RBSE Solutions Class 7 Science Chapter 5 Changes Around Us Physical and Chemical

Class 7 Science Curiosity Chapter 5 Question Answer (Exercise)

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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.
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (iii) and (iv)
Answer: (c) (i) and (iii)
In simple words: Physical changes mean the substance looks different but is still the same material. Its state might change (like water to ice), but it can still be changed back. For example, melting ice is a physical change, as it is still water.

🎯 Exam Tip: Remember that physical changes alter the form or appearance of a substance but not its chemical composition. Look for clues that indicate no new substance is formed.

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
(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
Answer:
(i) Stitching cloth to a shirt: Irreversible. Once a shirt is made from pieces of cloth, it cannot be easily changed back into cloth pieces without cutting or tearing. This transformation changes the form permanently.
(ii) Twisting of straight string: Reversible. A twisted string can be untwisted to become straight again. This change only alters the shape temporarily.
(iii) Making idlis from a batter: Irreversible. Idlis are cooked from batter, and they cannot be converted back into batter. Cooking usually causes irreversible chemical changes.
(iv) Dissolving sugar in water: Reversible. Water can be evaporated to get the sugar back, showing it is a physical change. This is a common way to separate mixtures.
(v) Drawing water from a well: Reversible. Water drawn from a well can be put back into the well. This action simply moves the water's location.
(vi) Ripening of fruits: Irreversible. Fruit ripening is a chemical change where starch turns into sugar and the fruit's structure changes. Ripened fruits cannot become unripe again.
(ix) Grinding wheat grains to flour: Irreversible. Grinding changes the shape and size of the wheat grains completely into flour. You cannot easily get the original grains back from the flour.
(x) Forming of soil from rocks: Irreversible. This is a very long process involving both chemical and physical changes to rocks. The soil formed cannot be converted back into rocks.
In simple words: Look if you can easily undo the change. If you can, it's reversible. If a new thing is made or the original thing is permanently changed, it's irreversible.

🎯 Exam Tip: To determine if a change is reversible or irreversible, imagine if you can get the original substance back simply by reversing the action. If a new substance is formed, the change is typically irreversible.

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)
(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)
Answer:
(i) True
(ii) False; Correct statement: Collecting water vapour by condensing involves a physical change, not a chemical change. Condensation is simply a change of state from gas to liquid.
(iii) True
(iv) True
In simple words: True statements are facts. For false statements, you need to write the correct fact to make it true.

🎯 Exam Tip: When correcting a false statement, focus on changing only the incorrect part to make the statement factually accurate, while keeping the rest of the sentence structure.

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.
(ii) Folding a handkerchief is a ______ change and can be ______.
Answer:
(i) Nalini observed that the handle of her cycle has got brown deposits. The brown deposits are due to rust and this is a chemical change. Rusting is a common example of oxidation.
(ii) Folding a handkerchief is a physical change and can be reversible. No new substance is formed when a handkerchief is folded.
In simple words: Fill in the missing words to complete the sentences correctly, thinking about what kind of change each event is.

🎯 Exam Tip: Pay attention to keywords in the blanks, like 'deposits' suggesting rust, or 'folding' implying a physical, reversible action, to choose the right terms.

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 both physical changes. This is because only the state of matter changes (liquid to solid, or liquid to gas), but no new substance is formed. The chemical identity of water, \( H_2O \), remains the same throughout these changes.
In simple words: When water turns into ice or steam, it's still water, just in a different form. So, these are physical changes.

🎯 Exam Tip: Remember that changes in state (solid, liquid, gas) are always physical changes because the substance's chemical makeup doesn't change.

Question 6. Is curdling of milk a physical or chemical change? Justify your statement.
Answer: Curdling of milk is a chemical change. This is because a new substance, curd, is formed due to chemical reactions involving the proteins in milk. This process is irreversible and creates a substance with new properties.
In simple words: Curdling milk makes a completely new food (curd) that you can't turn back into milk. This shows it's a chemical change.

🎯 Exam Tip: Look for signs of a chemical change like the formation of a new substance, change in color, odor, or release of gas or heat. Curdling demonstrates the formation of a new substance.

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, influenced by natural factors like air, rain, and temperature, involves both physical and chemical changes. When rocks break into smaller pieces, it is a physical change. However, when air, water, and gases react with minerals in the rocks, it leads to chemical changes, altering their composition. Therefore, soil formation is a combined (physical and chemical) reaction, showing how complex natural processes can be.
In simple words: Soil forms from rocks due to both physical breaking down and chemical reactions that change the rock minerals. So, it's both types of change working together.

🎯 Exam Tip: Many natural processes are a combination of both physical and chemical changes. Think about how the substance's size/shape changes (physical) and if its basic makeup changes (chemical).

Question 8. Read the following story titled 'Ecofriendly 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 material begin to decompose due to the action of bacteria and fungi, forming compost
Answer:
Title: Changes in Prithvi's Story
Chops vegetables, peels potatoes, and cuts fruits – Physical change (These actions only alter the size and shape of the food items)
Collecting peels in clay pot – Physical change (This is just gathering items, no chemical reaction)
Forming compost – Chemical change (Decomposition breaks down organic matter into new substances)
Blooming of flowers – Chemical change (The growth and development of flowers involve complex chemical processes)
In simple words: Physical changes alter how things look, while chemical changes make new things. Prithvi does both when cooking and composting.

🎯 Exam Tip: When evaluating changes, think about whether the original substance can easily be retrieved or if its fundamental nature has been altered to create something new.

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'.
Answer:
Classification of changes:
'A' (Physical changes) – Tearing of paper, Melting of ice, Folding of clothes. These changes only alter appearance.
'B' (Chemical changes) – Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar. These create new substances.
'C' (Both physical and chemical changes) – Burning of candle. The wax melts (physical) and burns (chemical).
In simple words: Physical changes are like tearing paper, chemical changes are like rusting metal, and some things like a burning candle show both at the same time.

🎯 Exam Tip: For Venn diagrams, clearly identify what each section represents. The intersection 'C' is key for changes that involve both physical and chemical aspects simultaneously.

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?
Answer: Lime water turns milky in cases (a) and (d). This happens because in both these experiments, carbon dioxide gas is released when acid reacts with baking soda. The carbon dioxide gas then turns the lime water milky. In case (a), we have:
(a) Vinegar + Baking soda \( \rightarrow \) \( CO_2 \) + Other substances
And in case (d), we have:
(d) Lemon juice + Baking soda \( \rightarrow \) \( CO_2 \) + Other substances
There will be no change in lime water in cases (b) and (c) because carbon dioxide gas is not emitted in those scenarios. This is a classic test for carbon dioxide presence.
In simple words: Lime water becomes milky when carbon dioxide gas is present. This gas is made when vinegar or lemon juice mixes with baking soda.

🎯 Exam Tip: The 'lime water test' is a key indicator for the presence of carbon dioxide. Remember that acids reacting with carbonates (like baking soda) typically produce \( CO_2 \).

Changes Around Us Physical and Chemical Class 7 Questions and Answers (Intext)

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Question 1. Can we arrange these changes into categories?
Answer: Yes, we can arrange the various changes happening around us into different categories. These include physical changes, chemical changes, desirable changes, undesirable changes, reversible changes, and irreversible changes. Categorizing helps us understand and study them better.
In simple words: Yes, we can put changes into groups like physical, chemical, good, bad, reversible, and irreversible.

🎯 Exam Tip: Always try to classify changes based on whether a new substance is formed (chemical) or just its form alters (physical), and if it can be easily undone (reversible).

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Question 2. Do you get the same paper back when you unfold the objects of paper?
Answer: No, you do not get the exact same paper back when you unfold a folded paper object. While it is a physical change, and the paper remains paper, it will have creases due to the folding. The presence of these creases means the paper's original smooth state is not fully restored. You can still use the paper for most things, but it won't be perfectly flat.
In simple words: No, the paper will have folds and creases, so it won't be exactly the same as before you folded it.

🎯 Exam Tip: Even in physical changes, some small alterations in form or texture, like creases from folding, can remain, showing that "reversible" doesn't always mean "perfectly restored."

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Question 4. What do you infer about the gas formed by mixing vinegar and baking soda?
Answer: When vinegar and baking soda are mixed, they react to form carbon dioxide gas. This is a chemical change, and the gas produced is evident by fizzing or bubbling. The reaction can be shown as:
Vinegar + Baking Soda \( \rightarrow \) Carbon dioxide + Other substances
This gas can turn lime water milky, confirming its identity.
In simple words: Mixing vinegar and baking soda makes carbon dioxide gas, which you can see as bubbles.

🎯 Exam Tip: Reactions that produce gas, visible as fizzing or bubbling, are often indicators of a chemical change, like the classic vinegar and baking soda experiment.

Question 5. Let us find out whether the presence of oxygen is essential for combustion.
Answer: Yes, the presence of oxygen is absolutely essential for combustion to occur. Combustion is a chemical process where a substance reacts rapidly with oxygen, usually producing heat and light. Without oxygen, most common combustion processes cannot sustain themselves, which is why smothering a fire with a blanket helps put it out.
In simple words: Yes, oxygen is needed for things to burn. Without it, fires cannot start or keep going.

🎯 Exam Tip: Remember the 'fire triangle' — fuel, oxygen, and heat are all necessary for combustion. Removing any one stops the burning process.

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Question 6. What changes take place when a candle is lit? Let us explore.
Answer: When a candle is lit, both physical and chemical changes take place. The wax melts and then solidifies again as it cools (physical change). Simultaneously, the wax vapor burns to produce new substances like carbon dioxide and water vapor (chemical change). This makes a candle burning a good example of simultaneous changes.
In simple words: When a candle burns, the wax melts and gets hard again (physical change), and some wax turns into gas and burns up (chemical change).

🎯 Exam Tip: A burning candle is a classic example of a phenomenon involving both physical (melting wax) and chemical (burning wax) changes at the same time.

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Question 7. Once something has undergone a change, can we get it back in its original form?
Answer: Whether something can be returned to its original form after a change depends on the type of change. Some changes are reversible, meaning the original form can be regained. For example, melting ice into water or changing water into water vapor can be reversed by freezing or condensation. However, other changes are irreversible, such as chopping vegetables or making popcorn, where the original form cannot be recovered. So, it depends on the nature of the change.
In simple words: Sometimes yes, sometimes no. If it's a simple change like melting ice, you can turn it back. But if you cook or chop something, you usually can't get the original back.

🎯 Exam Tip: Always consider if a new substance has been formed or if the change is permanent; this helps determine if a change is reversible or irreversible.