NCERT Solutions Class 7 Science Curiosity Chapter 05 Changes Around Us: Physical and Chemical

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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.
(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: (c) (i) and (iii)
In simple words: Only physical traits like shape or size change. No new thing is made.

Exam Tip: Remember that physical changes are usually easy to reverse, like melting ice back into water.

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: It cannot be reversed. Once you cut and sew the fabric, it is permanently changed into a shirt.
(ii) Twisting of straight string: It can be reversed. You can easily untwist the string to make it straight again.
(iii) Making idlis from a batter: It cannot be reversed. Cooking changes the batter chemically. You cannot get the raw batter back.
(iv) Dissolving sugar in water: It can be reversed. Heat the water to make it evaporate. This leaves the sugar crystals behind.
(v) Drawing water from a well: It can be reversed. This only changes the location of the water. You can pour it back down.
(vi) Ripening of fruits: It cannot be reversed. Chemical changes happen inside the fruit as it ripens. These cannot be undone.
(vii) Boiling water in an open pan: It can be reversed. The steam can be cooled down. Then you collect it back as liquid water.
(viii) Rolling up a mat: It can be reversed. You can unroll the mat to make it flat again.
(ix) Grinding wheat grains to flour: It cannot be reversed. This is a physical change. But we cannot put the tiny flour dust back into solid grains.
(x) Forming of soil from rocks: It cannot be reversed. This is a very slow process. It breaks down rocks over hundreds of years.
In simple words: Reversible changes can be undone easily, while irreversible changes are permanent and cannot go back to how they started.

Exam Tip: Look at whether a completely new thing is formed. If cooking or chemical reactions occur, the change is almost always 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. The wax must melt and turn into vapor before it can burn.
(ii) False. Correct statement: Condensing water vapor is a physical change. It only changes its state from a gas to a liquid.
(iii) True. When leaves decay into compost, bacteria break them down. This is a chemical reaction.
(iv) True. This reaction produces gas bubbles. It also creates entirely new substances.
In simple words: Physical changes do not make new things, but chemical changes create completely new substances.

Exam Tip: Always check if the state of matter is just changing (like steam turning back to water) - that is always a physical change.

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 ______.
(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.
Answer:
(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.
In simple words: Rusting, combustion, and burning are chemical changes that make new things. Folding is a simple physical change.

Exam Tip: For fill-in-the-blank questions, highlight or bold the answers so they stand out clearly to the examiner.

Question 5. Are the changes of water to ice and water to steam, physical or chemical? Explain.
Answer: These are physical changes.
Explanation: When water freezes into ice or boils into steam, only its form changes. It changes from liquid to solid, or liquid to gas. However, the substance is still water (\( \text{H}_2\text{O} \)). No new material is made. You can easily change them back to liquid water.
In simple words: Turning water into ice or steam only changes its shape or state. It is still water, so it is a physical change.

Exam Tip: State changes (like freezing, melting, boiling, and condensation) are always physical changes and are reversible.

Question 6. Is curdling of milk a physical or chemical change? Justify your statement.
Answer: Curdling of milk is a chemical change.
Justification: Curd is an entirely new substance. It has a different sour taste and a thicker feel. It also has a new chemical makeup. You cannot turn curd back into milk. This is because bacteria have changed the milk. This is a chemical reaction.
In simple words: Milk turning into curd is a chemical change. It makes a new sour substance that cannot be turned back into milk.

Exam Tip: If a process cannot be undone and produces a new substance with new properties (like curd), 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: Soil forming from rocks involves both physical and chemical changes.
Why: First, rocks break into smaller pieces. This is caused by hot and cold weather, freezing water, or plant roots. These are physical changes. Second, rocks react with water, air, or weak acids in rain. This changes what the rocks are made of. This is a chemical change. This whole process is called weathering.
In simple words: Making soil from rocks uses both types of changes. Cracking rocks is physical, but rain changing the rock minerals is chemical.

Exam Tip: The slow breaking down of rocks into soil by natural forces is called 'weathering' and is a great example of a combined physical and chemical process.

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: The correct choices from the story are:
- Chopping vegetables, peeling potatoes, and cutting fruits are physical changes.
- Collecting the seeds, fruits, and vegetable peels into a clay pot is a physical change.
- The decaying of fruits, vegetable peels, and other materials to form compost is a chemical change.
- Germinating seeds, growing plants, and blooming flowers are chemical changes because they involve life processes.

Suitable Titles:
- 'Prithvi's Green Garden'
- 'From Waste to Beautiful Flowers'
- 'Eco-friendly Gardening with Prithvi'
In simple words: Chopping and moving things are physical changes. Rotting into compost and growing into flowers are chemical changes.

Exam Tip: Any biological process like seed growth, decay, or digestion involves complex chemical reactions and is a 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’. (Venn Diagram) 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: The classification of these changes is shown below:
- Area A (Physical Changes only): Tearing of paper, Melting of ice, Folding of clothes.
- Area B (Chemical Changes only): Rusting, Curdling of milk, Ripening of fruits, Burning of magnesium, Mixing baking soda with vinegar.
- Area C (Both Physical and Chemical): Process of burning a candle.

Exam Tip: A burning candle melts wax (physical) and burns wax vapor (chemical) at the same time.

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? (Diagram shows reactions piped into lime water) (a) Vinegar and baking soda (b) Lemon juice and vinegar (c) Vinegar and common salt (d) Lemon juice and baking soda
Answer: Lime water turns milky in setup (a) and setup (d).
Why: Carbon dioxide gas makes lime water turn cloudy or milky. When you mix baking soda with an acid, like vinegar in (a) or lemon juice in (d), it creates carbon dioxide gas. In cases (b) and (c), mixing two acids together or mixing an acid with salt does not make this gas.
In simple words: Baking soda reacting with acids (vinegar or lemon juice) makes carbon dioxide gas, which turns lime water milky.

Exam Tip: The classic test for carbon dioxide gas is that it turns lime water milky due to the formation of insoluble calcium carbonate.

Class 7 Science Curiosity Chapter 5 Very Short Answer Type Questions

Question 11. What is a physical change?
Answer: A physical change is a change where only the size, shape, or state of a substance is altered. No new substance is created during this process.
In simple words: A change where the shape or state changes, but no new thing is made.

Exam Tip: Always specify that physical changes are generally reversible and do not produce new substances.

Question 12. What is a chemical change?
Answer: A chemical change is a process where one or more entirely new substances are created because of a chemical reaction.
In simple words: A change that makes a completely new substance.

Exam Tip: Keywords like 'new substance' and 'chemical reaction' must be included to get full marks.

Question 13. Give one example of a reversible change.
Answer: The melting of ice is an example of a reversible change. This is because you can easily freeze the liquid water back into solid ice.
In simple words: Ice melting is reversible because you can freeze the water back into ice.

Exam Tip: Most physical changes of state, like melting or freezing, are easy to reverse.

Question 14. Which gas turns lime water milky?
Answer: Carbon dioxide gas turns clear lime water milky. This change is a sign that a chemical reaction has taken place.
In simple words: Carbon dioxide gas is the one that makes lime water look milky.

Exam Tip: This is a standard chemical test. Make sure you spell 'carbon dioxide' correctly.

Question 15. What is ignition temperature?
Answer: Ignition temperature is the lowest temperature that a substance needs to reach in order to catch fire and begin burning.
In simple words: The lowest heat needed for something to catch fire and burn.

Exam Tip: For combustion to occur, a substance must be heated to its ignition temperature.

Class 7 Science Curiosity Chapter 5 Short Answer Type Questions

Question 16. How is a physical change different from a chemical change?
Answer: A physical change only alters the physical traits of a substance, like its shape or size, and can usually be undone. A chemical change produces completely new substances and is typically impossible to reverse.
In simple words: Physical changes can be undone and make no new things. Chemical changes make new things and cannot be undone.

Exam Tip: Use a table or T-chart to show this comparison in exams. It makes your answer very clear to the examiner.

Question 17. Why is rusting of iron considered a chemical change?
Answer: Rusting is a chemical change because iron reacts with air and water to form a brand new substance called iron oxide (rust). Since this reaction creates a new substance with different properties, it is chemical.
In simple words: Iron reacts with wet air to make a new brown stuff called rust. Making new stuff means it is a chemical change.

Exam Tip: In your answer, specify that both oxygen (from air) and moisture (water) are necessary for rusting to happen.

Question 18. What is combustion?
Answer: Combustion is a chemical reaction in which a fuel reacts with oxygen to produce heat and light. Classic examples are burning wood, burning kerosene, or burning magnesium ribbon.
In simple words: Combustion is when something burns in air and gives off heat and light.

Exam Tip: Always define combustion as a reaction with oxygen, and give at least one common example.

Question 19. How does burning of a candle involve both physical and chemical changes?
Answer: When a candle burns, two things happen. The melting of solid wax and its hardening are physical changes. Meanwhile, the wax vapor reacts with oxygen in the air and burns. This produces new substances like carbon dioxide and water vapor, which is a chemical change.
In simple words: Melting wax is a physical change, but when the wax vapor burns and turns into gases, that is a chemical change.

Exam Tip: Be precise: state that melting wax is physical, but burning wax is chemical. Do not just say 'melting and burning' without explaining which is which.

Question 20. What is the role of oxygen in combustion?
Answer: Oxygen is necessary because it supports combustion. A substance cannot catch fire or keep burning without it. For example, if you cover a lit candle with a glass jar, it goes out once the oxygen is used up.
In simple words: Oxygen is needed for things to burn. Without air, a fire will go out quickly.

Exam Tip: Mention the covered glass jar experiment as a simple, visual proof that combustion needs oxygen.

Class 7 Science Curiosity Chapter 5 Descriptive Answer Type Questions

Question 21. What are the three essential conditions for combustion?
Answer: For combustion to occur, three key conditions must be met:
1. A combustible substance, which acts as the fuel.
2. A continuous supply of oxygen (usually from the air).
3. Sufficient heat to raise the temperature of the fuel to its ignition point.
All three components must be present for a fire to burn. If you remove any one of these three elements, the fire will go out.
In simple words: To make a fire, you need three things: fuel to burn, air (oxygen), and heat. If you take away any of these, the fire stops.

Exam Tip: These three conditions are often called the 'Fire Triangle'. Remembering this visual concept can help you list all three conditions easily.

Question 22. Describe what happens when vinegar reacts with baking soda.
Answer: When you combine vinegar with baking soda, they undergo a rapid chemical reaction. This process releases bubbles of carbon dioxide gas. You can verify that a new substance is formed. This is because the gas turns clear lime water milky, which proves that a chemical change has taken place.
In simple words: Mixing vinegar and baking soda makes fizzy carbon dioxide gas. This gas turns lime water milky, showing a new substance was made.

Exam Tip: If asked to describe this, write down the word equation if you know it: Vinegar + Baking soda -> Carbon dioxide + Other substances.

Question 23. Why is burning of magnesium a chemical change?
Answer: When magnesium burns in air, it reacts with oxygen to form a completely new white powder called magnesium oxide. This reaction also produces a very bright light and intense heat. Because a brand new substance is created, this process is classified as a chemical change.
In simple words: Burning magnesium makes a new white ash called magnesium oxide. Since a new substance is made, it is a chemical change.

Exam Tip: Mention the bright white light and the formation of 'magnesium oxide' to secure full marks for this question.

Question 24. How do both physical and chemical changes occur in a burning candle?
Answer: Inside a burning candle, two actions happen at the same time. First, the solid wax melts into a liquid. Then it turns into wax vapor. This is a physical change because it is still wax. Second, this wax vapor reacts with oxygen in the air to burn. This produces new substances like carbon dioxide and water vapor, which is a chemical change. Therefore, both changes take place together.
In simple words: Melting wax is a physical change, but when the wax vapor burns and turns into gases, that is a chemical change.

Exam Tip: Make sure to clearly split your answer into two parts: the melting of wax (physical) and the burning of vapor (chemical).

Question 25. What is weathering of rocks? Explain with an example.
Answer: Weathering is the slow process where rocks break down into tiny soil particles over long periods. This happens through physical actions like wind, rain, and temperature shifts. It also happens through chemical reactions. For example, dark basalt rock can react with water and oxygen in the air. This turns the rock red as it forms iron oxide.
In simple words: Weathering is rocks breaking down into soil over a long time. For example, rocks can rust and turn red when wet air touches them.

Exam Tip: Define both the physical and chemical aspects of weathering, and use the basalt rock turning red as a concrete example.

Class 7 Science Curiosity Chapter 5 Exploring Questions

Question 26. Why are some changes considered undesirable?
Answer: We call certain changes undesirable because they cause harm or damage to our things or health. For example, rusting makes iron structures weak, and rotting makes food unsafe to eat. However, some changes that look bad, like rotting waste, are actually useful when they make compost for gardens.
In simple words: Undesirable changes are changes we do not want because they ruin things, like food spoiling or metal rusting.

Exam Tip: Provide clear examples of harmful changes (rusting, food spoiling) to support your explanation of why they are undesirable.

Question 27. Can chemical changes be prevented? Explain with an example.
Answer: Yes, we can prevent many chemical changes. For example, we can stop iron from rusting by painting it, putting oil on it, or coating it with zinc (called galvanizing). These methods keep water and air away from the metal, stopping the chemical reaction.
In simple words: Yes, we can stop chemical changes. We can paint iron so that air and water cannot touch it and make it rust.

Exam Tip: Learn the term 'galvanising' as it is a highly specific method examiners love to see on papers.

Question 28. Why is burning paper using sunlight a chemical change?
Answer: When you focus hot sunbeams onto paper using a magnifying glass, the paper heats up, catches fire, and burns. It turns into gray ash and releases smoke, which are completely new substances. Since this process is permanent and cannot be undone, it is a chemical change.
In simple words: Burning paper with sunlight turns it into ash and smoke. You cannot get the paper back, so it is a chemical change.

Exam Tip: Whenever paper is burned, regardless of how the fire started, it is a chemical change because ash and gases are produced.

Question 29. Is curdling of milk a physical or chemical change? Justify.
Answer: Curdling of milk is a chemical change. Inside the milk, special bacteria act on the proteins and acids to turn them into curd. This reaction creates a brand new substance, and you can never change curd back into liquid milk.
In simple words: Bacteria change the proteins in milk to make curd. Because it makes a new thing and cannot be undone, it is a chemical change.

Exam Tip: Be ready to explain that bacteria cause this change by acting on the milk's natural proteins.

Question 30. How does erosion represent a physical change?
Answer: Erosion is a physical change because it only breaks down rocks and soil and moves them to a new place using wind or water. No new substances are created in this process; only the shape and location of the dirt and rocks are altered.
In simple words: Erosion just breaks and moves rocks and soil around. No new materials are created, so it is a physical change.

Exam Tip: Remember that moving something to a new place (transportation) is a physical change, not a chemical one.

Question 31. Why do some changes around us happen quickly while others take years, as explained in Class 7 Science Curiosity Chapter 5?
Answer: According to the chapter, changes happen at many different speeds depending on the process. Changes like melting ice, boiling water, or lighting a candle take only seconds or minutes. These are physical changes or very rapid chemical ones. In contrast, processes like rocks breaking down, soil forming, or iron rusting can take weeks, years, or even hundreds of years. Natural forces like wind, rain, and temperature changes drive these slow processes. How fast a change happens depends on the substance, the heat around it, and the environment.
In simple words: Some changes like melting ice happen fast, while others like soil forming take years because they depend on slow natural forces.

Exam Tip: Classify changes into fast and slow categories, and use everyday examples for each to make your answer strong.

Question 32. Can one activity involve both physical and chemical changes at the same time in Class 7 Science Curiosity Chapter 5?
Answer: Yes, a single activity can involve both physical and chemical changes. The best example of this is a burning candle. When the wax melts, turns to vapor, and then cools back into solid wax, these are all physical changes. This is because the wax is still wax and no new substance is made. However, when the wax vapor burns by reacting with oxygen, it produces heat, light, and new substances like carbon dioxide and water vapor. This is a chemical change. This example shows that real-world changes can be complex. They often involve both types of changes at once.
In simple words: Yes, like when a candle burns: melting the wax is a physical change, but burning the wax vapor is a chemical change.

Exam Tip: The burning candle is the primary textbook example of a simultaneous physical and chemical change. Learn it thoroughly.

Question 33. How can we test for the presence of carbon dioxide gas using a simple classroom method, as taught in Class 7 Science Curiosity Chapter 5?
Answer: We can easily test for carbon dioxide gas using lime water. When we pass this gas through clear lime water, they react to form a white powder called calcium carbonate. This makes the liquid look milky or cloudy. This shows a chemical change has happened. To do this in class, you can mix vinegar and baking soda in a test tube to make the gas. Then, use a tube to pass that gas into lime water. When the lime water turns milky, it proves that carbon dioxide is present.
In simple words: Pass the gas through clear lime water. If the liquid turns milky, it means carbon dioxide gas is there.

Exam Tip: Be ready to explain that the milky look is caused by the formation of 'calcium carbonate' when carbon dioxide reacts with lime water.

Question. What types of changes do I need to remember in Class 7 Science Curiosity Chapter 5?
Answer: You need to focus on two major types of changes: physical and chemical. Physical changes do not create new substances and can usually be undone, like melting ice back into water. Chemical changes create completely new substances and are hard to reverse, like rusting metal or curdling milk. Keeping a few real-life examples in mind will help you spot them easily.
In simple words: Focus on physical changes (which make no new things and can be undone) and chemical changes (which make new things and cannot be undone).

Exam Tip: List at least three everyday examples of each type of change to write a complete answer in your exam.

Question. Is Class 7 Science Curiosity Chapter 5 difficult to understand?
Answer: No, this chapter is very easy to understand because it is based on everyday changes we see around us. By connecting concepts like physical and chemical changes to simple processes like melting ice, boiling water, and curdling milk, you can grasp the topics easily.
In simple words: No, this chapter is simple because it uses everyday examples like ice, water, and milk to explain the ideas.

Exam Tip: Focus on the differences between physical and chemical changes, as this is the most important concept in the chapter.

Question. How can I prepare for Class 7 Science Curiosity Chapter 5 in a better way?
Answer: You can prepare for this chapter effectively by performing or observing simple classroom experiments, such as mixing vinegar with baking soda. Make a table comparing physical and chemical changes, memorize their key characteristics, and practice identifying various real-life changes as reversible or irreversible.
In simple words: Practice by grouping everyday changes into physical and chemical categories, and write down their differences.

Exam Tip: Be thorough with the standard chemical tests, like using lime water to detect carbon dioxide, since they are frequently asked in exams.