Solves Exercises
1. Classify the changes involved in the following processes as physical or chemical changes:
(a) Photosynthesis
(b) Dissolving sugar in water
(c) Burning of coal
(d) Melting of wax
(e) Beating aluminium to make aluminium foil
(f) Digestion of food
Answers:
(a) Photosynthesis – Chemical change
(b) Dissolving sugar in water – Physical change
(c) Burning of coal – Chemical change
(d) Melting of wax – Physical change
(e) Beating aluminium to make aluminium foil – Physical change
(f) Digestion of food – Chemical change
2. 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
(b) Formation of manure from leaves is a physical change. (True/false)
Answer: False
(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
(e) Condensation of steam is not a chemical change. (True/false)
Answer: 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(CaCO3).
(b) The chemical name of baking soda is Sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are galvanization and painting.
(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:
• When baking soda is mixed with lemon juice, bubbles are formed with the evaluation of carbon dioxide gas. This change is a chemical change.
• In this chemical reaction, Lemon juice(Citric acid) mixed with baking soda(sodium hydrogen carbonate) which results in the evolution of carbon dioxide gas.
Lemon juice (Citric acid) + Baking soda → Carbon dioxide + Other substances
5. 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 in the candle and Vaporisation.
Chemical change – Burning of wax vapours.
Another example – Food eating in which both the physical and chemical changes happens simultaneously.
Physical Change – Breaking of food into smaller particles.
Chemical Change – Digestion of food.
6. How would you show that setting of curd is a chemical change?
Answer:
• When the curd formed, a new substance, lactic acid is obtained from the lactose present in the milk.
• Once the curd form from the milk it does not reversed back to the milk.
• That means, setting of curd is irreversible process.
• Also, chemical change is a irreversible process.
• Therefore, setting of curd is a chemical change.
7. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.
Answer:
• When we burn wood, a new substance is obtained which is a coal. Hence, burning of wood is a chemical change.
• When we cut wood into small pieces, no new substance formed. The changes occurs in shape and size only. Hence, cutting of wood is a physical Change.
8. Describe how crystals of copper sulphate are prepared.
Answer: The crystals of copper sulphate are prepared by the process of Neutralisation. The process of formation of crystals of copper sulphate is as follows:
• A cupful of water is taken in a beaker and a few drops of dilute sulphuric acid added in it. Then after water get heated.
• When it starts boiling, added copper sulphate powder while stirring continuously.
• Continuously added copper sulphate powder till no more powder can be dissolved.
• Then, filtered the solution in a China dish and allowed to cool.
• The solution shouldn’t kept disturbed.
• Slowly, the crystals of copper sulphate separate out.
9. Explain how painting of an iron gate prevents it from rusting.
Answer:
• Rusting occurs in the presence of air water(moisture) and air(oxygen).
• By painting of an iron gate, we prevent the gate contacts from air and moisture which are present in the atmosphere.
• By this way, painting of an iron gate prevents it from rusting.
10. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Answer:
• Rusting occurs in the presence of moisture and air.
• In coastal areas, the quantity of moisture present in the air is higher than the moisture present in deserts.
• Therefore, rusting of iron objects is faster in coastal areas than in deserts.
11. The gas we use in the kitchen is called liquified petroleum gas (LPG). In the cylinder it exist 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: (iii) both processes A and B are chemical changes.
12. 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.
Apex Class 
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