This MCQ module is based on: Reversible and Irreversible Changes
TOPIC 17 OF 46
Reversible and Irreversible Changes
🎓 Class 7
Science
CBSE
Theory
Ch 5 — Changes Around Us: Physical and Chemical
⏱ ~14 min
🌐 Language: [gtranslate]
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5.6 Reversible vs Irreversible Changes
Look at any change around you and ask one simple question: "Can I get the original thing back?" The answer splits all changes into two useful groups.
(a) Reversible Change
A reversible change is one we can undo. The same substance comes back when we reverse the conditions. Most physical changes are reversible.
- Water in the freezer becomes ice (reverse by warming → water back).
- Stretching a rubber band (release it → original length).
- Dissolving salt in water (evaporate water → solid salt back).
- Folding a cloth, inflating a balloon, wet clay drying in the sun.
(b) Irreversible Change
An irreversible change cannot be easily undone. Once it happens, the original substance is gone. Most chemical changes are irreversible.
- Burning paper — ash cannot turn back into paper.
- Cooking an egg — the hard white/yellow cooked egg cannot become raw again.
- Ripening of fruits — a sweet ripe mango cannot become a sour raw one.
- Baking a cake from flour, eggs and sugar.
Watch out for exceptions! The link "physical = reversible, chemical = irreversible" works most of the time but is not absolute. In some laboratory reactions, a chemical change can also be reversed by changing conditions. Always check what is actually happening to the substance.
5.7 Some Changes Are Both Physical AND Chemical
In real life many processes are not "either/or" — they mix the two types together. Look at these familiar examples:
(a) Burning a Candle
As the flame heats the wax near the wick, solid wax melts into liquid wax — a physical change. This liquid wax is drawn up the wick and burns in oxygen to form CO₂ and water vapour — a chemical change.
(b) Cooking Rice
Water boils and turns into steam — a physical change. At the same time the starch inside the rice grains absorbs water and breaks down into simpler substances that are softer and tastier — a chemical change. That is why cooked rice can never become raw again.
(c) Digestion of Food
Chewing breaks food into smaller pieces — physical. The enzymes in saliva and the stomach then break down starch, proteins and fats into simpler substances that the body can absorb — chemical. Both happen at once, every time you eat!
Quick Comparison Table
| Feature | Physical Change | Chemical Change |
|---|---|---|
| New substance formed? | No | Yes |
| Change in shape/state | Usual | Not the main change |
| Reversible? | Usually yes | Usually no |
| Mass stays the same? | Yes | Usually yes (but different substance) |
| Examples | Ice melting, dissolving sugar, tearing paper | Rusting, cooking, burning, rotting |
Changes That Involve Energy
Every change involves energy — either heat is given out or heat is absorbed.
Exothermic Changes — Release Heat
When a process releases heat into its surroundings, it is called exothermic. The container or surroundings feel warmer.
- Burning of fuels — wood, LPG, petrol
- Neutralisation — acid reacting with a base
- Respiration in living cells
- Dissolving calcium chloride in water (gets hot)
Endothermic Changes — Absorb Heat
When a process takes in heat from the surroundings, it is called endothermic. The surroundings feel cooler.
- Melting of ice (it absorbs heat from the glass and the drink becomes cold!)
- Cooking food on a stove (absorbs heat from the flame)
- Photosynthesis in green plants (absorbs the Sun's energy)
- Dissolving ammonium chloride in water (gets cold)
Changes All Around Us — Everyday Examples
Once you start looking, changes hide everywhere. Let us take a tour:
In the Kitchen
- Chemical: cooking dal, baking bread, frying samosas, curdling milk, fermenting dosa batter.
- Physical: cutting onions, grinding spices, boiling water, freezing ice cream, beating curd.
In Nature
- Chemical: leaves changing colour in autumn, fruit ripening, wood decaying, photosynthesis.
- Physical: rain falling, snow forming, clouds condensing, wind carrying sand, river freezing.
In Industry
- Chemical: making steel in a furnace, brewing, making soap from oil and alkali.
- Physical: extracting salt from sea water, forging hot metal into tools, moulding plastic toys.
Interactive: Change Detective
Read each everyday scenario and decide whether it is a Physical, Chemical, or Both.
Score: 0 / 5
Competency-Based Questions L4 Analyse
Riya watches her grandmother make mango pickle: raw mango pieces are sliced (step 1), mixed with salt and oil (step 2) and left in sunlight for a week. Slowly, the pieces soften, darken and develop a tangy flavour (step 3). In the same week, Riya's brother prepares ice water by freezing water in trays (step 4) and later melts a few cubes back to water (step 5).
1. Which steps from 1 to 5 are only physical changes?
(b) — Slicing (1), freezing (4) and melting (5) are physical: shape/state changes without a new substance.
2. Step 3 is a chemical change. List any two signs that confirm this.
(i) Change in colour (raw green → darker pickle), (ii) change in taste & smell (new tangy flavour), and (iii) new substance formed that cannot become raw mango again.
3. Riya's freezer-and-melt cycle (steps 4–5) is a good example of a __________ change.
Reversible (physical) change — the same water moves between liquid and solid forms without anything new forming.
4. Respiration happens inside Riya's cells all day long. Is it exothermic or endothermic? Why is the answer useful? L4
Respiration is exothermic — it releases heat. This is the very warmth that keeps Riya's body at ~37 °C even on a cold morning.
5. The making of pickle is irreversible. Explain why, in your own words.
The mango pieces undergo a chemical change — new substances (acids, flavours) are formed by the action of salt, oil and sunlight. We cannot remove these new substances and get raw mango back, so the change is irreversible.
Assertion–Reason Questions
Choose: (A) Both A and R true, R explains A. (B) Both true, R does not explain A. (C) A true, R false. (D) A false, R true.
A: Melting of ice is an endothermic change.
R: Ice absorbs heat from the surroundings to turn into water.
(A) — That is exactly why ice in a glass makes the drink feel cold.
A: Cooking rice is a purely physical change.
R: Water changes from liquid to steam during cooking.
(D) — A is false: cooking rice is both physical (boiling water) and chemical (starch changes). R alone is true.
A: Burning of LPG is an exothermic chemical change.
R: It releases heat and forms new substances like CO₂ and water vapour.
(A) — R gives two perfect reasons why burning LPG is exothermic AND chemical.
Frequently Asked Questions — Reversible and Irreversible Changes
What does the topic 'Reversible and Irreversible Changes' cover in Class 7 Science?
The topic 'Reversible and Irreversible Changes' is part of NCERT Class 7 Science Chapter 5 — Changes Around Us: Physical and Chemical. It covers the key ideas of reversible change, irreversible change, melting, freezing, cooking, burning, examples, 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 'Reversible and Irreversible Changes' important for Class 7 NCERT Science?
'Reversible and Irreversible Changes' 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 reversible 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 Reversible and Irreversible Changes?
The key concepts in 'Reversible and Irreversible Changes' for Class 7 Science are: reversible change, irreversible change, melting, freezing, cooking, burning, examples. 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 Reversible and Irreversible Changes taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Reversible and Irreversible Changes' 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.
What real-life examples of reversible change can Class 7 students observe at home?
Class 7 students can observe reversible change at home in many simple ways linked to 'Reversible and Irreversible Changes'. Kitchens, school bags, playgrounds and the night sky are full of examples that connect to NCERT Chapter 5 — Changes Around Us: Physical and Chemical. For instance, students can check labels on food and cleaning products, watch changes while cooking, or observe the Sun and Moon across a week. Keeping a small science diary — noting the date, what was observed and a quick sketch — turns everyday life into a science lab. These real-life connections make concepts stick and prepare students well for competency-based questions in CBSE Class 7 Science.
How does 'Reversible and Irreversible Changes' connect to other chapters of Class 7 Science?
'Reversible and Irreversible Changes' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of reversible 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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