3. What happens when carbon dioxide is passed through lime water?
Answer: C. It becomes milky.
4. Which gas supports combustion?
Answer: B. Oxygen.
5. The minimum temperature at which a substance catches fire is called:
Answer: C. Ignition temperature.
6. Which is a chemical change?
Answer: C. Burning magnesium.
7. Which is an example of erosion?
Answer: B. Movement of sediments by flowing water.
8. Which change involves both physical and chemical changes?
Answer: C. Burning a candle.
Very Short Answer Questions
9. What is a physical change?
Answer: A change in which no new substance is formed and only physical properties such as shape, size or state change.
10. What is a chemical change?
Answer: A change in which one or more new substances are formed.
11. What is combustion?
Answer: Combustion is a chemical reaction in which a substance reacts with oxygen and produces heat and light.
12. What is ignition temperature?
Answer: It is the minimum temperature at which a substance catches fire.
13. Give two examples of reversible changes.
Answer: Melting of ice and evaporation of water.
14. What is erosion?
Answer: Erosion is the breakdown and movement of rocks, soil or sediments by wind or flowing water.
Short Answer Questions
15. Give three examples of physical changes.
Answer: Melting ice, cutting paper and folding clothes.
16. Give three examples of chemical changes.
Answer: Rusting of iron, burning magnesium and curdling of milk.
17. Why does lime water turn milky when we blow air into it?
Answer: Exhaled air contains carbon dioxide. Carbon dioxide reacts with lime water and forms insoluble calcium carbonate, making it milky.
18. Why is rusting of iron a chemical change?
Answer: Rusting forms a new substance called rust or iron oxide. Therefore, it is a chemical change.
19. What are the three requirements for combustion?
Answer: Fuel, oxygen and sufficient heat to reach the ignition temperature.
20. Differentiate between reversible and irreversible changes.
Answer: In a reversible change, the original substance can be obtained again. In an irreversible change, the original substance cannot be obtained easily.
21. What is weathering?
Answer: Weathering is the physical and chemical breakdown of rocks that contributes to soil formation.
Long Answer Questions
22. Differentiate between physical and chemical changes.
Physical Change: No new substance is formed. Examples are melting ice and cutting paper.
Chemical Change: One or more new substances are formed. Examples are rusting iron and burning magnesium.
Chemical Change: One or more new substances are formed. Examples are rusting iron and burning magnesium.
23. Explain how burning a candle involves both physical and chemical changes.
Answer: When a candle burns, wax melts and evaporates, which are physical changes. The wax vapour burns and forms new substances, which is a chemical change.
24. How can you show that oxygen is necessary for combustion?
Answer: Light two candles. Cover one candle with a glass. The covered candle goes out when the available oxygen is used up, while the uncovered candle continues to burn. This shows that oxygen is necessary for combustion.
25. What are the three conditions necessary for combustion?
Answer: The three conditions are:
1. Fuel
2. Oxygen
3. Heat sufficient to reach the ignition temperature.
If any one condition is removed, combustion stops.
1. Fuel
2. Oxygen
3. Heat sufficient to reach the ignition temperature.
If any one condition is removed, combustion stops.
26. Differentiate between weathering and erosion.
Answer: Weathering breaks down rocks into smaller particles. Erosion involves the movement of these sediments by natural forces such as wind and flowing water.
Case Study Based Questions
27. A student blows air into lime water and observes that it becomes milky. Which gas is responsible?
Answer: Carbon dioxide (CO₂).
28. What is the white substance formed when carbon dioxide reacts with lime water?
Answer: Calcium carbonate.
29. What physical and chemical changes occur when a candle burns?
Answer: Melting and evaporation of wax are physical changes. Burning of wax vapour is a chemical change.
30. Why does paper catch fire only after sufficient heating?
Answer: Paper must reach its ignition temperature before it catches fire. Combustion also requires oxygen and fuel.
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