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There are various changes that we observe around us every day, like the ripening of fruits, rusting of iron, freezing of water, burning of candles, etc. Out of these, some changes are Physical changes, while others are chemical changes, some are irreversible changes, while others are reversible changes, some changes are fast, while others are slow. In these chapters, students are going to learn about these changes and why some are called physical while others are chemical.
NCERT Solutions for Class 7 Science of Chapter 5 are prepared by subject experts and offer a systematic and structured approach for the exercise problems. These NCERT solutions help students to prepare well for exams by providing detailed solutions to all the exercise questions given in the book.
You can download the detailed solution to all the exercise questions from the Download PDF icon given below.
Question 1: Which of the following statements are the characteristics of a physical change?
(i) The state of the substance may or may not change.
(ii) A substance with different properties is formed.
(iii) No new substance is formed.
(iv) The substance undergoes a chemical reaction.
(a) (i) and (ii)
(c) (i) and (iii)
(b) (ii) and (iii)
(d) (iii) and (iv)
Answer:
(c) (i) and (iii)
(i) The state of the substance may or may not change - Physical changes can involve changes in state (like melting or freezing), but no new substance is formed.
(iii) No new substance is formed - In a physical change, the substance remains the same, only its form or state changes.
Question 2: Predict which of the following changes can be reversed and which cannot be reversed. If you are not sure, you may write that down. Why are you not sure about these?
(i) Stitching cloth to a shirt
Answer (i): Stitching cloth to a shirt - Cannot be reversed: Once stitched, it is hard to undo without damage.
(ii) Twisting of straight string
Answer(ii): Twisting of straight string - Can be reversed: The string can be untwisted.
(iii) Making idlis from a batter
Answer(iii):
Making idlis from a batter - Cannot be reversed: The batter cannot be returned to its original form after steaming.
(iv) Dissolving sugar in water
Answer(iv): Dissolving sugar in water - Can be reversed: Sugar can be recovered by evaporating water.
(v) Drawing water from a well
Answer(v): Drawing water from a well - Can be reversed: Water can be returned to the well.
(vi) Ripening of fruits
Answer(vi): Ripening of fruits - Cannot be reversed: Once ripe, fruits cannot go back to unripe.
(vii) Boiling water in an open pan
Answer(vii): Boiling water in an open pan - Can be reversed: Evaporated water can be obtained back by condensation.
(viii) Rolling up a mat
Answer(viii): Rolling up a mat - Can be reversed: The mat can be unrolled.
(ix) Grinding wheat grains to flour
Answer(ix): Grinding wheat grains to flour - Cannot be reversed: Flour cannot be turned back into grains.
(x) Forming of soil from rocks
Answer(x): Forming of soil from rocks - Cannot be reversed: Soil formation is a slow process and cannot form rock back.
Question 3: State whether the following statements are True or False. In case a statement is False, write the correct statement.
1. Melting of wax is necessary for burning a candle. (True/False)
Answer 1: True
2. Collecting water vapour by condensing involves a chemical change. (True/False)
Answer 2: False
Correct statement: Collecting water vapour by condensing involves a physical change.
3. The process of converting leaves into compost is a chemical change. (True/False)
Answer 3: True
4. Mixing baking soda with lemon juice is a chemical change. (True/False)
Answer 4: True
Question 4. Fill in the blanks in the following statements:
(i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to—-----------and this is a —----- change.
Answer: rusting, chemical
(ii) Folding a handkerchief is a —----------change and can be —---------.
Answer: physical, reversed
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called —-------- and this is a —------- change.
Answer: combustion, chemical
(iv) Magnesium, when burnt in air, produces a substance called —-------. The substance formed is —--------- in nature. Burning of magnesium is a —----------change.
Answer: magnesium oxide, basic, chemical
Question 5: Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer: Both, the change of water to ice and water to steam are physical changes. In both processes, the chemical composition of water remains the same; only its state changes from liquid to solid or from liquid to gas. No new substance is formed which is a characteristic of physical changes.
Question 6: Is curdling of milk a physical or chemical change? Justify your statement.
Answer: The curdling of milk is a chemical change because, during this process, milk reacts with acid or bacteria, forming new substances like curd. This change cannot be reversed, which makes it a chemical change.
Question 7: Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Answer: The formation of soil from rocks involves both physical and chemical changes. Natural factors like wind, rain and temperature break down rocks into smaller pieces (physical change), while chemical processes, like weathering, also change the minerals in the rocks (chemical change). Both types of changes work together to form soil.
Question 8: Read the following story titled 'Eco-friendly Prithin', and tick the most appropriate option(s) given in the brackets. Provide a suitable title of your choice for the story.
Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes/chemical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change/ chemical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (physical change/chemical change). He decides to plant seeds in the compost and water them regularly. After a few days, he notices that the seeds begin to germinate and small plants start to grow, eventually blooming into colourful flowers (physical change/chemical change). His efforts are appreciated by all his family members.
Answer: Prithvi's Green Kitchen
1. Prithvi chops vegetables, peels potatoes, and cuts fruits are physical changes
2. He collects the seeds, fruits, and vegetable peels into a clay pot are physical change
3. The fruits, vegetable peels, and other materials decompose into compost are chemical change
5. Seeds germinate and grow into plants are chemical change
Question 9: Some changes are given here. Write physical changes in the area marked 'A' and chemical changes in the area marked 'B'. Enter the changes which are both physical and chemical in the area marked 'C
Process of burning a candle; Tearing of paper; Rusting; Curdling of milk; Ripening of fruits; Melting of ice; Folding of clothes; Burning of magnesium and Mixing baking soda with vinegar.
Answer 9: A (Physical changes)
Tearing of paper
Melting of iron
Folding of clothes
B (Chemical Changes)
Rusting
Curdling of milk
Ripening of fruits
Burning of magnesium
Mixing baking soda with vinegar
C ( Both physical and chemical changes)
Process of burning of candle
Question 10: The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why?
Answer: In Figure (a), when vinegar reacts with baking soda, carbon dioxide gas is released. This carbon dioxide gas travels through the straw into the test tube with lime water, where it reacts to form calcium carbonate, a white solid substance that makes the lime water appear milky.
To solve questions from the chapter, students must keep the points given below in mind:
1) Understand the basic concepts like:
Physical changes
Chemical changes
Rusting
Combustion
Erosion
2) Read the question carefully
Try to identify what is given by identifying key terms given in the question, like physical changes, chemical changes, combustion, etc.
Determine what we need to find out.
3) Try to understand questions by connecting them with real-life examples
Change in the length of a rubber band by stretching, which is a physical change
Burning of a candle, which is both a physical as well as chemical change
Ripening of fruits, which is a chemical change
4) Also, use some common tricks, such as paying attention to key differences like physical and chemical changes.
5). Revise notes timely and practice previous year question papers of the exam, as well as practice mock tests accordingly.
1) A substance may change in appearance but remain the same
2) A substance may change in appearance and not remain the same
3) Some other processes involving chemical changes
Rusting
Combustion
4) Can physical and chemical changes occur in the same process
5) Are changes permanent
6) Are all changes desirable
7) Some slow natural changes
Weathering of rocks
Erosion
Question 1. Which of the following is a chemical change?
1) Melting of wax
2) Tearing of paper
3) Burning of wood
4) Dissolving sugar in water
Answer. Burning produces new substances like ash and gases, making it a chemical change.
Hence, the correct answer is option (3).
Question 2. Which of the following changes is reversible?
1) Rusting of iron
2) Cooking of rice
3) Freezing of water
4) Burning of a candle
Answer: Water can freeze into ice and melt back, making it a reversible physical change.
Hence, the correct answer is option (3).
Question 3. Is a chemical change reversible?
Answer: Typically, chemical changes are not easily reversible. While some chemical reactions can be reversed under specific conditions, it usually requires additional energy or other chemical reactions. Therefore, It is difficult.
The chapter-wise solutions are given below:
The NCERT books and syllabus links for class 7 are given below:
Also Read
A physical change is a change in the form of a substance; it doesn't change the chemical composition of the substance. The substance is still the same material, even if it looks different.
Examples of physical changes?
Melting ice: Water changes from solid to liquid.
Boiling water: Water changes from liquid to gas.
Cutting a piece of paper: Paper is still paper, just in smaller pieces.
Dissolving sugar in water: Sugar's molecules are still sugar, just dispersed.
Freezing water.
Many physical changes are reversible. This means you can change the substance back to its original form. For example, you can freeze liquid water back into ice. However, some physical changes, like cutting paper, are hard to reverse completely.
In a chemical change, a new substance is formed with a different chemical composition from the original substance. This process involves breaking and forming chemical bonds.
Examples of chemical changes
Burning wood: Wood combines with oxygen to form ash, carbon dioxide, and water.
Rusting of iron: Iron reacts with oxygen and water to form iron oxide, or rust.
Cooking an egg: The proteins denature and change structure.
Baking a cake: Ingredients react to form a new, complex mixture.
Digesting Food: The body breaks down food to release energy.
Photosynthesis: Plants create food.
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