Before trying to answer the NCERT exercise questions, you should have a thorough understanding of class 7 Science chapter 5 i.e. ‘Physical and Chemical Changes‘.
If you haven’t understood the chapter yet, then worry not you can go through it easily and develop a crystal clear understanding of various concepts from our notes whose link has been provided below. ⤵️
NCERT Exercise Questions & Answers
Que. 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
Ans. 1 (a) Chemical Change
(b) Physical Change
(c) Chemical Change
(d) Physical Change
(e) Physical Change
(f) Chemical Change
Que. 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)
(b) Formation of manure from leaves is a physical change. (True/False)
(c) Iron pipes coated with zinc do not get rusted easily. (True/False)
(d) Iron and rust are the same substances. (True/False)
(e) Condensation of steam is not a chemical change. (True/False)
Ans. 2 (a) Cutting a log of wood into pieces is a chemical change. (False). Cutting a log of wood into pieces is a physical change.
(b) Formation of manure from leaves is a physical change. (False). The formation of manure from leaves 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). Iron and rust are two different substances.
(e) Condensation of steam is not a chemical change. (True)
Que. 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 __________.
(b) The chemical name of baking soda is___________.
(c) Two methods by which rusting of iron can be prevented are _____________and _______________.
(d) Changes in which only ________________ properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called _____________ changes.
Ans. 3 (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 or sodium hydrogen carbonate.
(c) Two methods by which rusting of iron can be prevented are painting and galvanisation.
(d) Changes in which only the physical properties of a substance change are called physical changes.
(e) Changes in which new substances are formed are called chemical changes.
Que. 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.
Ans. 4 When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas is a type of chemical change because baking soda is a base and lemon juice is an acid together they lead to the formation of a salt, water and carbon dioxide gas.
Baking soda + Lemon juice ——> Carbon dioxide + water + salt
Que. 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.
Ans. 5 When a candle burns both physical and chemical changes take place. The candle wax is melting and vaporising which is a physical change and the wick along with some wax of the candle is burning, forming carbon dioxide gas along with other substances is a chemical change.
Another example of a process in which both chemical and physical changes take place is the digestion of food:
- The mechanical breakdown of food by our teeth is a physical change.
- And, the enzymatic digestion of food is a chemical process or change.
One more such example is: 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 which is a physical change, then it burns and producing carbon dioxide is a chemical change.
Que. 6 How would you show that the setting of curd is a chemical change?
Ans. 6 Curd is formed from the milk after adding some sour materials to it. The setting of curd is a chemical change because we can’t get back our milk from the curd. Both curd and milk are different products that have different properties like taste, smell, texture, etc.
Que. 7 Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Ans. 7 Both the burning of wood and chopping it into small pieces are different types of changes.
- Burning of wood is a chemical change because burning of wood leads to the production of new substances like ash or charcoal, carbon dioxide and smoke.
- The chopping of wood into smaller pieces is a physical change because only the physical properties of the wood are changing, no new substances are forming after chopping the wood.
Que. 8 Describe how crystals of copper sulphate are prepared.
Ans. 8 We can prepare the crystals of copper sulphate by following this simple procedure:
- Fill a beaker 3/4 with distilled water and add a few drops of dilute sulphuric acid to the beaker.
- Heat the beaker till the water starts to boil.
- Now, add copper sulphate powder slowly into the beaker with constant stirring.
- Continue adding copper sulphate powder till the solution saturates.
- Now filter the solution through a filter paper and allow it to cool down.
- Don’t disturb the solution when it’s cooling.
- Look at the solution after some time, you will be able to spot or see some really beautiful blue-coloured copper sulphate crystals.
Que. 9 Explain how the painting of an iron gate prevents it from rusting.
Ans. 9 For rusting an iron gate needs to be in constant touch or contact with water and air (oxygen) but by painting it, we create a protective layer between the iron gate and the surroundings that breaks the contact of the iron gate with water and air hence, preventing it from rusting.
Que. 10 Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans. 10 There are mainly two factors on which the rate of rusting of iron depends i.e. the amount of oxygen and amount of water it is in contact with. The amount of oxygen in the atmosphere is more or less the same in all places on Earth but the amount of water varies. As in our case, the amount of water or moisture in the air is more in the coastal region than in deserts and that’s why the rate of rusting of iron objects is faster in coastal areas than in deserts.
Que. 11 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.
Ans. 11 (ii) Process-B is a chemical change. Because burning of LPG gas produces carbon dioxide, whereas when LPG comes out from the cylinder it becomes a gas is a type of physical change.
Que. 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.
Ans. 12 (iii) Both processes A and B are chemical changes.
Credits & References
- Class 7th Science NCERT Textbook
- Image by Myriams-Fotos from Pixabay
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