Class 7 Science Chapter- 5 Changes Around Us: Physical and Chemical Questions and Answers
Let Us Enhance Our Learning Page no: 69
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) (b) (ii) and (iii)
(c) (i) and (iii) (d) (iii) and (iv)
Answer: The correct option is (c) (i) and (iii).
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 : Cannot be reversed. Once stitched, the cloth cannot return to its original form without cutting.
(ii) Twisting of straight string : The string can be untwisted back to its original state.
(iii) Making idlis from a batter : Once cooked, the batter cannot return to its original form.
(iv) Dissolving sugar in water : The sugar can be recovered by evaporating the water.
(v) Drawing water from a well : Once drawn, the water cannot return to the well.
(vi) Ripening of fruits : Once ripened, fruits cannot return to their unripe state.
(vii) Boiling water in an open pan : The water can be condensed back into liquid form.
(viii) Rolling up a mat : The mat can be unrolled back to its original state.
(ix) Grinding wheat grains to flour : Once ground, the flour cannot return to its original grain form.
(x) Forming of soil from rocks : This is a natural process that cannot revert to the original rocks.
Question: 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)
Answer: True.
Explanation: The wax must melt to be drawn up the wick and vaporize, allowing it to burn and produce light.
(ii) Collecting water vapour by condensing involves a chemical change. (True/False)
Answer: False
Correct Statement: Collecting water vapour by condensing involves a physical change.
Explanation: During condensation, water changes from a gas to a liquid without altering its chemical composition.
(iii) The process of converting leaves into compost is a chemical change. (True/False)
Answer: True
Explanation: The decomposition of leaves involves a chemical change as microorganisms break down the organic matter, resulting in new substances.
(iv) Mixing baking soda with lemon juice is a chemical change. (True/False)
Answer: True
Explanation: This reaction produces carbon dioxide gas, indicating a chemical change has occurred as new substances are formed.
Page no: 70
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 , white powder , chemical.
Explanation of Key Terms :-
1. Rusting: This is a chemical change that occurs when iron reacts with oxygen and moisture, forming iron oxide (rust).
2. Physical Change: A change that affects one or more physical properties of a substance without altering its chemical composition. Folding is an example of such a change.
3. Combustion: This is a chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light.
4. Magnesium Oxide: A white powder that is formed when magnesium burns in air, indicating a chemical change has occurred.
Question: 5. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer: The changes of water to ice and water to steam are physical changes.
1. Water to Ice: This process is called freezing. When water cools down, its molecules lose energy and come together to form a solid structure (ice). No new substance is created; the chemical composition of water (H₂O) remains the same.
2. Water to Steam: This process is known as vaporization. When water is heated, its molecules gain energy and move apart to become water vapor (steam). Again, the chemical composition remains unchanged.
In both cases, the changes are reversible: ice can melt back into water, and steam can condense back into water.
Question: 6. Is curdling of milk a physical or chemical change? Justify your statement.
Answer: Curdling of milk is a chemical change.
1. During curdling, the proteins in milk (casein) react with acids (like lemon juice or vinegar) or with the natural bacteria present in milk. This reaction causes the proteins to coagulate and form curds, while the liquid whey separates.
2. A new substance (curds) is formed, which has different properties from the original milk. This process cannot be reversed to obtain the original milk, further indicating that it is 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 due to natural factors like wind and rain involves both physical and chemical changes, but primarily it is a physical change.
1. Physical Change: Weathering breaks down rocks into smaller particles through physical processes like temperature changes, freezing and thawing, and erosion caused by wind and water. These processes do not alter the chemical composition of the rocks; they merely change their size and shape.
2. Chemical Change: However, chemical weathering can also occur when water or acids react with minerals in the rocks, leading to changes in their chemical composition. For example, iron in rocks can oxidize when exposed to water and air, forming rust.
In summary, while the initial breakdown of rocks into soil is primarily a physical change, chemical changes can also contribute to soil formation.
Question: 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 flowers (physical change/chemical change). His efforts are appreciated by all his family members.
Answer: Title Suggestion: “Prithvi’s Sustainable Kitchen”
Options :-
1. Prithvi is preparing a meal in the kitchen. He chops vegetables, peels potatoes, and cuts fruits – (Physical changes).
2. He collects the seeds, fruits, and vegetable peels into a clay pot – (Physical change).
3. The fruits, vegetable peels, and other materials begin to decompose due to the action of bacteria and fungi, forming compost – (Chemical change).
4. 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).
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: Area A (Physical Changes):
- Tearing of paper
- Melting of ice
- Folding of clothes
Area B (Chemical Changes):
- Rusting
- Curdling of milk
- Burning of magnesium
- Mixing baking soda with vinegar
Area C (Both Physical and Chemical Changes):
- Process of burning a candle
- Ripening of fruits
Page no: 71
Question: 10. The experiments shown in Fig. 5.11 a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why?
1. Experiment (a): Vinegar and Baking Soda
– Observation: Lime water turns milky.
– Reason: The reaction produces carbon dioxide gas, which reacts with lime water.
2. Experiment (b): Vinegar and Common Salt
– Observation: Lime water does not turn milky.
– Reason: No carbon dioxide is produced in this reaction.
3. Experiment (c): Lemon Juice and Vinegar
– Observation: Lime water does not turn milky.
– Reason: Similar to experiment (b), no carbon dioxide is produced.
4. Experiment (d): Lemon Juice and Baking Soda
– Observation: Lime water turns milky.
– Reason: The reaction produces carbon dioxide, which reacts with lime water.
Conclusion:-
1. Lime water turned milky in experiments (a) and (d) due to the formation of carbon dioxide gas, which reacts with lime water to produce calcium carbonate.
2. In experiments (b) and (c), lime water did not turn milky because carbon dioxide was not produced.