Exercise Questions and Answers
1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis – Chemical change
(b) Dissolving sugar in water – Physical change
(c) Burning of coal – Chemical change
(d) Melting of wax – Physical change
(e) Digestion of food – Chemical change
(f) Beating aluminium to make aluminium foil – Physical change
2. State whether the following statements are true or false.
(a) Cutting a log of wood into pieces is a chemical change. False
Correction: Cutting wood is a physical change.
(b) Formation of manure from leaves is a physical change. False
Correction: Formation of manure is a chemical change.
(c) Iron pipes coated with zinc do not get rusted easily. True
(d) Iron and rust are the same substances. False
Correction: Iron and rust are different substances.
(e) Condensation of steam is not a chemical change. True
3. Fill in the blanks in the following statements:
(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of calcium carbonate.
(b) The chemical name of baking soda is sodium bicarbonate.
(c) Two methods by which rusting of iron can be prevented are painting and galvanization.
(d) Changes in which only physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.
4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it? Explain.
Answer: It is a chemical change. The reaction between baking soda and lemon juice produces carbon dioxide gas, which forms bubbles and indicates the formation of a new substance.
5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a process where both types of changes occur.
Answer:
- Physical change: Melting of wax.
- Chemical change: Burning of wax to produce carbon dioxide and water.
Another example: Cooking food involves physical changes like water evaporation and chemical changes like food being cooked.
6. How would you show that the setting of curd is a chemical change?
Answer: The setting of curd is a chemical change because milk is transformed into curd by bacteria, forming new substances like lactic acid. This change is irreversible.
7. Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Answer: Cutting wood is a physical change as no new substance is formed, and the wood retains its properties. Burning wood is a chemical change because it forms new substances like ash, carbon dioxide, and heat.
8. Describe how crystals of copper sulfate are prepared.
Answer:
- Dissolve copper sulfate in boiling water.
- Add a few drops of dilute sulfuric acid to the solution.
- Filter the solution to remove impurities.
- Allow the solution to cool undisturbed.
- Blue crystals of copper sulfate will form.
9. Explain how painting an iron gate prevents it from rusting.
Answer: Painting an iron gate prevents it from rusting by creating a barrier that prevents contact with oxygen and moisture, which are essential for rusting.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: Rusting is faster in coastal areas because the air has high humidity and salt, which accelerate the rusting process. Deserts are dry with low moisture, slowing down rusting.
11. The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out of the cylinder, it becomes a gas (Change A), and then it burns (Change B). Choose the correct statement.
Answer: (ii) Process B is a chemical change.
12. Anaerobic bacteria digest animal waste and produce biogas (Change A). The biogas is then burnt as fuel (Change B). Choose the correct statement.
Answer: (iii) Both processes A and B are chemical changes.
Extended Learning — Activities and Projects
1. Describe two changes that are harmful. Explain why you consider them harmful. How can you prevent them?
Answer:
- Rusting of iron: It weakens structures and leads to monetary loss. Prevention: Painting or galvanization.
- Food spoilage: It wastes food and can cause health issues. Prevention: Proper storage and refrigeration.
2. Experiment with iron nails and water bottles.
- Fill three bottles:
- Bottle A: Tap water.
- Bottle B: Boiled water.
- Bottle C: Boiled water with oil on top.
- Place nails in each bottle and leave them for a few days.
- Observation:
- Bottle A: Nails rust due to oxygen and water.
- Bottle B: No rust as boiled water has no oxygen.
- Bottle C: No rust as oil prevents contact with air.
3. Prepare crystals of alum.
- Dissolve alum in boiling water until saturated.
- Filter the solution to remove impurities.
- Let the solution cool undisturbed.
- Alum crystals will form as the water evaporates.
4. Collect information about fuels used for cooking.
Answer: Common fuels include LPG, kerosene, wood, and biogas. LPG and biogas are less polluting because they burn cleaner and release fewer harmful gases. Wood and kerosene produce more smoke and pollutants.
1. What is a physical change?
Answer: A physical change is a change in the shape, size, color, or state of a substance without forming a new substance. It is usually reversible.
2. What is a chemical change?
Answer: A chemical change is a change in which new substances are formed with properties different from the original substances. It is usually irreversible.
3. Give an example of a physical change.
Answer: Melting ice into water is a physical change.
4. Give an example of a chemical change.
Answer: Burning magnesium ribbon to form magnesium oxide is a chemical change.
5. What is rusting?
Answer: Rusting is the process where iron reacts with oxygen and water to form a brown substance called rust.
6. Write the chemical equation for rusting.
Answer: Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Rust (Fe₂O₃).
7. How can rusting be prevented?
Answer: Rusting can be prevented by applying paint, grease, or by galvanizing iron articles.
8. What is galvanization?
Answer: Galvanization is the process of coating iron with zinc to prevent rusting.
9. What is crystallization?
Answer: Crystallization is the process of forming pure and large crystals of a substance from its solution.
10. Is crystallization a physical or chemical change?
Answer: Crystallization is a physical change.
11. What are the signs of a chemical change?
Answer: Signs include heat or light production, gas formation, color change, sound production, or a change in smell.
12. Why does a cut apple turn brown?
Answer: A cut apple turns brown due to a chemical reaction between the apple and oxygen in the air.
13. What happens when magnesium ribbon burns?
Answer: It forms magnesium oxide and gives off a bright white light.
14. Is dissolving sugar in water a physical or chemical change?
Answer: It is a physical change.
15. What happens when vinegar reacts with baking soda?
Answer: It produces carbon dioxide gas, water, and a new substance.
16. What is the standard test for carbon dioxide gas?
Answer: Passing carbon dioxide through lime water turns it milky.
17. Is boiling water a physical or chemical change?
Answer: Boiling water is a physical change.
18. Why does milk turn sour?
Answer: Milk turns sour due to a chemical change caused by the formation of lactic acid.
19. What happens during the explosion of fireworks?
Answer: Fireworks produce light, heat, sound, and gases, which are all signs of a chemical change.
20. What happens when copper sulfate reacts with iron?
Answer: Iron displaces copper from copper sulfate to form iron sulfate and copper.
21. Write the chemical reaction for copper sulfate and iron.
Answer: Copper sulfate (CuSO₄) + Iron (Fe) → Iron sulfate (FeSO₄) + Copper (Cu).
22. Is tearing paper a physical or chemical change?
Answer: Tearing paper is a physical change.
23. Is burning paper a physical or chemical change?
Answer: Burning paper is a chemical change.
24. Why do ships rust faster in seawater?
Answer: Seawater contains salts that speed up the rusting process.
25. What is the role of the ozone layer?
Answer: The ozone layer absorbs harmful ultraviolet radiation from the sun, protecting life on Earth.
26. Is photosynthesis a chemical change?
Answer: Yes, photosynthesis is a chemical change as new substances like glucose and oxygen are formed.
27. Is digestion a physical or chemical change?
Answer: Digestion is a chemical change as food is broken down into simpler substances.
28. What happens when food spoils?
Answer: Spoiling of food is a chemical change, often producing a foul smell and harmful substances.
29. Why is stainless steel rust-resistant?
Answer: Stainless steel contains chromium, nickel, and other metals, which make it resistant to rusting.
30. What is a common characteristic of all chemical changes?
Answer: Chemical changes result in the formation of one or more new substances with different properties.
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