CBSE Science Class 7- Chapter 6- Physical and Chemical changes- NCERT Exercise Solution (Question-Answer) is provided below. Total 12 Questions are in this NCERT Chapter Exercise, all are solved here.
Q1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
Answer: Chemical change
(b) Dissolving sugar in water
Answer: Physical change
(c) Burning of coal
Answer: chemical change
(d) Melting of wax
Answer: Physical change
(e) Beating aluminium to make aluminium foil
Answer: Physical change
(f) Digestion of food
Answer: Chemical change
Q2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.
(a) Cutting a log of wood into pieces is a chemical change. (True/False)
Answer: False
Correct statement: Cutting a log of wood into pieces is a physical change.
(b) Formation of manure from leaves is a physical change. (True/False)
Answer: False
Correct statement: Formation of manure from leaves is a chemical change.
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
Answer: True
(d) Iron and rust are the same substances. (True/False)
Answer: False
Correct statement: Iron and rust are different chemical substances.
(e) Condensation of steam is not a chemical change. (True/False)
Answer: True
Q3. 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 ____________________.
Answer: Calcium carbonate (CaC03)
(b) The chemical name of baking soda is _________.
Answer: Sodium hydrogen carbonate (NaHCO3)
(c) Two methods by which rusting of iron can be prevented are ____________ and __________________.
Answer: painting, galvanization
(d) Changes in which only ___________ properties of a substance change are called physical changes.
Answer: physical
(e) Changes in which new substances are formed are called _________ changes.
Answer: chemical
Q4. 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: When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. It is a type of chemical change because some new substances are formed in this reaction.
Baking soda + Lemon juice → Carbon dioxide + other substances
Q5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.
Answer: 👉When a candle burns, both physical and chemical changes take place:
Physical change: Melting of wax.
Chemical change: Burning of wax vapour near the flame gives new substances such as carbon dioxide, carbon soot, heat, light and water vapour.
👉 Another example: The burning of wood is also a physical and chemical change both.
Physical change: Water present in wood changes into vapour.
Chemical changes: Burning of wood produces new substances like CO2, H2O, CO Etc
👉 LPG, digestion of food are also the example in which both physical and chemical changes takes place.
Q6. How would you show that setting of curd is a chemical change?
Answer: The setting of curd is a chemical change because the property of milk and curd are different. Curd cannot be turned back to the milk.
Q7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer: Burning of wood is a chemical change because new substances are formed in this process and cannot be turned back to the original/ previous substance. The new substance formed has its own different property.
Cutting of wood into small pieces is a physical change because the nature of wood does not change. No new substance is formed in this process.
Q8. Describe how crystals of copper sulphate are prepared.
Answer: Crystal of copper sulphate are prepared by crystallization method:
👉 Steps:
1. Take water into a beaker.
2. Now, add few drops of dilute sulphuric acid into water.
3. Heat the water until it starts boiling. Now, add the powder of copper sulphate into the water with continuous stirring the solution.
4. After some time it is observed that the copper sulphate gets deposited into the bottom of the beaker. Then filter the solution and left for cooling.
5. Finally, the crystal of Copper sulphate is obtained.
Q9. Explain how the painting of an iron gate prevents it from rusting.
Answer: Rusting of iron takes place in the presence of water (moisture) and oxygen. If we paint the iron gate, it can be prevented by rusting because now, there is no direct contact between the iron particles and moisture present in the air.
Q10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer: We know rusting of iron takes place in the presence of water (moisture) and oxygen. In the coastal area, humidity is more in comparison to the desert area. Hence, the process of rusting is faster in coastal area than in the desert.
Q11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder, it exists as a liquid. When it comes out from the cylinder it becomes a gas (Change – A) then it burns (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change.
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change.
Answer: (ii) Process – B is a chemical change.
Q12. Anaerobic bacteria digest animal waste and produce biogas (Change – A). The biogas is then burnt as fuel (Change – B). The following statements pertain to these changes. Choose the correct one.
(i) Process – A is a chemical change.
(ii) Process – B is a chemical change
(iii) Both processes A and B are chemical changes.
(iv) None of these processes is a chemical change
Answer: (iii) Both processes A and B are chemical changes.
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