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NCERT Class 7 Science Chapter 5 Changes Around Us: Physical and Chemical
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Changes Around Us: Physical and Chemical
Chapter: 5
Page No – 58
Table 5.1: Some changes observed around us.
| S.No. | Change | Observation(s) |
| 1 | Melting ice cubes. | |
| 2 | Chopping vegetables. | |
| 3 | Boiling water. | |
| 4 | Making popcorn from corn. | |
| 5 | Cutting a piece of paper. | |
| 6 | Adding beetroot extract to water. | |
| 7 | Burning wood | |
| 8 | Drying wet clothes. | |
| 9 | Making small balls of dough. | |
| 10 | Rolling small balls of dough into chapatis. | |
| 11 | Rusting of an iron gate. |
Ans:
| S.No. | Change | Observation(s) |
| 1 | Melting ice cubes. | Solid ice turns into liquid water. |
| 2 | Chopping vegetables. | Vegetables are cut into smaller pieces. The size and shape of the vegetables change. |
| 3 | Boiling water. | Liquid water turns into water vapor |
| 4 | Making popcorn from corn. | Hard, small kernels of corn heat up and explode, forming larger, fluffy popcorn. |
| 5 | Cutting a piece of paper. | The shape and size of the paper change. No new substance is formed. |
| 6 | Adding beetroot extract to water. | The water changes color to a shade of red or pink, depending on the concentration of the beetroot extract. |
| 7 | Burning wood | The wood is converted into ash and carbon dioxide, among other substances. |
| 8 | Drying wet clothes. | Water in the wet clothes evaporates and turns into water vapor, which disperses into the air. |
| 9 | Making small balls of dough. | A larger mass of dough is divided and shaped into smaller, spherical pieces. |
| 10 | Rolling small balls of dough into chapatis. | The spherical dough balls are flattened into circular or irregular shapes. The shape changes, but the dough’s composition remains the same. |
| 11 | Rusting of an iron gate. | You would observe a reddish-brown flaky substance (rust) forming on the surface of the iron. The iron becomes weaker over time. |
Page No – 66
Table 5.2: Can changes be reversed?
| S.No. | Change | The original state can be brought back (Yes/No) |
| 1 | Melting ice cubes | Yes |
| 2 | Chopping vegetables | No |
| 3 | Boiling water | Yes |
| 4 | Making popcorn from corn | No |
| 5 | Folding a piece of paper | |
| …. | ||
| 12 |
Ans:
| S.No. | Change | The original state can be brought back (Yes/No) |
| 1 | Melting ice cubes | Yes |
| 2 | Chopping vegetables | No |
| 3 | Boiling water | Yes |
| 4 | Making popcorn from corn | No |
| 5 | Folding a piece of paper | Yes |
| 6 | Dissolving sugar in water | Yes |
| 7 | Burning a piece of paper | No |
| 8 | Stretching a rubber band | Yes |
| 9 | Curdling of milk | No |
| 10 | Growth of a plant | No |
| 11 | Iron rusting | No |
| 12 | Cooking rice | No |
| Let Us Enhance Our Learning |
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).
(b) (ii) and (iii).
(c) (i) and (iii).
(d) (iii) and (iv).
Ans: (c) (i) and (iii).
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.
Ans: Cannot be reversed.
(ii) Twisting of straight string.
Ans: Can be reversed.
(iii) Making idlis from a batter.
Ans: Cannot be reversed.
(iv) Dissolving sugar in water.
Ans: Can be reversed.
(v) Drawing water from a well.
Ans: Can be reversed.
(vi) Ripening of fruits.
Ans: Cannot be reversed.
(vii) Boiling water in an open pan.
Ans: Can be reversed.
(viii) Rolling up a mat.
Ans: Can be reversed
(ix) Grinding wheat grains to flour.
Ans: Cannot be reversed.
(x) Forming of soil from rocks.
Ans: Cannot be reversed.
3. State whether the following statements are True or False. In case a statement is False, write the correct statement.
(i) Melting of wax is necessary for burning a candle. (True/False)
Ans: True.
(ii) Collecting water vapour by condensing involves a chemical change. (True/False)
Ans: False
Correct Statement: Collecting water vapour by condensing involves a physical change.
(iii) The process of converting leaves into compost is a chemical change. (True/False)
Ans: True.
(iv) Mixing baking soda with lemon juice is a chemical change. (True/False)
Ans: True.
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.
Ans: Rusting,chemical change.
(ii) Folding a handkerchief is a ___________ change and can be________.
Ans: Physical change,Reversed.
(iii) A chemical process in which a substance reacts with oxygen with evolution of heat is called _________, and this is a __________ change.
Ans: Combustion,chemical change.
(iv) Magnesium, when burnt in air, produces a substance called __________. The substance formed is _________ in nature. Burning of magnesium is a _________ change.
Ans: Magnesium oxide, Basic, chemical change.
5. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Ans: The changes of water to ice and water to steam are physical changes. This is because only the form or state of water changes, not the substance itself. In all three forms ice, water, and steam the chemical composition remains the same (H₂O). These changes can also be reversed, like melting ice back into water or condensing steam into water, which clearly shows they are physical and not chemical changes.
6. Is curdling of milk a physical or chemical change? Justify your statement.
Ans: Curdling of milk is a chemical change because a new substance is formed. When milk turns into curd, the bacteria or acid cause the milk proteins to break down and form curd. This change cannot be reversed, and the milk does not remain in its original form. That’s why it is called a chemical change.
7. Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Ans: The change from rocks to soil due to wind, rain, and other natural factors is both a physical and a chemical change. It is a physical change because the rocks break into smaller pieces without changing what they’re made of. At the same time, it is a chemical change because rainwater and air can react with the minerals in the rocks and form new substances.
8. Read the following story titled ‘Eco-friendly Prithvi’, 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 f l owers (physical change/chemical change). His eff orts are appreciated by all his family members.
Ans: Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits (physical changes). He collects the seeds, fruits, and vegetable peels into a clay pot (physical change). The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (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 (chemical change). His efforts are appreciated by all his family members.
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.
Ans: A – Physical Changes:
(i)Tearing of paper.
(ii) Melting of ice.
(iii) Folding of clothes.
B – Chemical Changes:
(i) Rusting.
(ii) Curdling of milk.
(iii) Ripening of fruits.
(iv) Burning of magnesium.
(v) Mixing baking soda with vinegar.
C – Both Physical and Chemical Changes:
(i) Burning of a candle.
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?
Ans: Lime water turns milky due to the formation of calcium carbonate (CaCO₃), which occurs when carbon dioxide (CO₂) gas is passed through lime water (calcium hydroxide solution).
(a) Vinegar and baking soda: This reaction produces CO₂.
Result: Lime water turns milky.
(b) Lemon juice and vinegar: Both are acids, and no gas-producing reaction occurs.
Result: Lime water does not turn milky.
(c) Vinegar and common salt: No CO₂ is released.
Result: Lime water does not turn milky.
(d) Lemon juice and baking soda: This combination releases CO₂.
Result: Lime water turns milky.
Lime water turns milky in (a) and (d) because carbon dioxide is produced in those reactions.
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