This MCQ module is based on: Changes Around Us
TOPIC 18 OF 46
Changes Around Us
🎓 Class 7
Science
CBSE
Theory
Ch 5 — Changes Around Us: Physical and Chemical
⏱ ~8 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_7" science_domain="chemistry" difficulty="basic"]
Chapter 5 — Summary
- A change happens when something in a material is altered — its shape, size, state, colour, smell or very substance.
- A physical change changes only physical properties (shape, size, state). The substance is the same and the change is usually reversible.
- A chemical change produces one or more new substances with different properties. Signs include colour change, gas release, heat/light change, precipitate or new smell. These changes are usually irreversible.
- Many real processes (candle burning, cooking rice, digestion) show both types of changes at the same time.
- Rusting of iron needs iron + oxygen + water together. It is prevented by painting, oiling, galvanising, chrome plating or using stainless steel.
- Crystallisation produces pure crystals from a saturated solution on cooling (CuSO₄, salt from sea water). It is a reversible, physical change.
- Combustion is a chemical change — a fuel burns in oxygen, giving heat and light. The fire triangle (fuel + oxygen + heat above ignition temperature) must be complete.
- To put out a fire, we remove any one side of the triangle. Water cools; CO₂ & foam cut off oxygen. Never use water on electrical or oil fires.
- Exothermic changes release heat (burning, respiration); endothermic changes absorb heat (melting ice, photosynthesis).
Key Terms Glossary
Physical changeShape/size/state changes; same substance; usually reversible.
Chemical changeNew substance with new properties; usually irreversible.
Reversible changeCan be undone; starting material returns.
Irreversible changeCannot be undone; original substance lost.
CombustionBurning in oxygen, giving heat and light.
Ignition temperatureLowest temperature at which a fuel catches fire.
FuelA combustible substance that burns in air.
Fire triangleFuel + oxygen + heat — all three needed for fire.
RustingSlow chemical attack of moist air on iron.
GalvanisationCoating iron with zinc to stop rusting.
CrystallisationPure crystals forming from a saturated solution.
Saturated solutionA solution that cannot dissolve more solute.
ExothermicChange that gives out heat.
EndothermicChange that takes in heat.
Lime water testMilky colour confirms CO₂ gas.
PrecipitateA new insoluble solid formed in a liquid.
NCERT Exercises — Full Solutions
1. Classify each of the following as a physical (P) or a chemical (C) change:
(a) Ice melting (b) Iron rusting (c) Stretching a rubber band (d) Burning of wood (e) Dissolving sugar in water.
(a) Ice melting (b) Iron rusting (c) Stretching a rubber band (d) Burning of wood (e) Dissolving sugar in water.
(a) P — solid water → liquid water, no new substance. (b) C — reddish-brown rust forms. (c) P — only shape changes. (d) C — ash, smoke and gases are new substances. (e) P — sugar can be recovered by evaporating the water.
2. Name any three changes from daily life that are chemical in nature.
Any three of: (i) cooking food (rice, dal, roti), (ii) milk turning into curd, (iii) burning of LPG in a gas stove, (iv) rusting of an iron tool, (v) ripening of fruits, (vi) digestion of food.
3. What is combustion? Name any three substances that can undergo combustion.
Combustion is a chemical process in which a substance burns in the presence of oxygen and produces heat and light. Three combustible substances: wood, LPG (cooking gas), paper (also kerosene, petrol, wax, coal).
4. Explain the fire triangle with a neat diagram. Why does a fire go out if any one side is missing?
5. Why does water help put out most fires? Why should water NEVER be used to put out electrical fires?
Water has a high capacity to absorb heat, so when poured on burning wood or paper it cools the fuel below its ignition temperature and the fire dies. However, ordinary water conducts electricity. On an electrical fire (wiring, appliances), water can carry a live current up to the person holding the bucket, giving a fatal shock. Water also makes oil fires worse because oil floats on water and the burning oil spreads. For electrical and oil fires, use a CO₂ or foam extinguisher instead.
6. List any three methods by which rusting of iron can be prevented.
(i) Painting, greasing or oiling the iron to form a barrier that keeps oxygen and moisture away. (ii) Galvanisation — coating iron with zinc. (iii) Alloying iron with chromium and nickel to make stainless steel, which does not rust. (Also acceptable: chrome plating, tin plating.)
7. In experiments shown in Fig. 5.11 (a–d), a gas is collected in each case and passed through lime water. In which case(s) does the lime water turn milky and why?
Lime water turns milky only in the cases where carbon dioxide (CO₂) is produced — for example, when a candle burns, when baking soda reacts with vinegar, or when a person breathes out. CO₂ reacts with lime water (calcium hydroxide) to form a white insoluble precipitate of calcium carbonate (CaCO₃), giving the milky appearance. In experiments where no CO₂ is formed (like a simple dissolving or melting), lime water stays clear.
8. Is the burning of a candle a physical change or a chemical change? Justify with reasons.
Burning of a candle is actually both. (i) The solid wax near the flame melts into liquid wax — a physical change (reversible; wax is still wax). (ii) Wax vapour then burns in oxygen to form carbon dioxide, water vapour, heat and light — a chemical change (new substances, irreversible). So the single event shows both physical and chemical change happening together.
9. Exploratory: Write a secret message on a piece of paper using lemon juice and a thin stick. Let the paper dry. Then hold the paper gently above a lit candle (without burning it). What do you observe and why?
When the paper is warmed, the hidden letters slowly turn brown and the message appears! Lemon juice contains citric acid and sugars. When heated, these compounds react and get partly burnt/charred before the surrounding paper does. This chemical change produces brown substances that reveal the message. It is a chemical change because a new brown material is formed and cannot be removed easily.
10. Can we reverse the effects of news events like landslides or the breaking of rocks? Similarly, why is the ripening of fruits beyond our control once it starts?
No — once a landslide has sent tonnes of soil and rock down a slope, or once a huge rock has split and fallen, we cannot put the mountain back together exactly the way it was. These are irreversible changes in nature. In the same way, when a fruit ripens, enzymes break down the starch and acids into softer, sweeter substances — new substances form. That is a chemical change and cannot be reversed; a ripe mango cannot become a raw mango again. Ripening also continues on its own once it begins, because the chemical reactions are self-triggering.
11. Spend 10 minutes observing activities in your kitchen. List five changes and classify each as physical or chemical, giving a reason.
Sample answers: (i) Boiling water — physical (only state changes). (ii) Cooking rice — chemical (starch changes; cannot uncook). (iii) Cutting vegetables — physical (only size/shape). (iv) Milk turning into curd — chemical (new substance with sour taste). (v) Freezing water into ice — physical (reversible). (vi) Burning gas on the stove — chemical (LPG → CO₂ + water + heat). Accept any five correct entries.
12. Short answer: Why is the dissolving of common salt in water considered a physical change even though the salt disappears from view?
The salt has not disappeared — its tiny particles have simply spread out evenly between the water particles. The salt is still chemically salt; it has the same taste and can be recovered unchanged by evaporating the water. Since no new substance is formed and the change is reversible, dissolving is a physical change.
Wrap-up: Changes are the heartbeat of the world around us — from the rusting gate in your lane to the ripe guavas in your garden, from the flame of a diya to the crystals of salt on your dinner plate. Every time you see a change, ask: Is it physical or chemical? Can I reverse it? Which signs do I see? That is how scientists have unlocked the secrets of matter for centuries.
Frequently Asked Questions — Changes Around Us — Chapter 5 Exercises
What does the topic 'Changes Around Us — Chapter 5 Exercises' cover in Class 7 Science?
The topic 'Changes Around Us — Chapter 5 Exercises' is part of NCERT Class 7 Science Chapter 5 — Changes Around Us: Physical and Chemical. It covers the key ideas of physical change, chemical change, NCERT exercises, MCQ, short answer, explained through everyday examples, labelled diagrams and hands-on activities drawn from the NCERT Curiosity textbook. Students learn not just definitions but also the reasoning behind each concept so they can answer competency-based questions and assertion–reason items. The lesson helps Class 7 students build a strong base for higher classes by linking each idea to real observations at home, school and in nature, and by preparing them for CBSE school assessments and Olympiads.
Why is 'Changes Around Us — Chapter 5 Exercises' important for Class 7 NCERT Science?
'Changes Around Us — Chapter 5 Exercises' is important because it builds core scientific thinking that Class 7 students will use throughout middle and secondary school. NCERT Chapter 5 — Changes Around Us: Physical and Chemical — introduces physical change and related ideas that appear again in Class 8, 9 and 10 Science. Mastering this subtopic helps students read labels and safety signs, understand news about science and technology, and perform better in CBSE school exams. The chapter also encourages curiosity and evidence-based thinking — skills that support the National Education Policy (NEP) 2020 focus on conceptual understanding and competency-based learning.
What are the key concepts students should remember from Changes Around Us — Chapter 5 Exercises?
The key concepts in 'Changes Around Us — Chapter 5 Exercises' for Class 7 Science are: physical change, chemical change, NCERT exercises, MCQ, short answer. Students should be able to define each term in their own words, give at least one everyday example, and explain how the concept connects to other chapters in NCERT Class 7 Science. For example, linking the idea to daily life — in the kitchen, classroom or outdoors — makes revision easier. Writing short notes, drawing labelled diagrams and solving the NCERT in-text and exercise questions for Chapter 5 will help students retain these concepts for unit tests and the annual CBSE examination.
How is Changes Around Us — Chapter 5 Exercises taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Changes Around Us — Chapter 5 Exercises' using an inquiry-based approach with Predict–Observe–Explain activities. Students are asked to make a guess first, then perform a simple experiment with safe, easily available materials, and finally explain what they observed. This matches the NEP 2020 focus on learning by doing. For Chapter 5 — Changes Around Us: Physical and Chemical — the textbook includes hands-on tasks, labelled diagrams and questions that build Bloom's Taxonomy skills from Remember (L1) to Create (L6). Teachers use these activities, along with competency-based questions (CBQs) and assertion–reason items, to check real understanding rather than rote memorisation.
How should Class 7 students prepare for the Chapter 5 exercises?
To prepare for the Chapter 5 — Changes Around Us: Physical and Chemical — exercises in NCERT Class 7 Science, students should first revise the theory in Parts 1–3 and make a short list of definitions and diagrams for physical change, chemical change, NCERT exercises, MCQ, short answer. Next, attempt each exercise question on their own before checking the solution. Pay extra attention to MCQs, assertion–reason questions and short-answer items, as these appear in CBSE competency-based tests. Practising with the NCERT Curiosity textbook, the exemplar questions, and the MyAiSchool practice bank helps Class 7 students score better in unit tests and the annual examination.
How does 'Changes Around Us — Chapter 5 Exercises' connect to other chapters of Class 7 Science?
'Changes Around Us — Chapter 5 Exercises' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of physical change appear again when students study related topics like heat, light, changes, life processes and Earth-Sun-Moon. For example, understanding this subtopic helps in building mental models for later chapters and for Class 8, 9 and 10 Science. Teachers often use cross-chapter questions in CBSE examinations to test whether students can apply what they learned in Chapter 5 — Changes Around Us: Physical and Chemical — to new situations. This integrated approach matches the NEP 2020 and NCF 2023 focus on holistic, competency-based learning.
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