Changes Around Us: Physical and ChemicalClass 7 Science NCERT Solutions
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Q1Let Us Enhance Our Learning
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)
Solution
Answer: (c) (i) and (iii)
Explanation:
A physical change is a change in the physical properties of a substance, such as its shape, size, or state (solid, liquid, gas), without forming a new substance.
- Statement (i) is correct because the state can change (e.g., ice melting to water) or remain the same (e.g., cutting paper).
- Statement (iii) is correct as it is the defining characteristic of a physical change that no new substance is formed.
- Statements (ii) and (iv) describe a chemical change, where a new substance is formed through a chemical reaction.
Q2Let Us Enhance Our Learning
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
(ii)
Twisting of straight string
(iii)
Making idlis from a batter
(iv)
Dissolving sugar in water
(v)
Drawing water from a well
(vi)
Ripening of fruits
(vii)
Boiling water in an open pan
(viii)
Rolling up a mat
(ix)
Grinding wheat grains to flour
(x)
Forming of soil from rocks
Solution
The classification of the given changes is as follows:
(i)
Stitching cloth to a shirt: This can be reversed by unpicking the stitches.
(ii)
Twisting of straight string: This can be reversed by untwisting the string.
(iii)
Making idlis from a batter: This cannot be reversed. The batter is cooked, which is a chemical change, and it cannot be turned back into batter.
(iv)
Dissolving sugar in water: This can be reversed by evaporating the water, which will leave the sugar behind.
(v)
Drawing water from a well: This can be reversed by pouring the water back into the well.
(vi)
Ripening of fruits: This cannot be reversed. Ripening is a complex chemical change, and a ripe fruit cannot become raw again.
(vii)
Boiling water in an open pan: This can be reversed. The water turns into steam (evaporation), which can be collected and condensed back into liquid water.
(viii)
Rolling up a mat: This can be reversed by unrolling the mat.
(ix)
Grinding wheat grains to flour: This cannot be reversed. It is not possible to reassemble the fine flour particles back into whole wheat grains.
(x)
Forming of soil from rocks: This cannot be reversed. It is a very slow process involving weathering and erosion over thousands of years.
Q3Let Us Enhance Our Learning
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)
(ii)
Collecting water vapour by condensing involves a chemical change. (True/False)
(iii)
The process of converting leaves into compost is a chemical change. (True/False)
(iv)
Mixing baking soda with lemon juice is a chemical change. (True/False)
Solution
(i)
True. The heat from the flame melts the wax, which then travels up the wick as a liquid and vaporizes to burn.
(ii)
False.
Correct statement: Collecting water vapour by condensing involves a physical change. Condensation is a change of state from gas (vapour) to liquid, and no new substance is formed.
(iii)
True. The decomposition of leaves by microorganisms to form compost is a chemical change, as new substances are formed.
(iv)
True. Mixing baking soda with lemon juice (an acid) causes a chemical reaction that produces carbon dioxide gas and other new substances.
Q4Let Us Enhance Our Learning
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.
(ii)
Folding a handkerchief is a _____ change and can be _____ .
(iii)
A chemical process in which a substance reacts with oxygen with evolution of heat is called _____ , and this is a _____ change.
(iv)
Magnesium, when burnt in air, produces a substance called _____ . The substance formed is _____ in nature. Burning of magnesium is a _____ change.
Solution
(i)
Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to rusting, and this is a chemical change.
(ii)
Folding a handkerchief is a physical change and can be reversed.
(iii)
A chemical process in which a substance reacts with oxygen with evolution of heat is called combustion, and this is a chemical change.
(iv)
Magnesium, when burnt in air, produces a substance called magnesium oxide. The substance formed is basic in nature. Burning of magnesium is a chemical change.
Q5Let Us Enhance Our Learning
Are the changes of water to ice and water to steam, physical or chemical? Explain.
Solution
The changes of water to ice (freezing) and water to steam (boiling/evaporation) are physical changes.
Explanation:
In both processes, only the state of the substance changes, but its chemical composition remains the same. Ice, liquid water, and steam are all chemically water (). No new substance is formed. These changes are also reversible. Ice can be melted back to water, and steam can be condensed back to water. Therefore, they are classified as physical changes.
Q6Let Us Enhance Our Learning
Is curdling of milk a physical or chemical change? Justify your statement.
Solution
Curdling of milk is a chemical change.
Justification:
When milk turns into curd, a new substance with different properties is formed. The taste, smell, and chemical composition of curd are different from those of milk. This transformation is caused by a chemical reaction involving bacteria that convert lactose (milk sugar) into lactic acid. Since a new substance is formed and the process is irreversible (curd cannot be turned back into milk), it is a chemical change.
Q7Let Us Enhance Our Learning
Natural factors, such as wind, rain, etc., help in the formation of soil from rocks. Is this change physical or chemical and why?
Solution
The formation of soil from rocks, known as weathering, involves both physical and chemical changes.
Explanation:
- Physical Changes: Large rocks are broken down into smaller pieces by physical forces like temperature changes (expansion and contraction), freezing of water in cracks, and the action of wind and flowing water. This is a physical change because only the size of the rock particles changes, not their chemical composition.
- Chemical Changes: Water and chemicals present in it can react with the minerals in the rocks, changing their chemical composition. For example, iron-containing rocks can react with oxygen and water to form iron oxide (rust), which has a different colour and properties. This is a chemical change.
Therefore, soil formation is a complex process that includes both physical and chemical changes.
Q8Let Us Enhance Our Learning
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 mealin 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.
Solution
Suitable Title: Prithvi's Green Kitchen
The most appropriate options are:
-
He chops vegetables, peels potatoes, and cuts fruits (physical changes). Reason: Only the shape and size of the vegetables and fruits are changing; no new substance is formed.
-
He collects the seeds, fruits, and vegetable peels into a clay pot (physical change). Reason: This is simply mixing different items together without any chemical reaction.
-
The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost (chemical change). Reason: Decomposition is a chemical process where complex organic materials are broken down into simpler new substances (compost).
-
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). Reason: Growth is a complex biological process involving many chemical reactions (like photosynthesis) to produce new cells and tissues.
Q9Let Us Enhance Our Learning
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.
Solution
The changes are classified as follows:
-
A (Physical Changes only):
- Tearing of paper
- Melting of ice
- Folding of clothes
-
B (Chemical Changes only):
- Rusting
- Curdling of milk
- Ripening of fruits
- Burning of magnesium
- Mixing baking soda with vinegar
-
C (Both Physical and Chemical Changes):
- Process of burning a candle
Q10Let Us Enhance Our Learning
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?
Solution
Lime water turns milky in the presence of carbon dioxide () gas. This happens because carbon dioxide reacts with lime water (calcium hydroxide) to form calcium carbonate, a white insoluble substance.
Based on the experiments shown:
-
Case (a): A person is blowing exhaled air into lime water. Exhaled air is rich in carbon dioxide. Therefore, the lime water will turn milky.
-
Case (b): Air is being pumped into lime water. Normal air contains a very small amount of carbon dioxide (about 0.04%). This is usually not enough to cause a noticeable change in the short duration of an experiment. Therefore, the lime water will likely not turn milky, or the change would be negligible.
-
Case (c): Gas produced from the reaction of vinegar and baking soda is passed through lime water. This reaction produces a large amount of carbon dioxide gas. Therefore, the lime water will turn milky.
-
Case (d): Gas produced from heating sugar is passed through lime water. When sugar is heated strongly, it undergoes combustion, which produces carbon dioxide and water. Therefore, the lime water will turn milky.
Conclusion: Lime water turned milky in cases (a), (c), and (d) because carbon dioxide gas was produced and passed through it in each of these setups.