This MCQ module is based on: Rusting, Crystallisation and Combustion
TOPIC 16 OF 46
Rusting, Crystallisation and Combustion
🎓 Class 7
Science
CBSE
Theory
Ch 5 — Changes Around Us: Physical and Chemical
⏱ ~14 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_7" science_domain="chemistry" difficulty="basic"]
5.3 Rusting of Iron
Have you ever noticed the flaky reddish-brown coating on an old iron gate, a wet bicycle chain, or a cooking pan left overnight in water? That coating is called rust, and the process that produced it is rusting — a slow but important chemical change that quietly damages millions of rupees worth of iron goods every year.
Iron rusts only when both oxygen (from air) and moisture (water) are present together. The overall reaction can be written simply as:
Iron + Oxygen + Water → Rust (Fe₂O₃·nH₂O)
Activity 5.5 — What Does Iron Really Need to Rust? L4 Analyse
Take three identical test tubes. Place a clean, shiny iron nail in each.
- Tube A — boiled (air-free) water + a layer of oil on top → nail sees water only
- Tube B — dry calcium chloride (absorbs moisture) + cork → nail sees dry air only
- Tube C — ordinary tap water, open to air → nail sees water + air
Predict: Which nail(s) will rust after a week?
Only Tube C shows rust! Tube A has water but no oxygen; Tube B has air but no moisture. Tube C has both — so only there does iron rust. Conclusion: rusting requires iron + oxygen + water together.
Preventing Rust
Since rust needs air + moisture, all prevention methods work by keeping air or water away from iron, or by using metals that do not rust.
- Painting, greasing or oiling — creates a thin barrier between iron and the air. That is why gates, bicycle chains and machinery are painted or oiled.
- Galvanisation — coating iron with a layer of zinc. Zinc reacts with air to form a protective layer that shields the iron underneath. Used for buckets, pipes and roofing sheets.
- Chrome plating — a shiny chromium coating on iron (common on bicycle handle bars and taps).
- Making alloys — mixing iron with carbon, chromium and nickel gives stainless steel, which does not rust at all (used in kitchen utensils).
5.4 Crystallisation — Growing Pure Crystals
Have you watched salt flats sparkle near the sea, or sugar grow into tiny glittery cubes from syrup? This is crystallisation, a gentle physical process that gives us very pure crystals.
Activity 5.6 — Growing Blue Copper Sulphate Crystals L3 Apply
Set-up (with teacher):
- Take a beaker of warm water.
- Keep adding copper sulphate powder and stirring until no more dissolves — you now have a saturated solution.
- Filter it to remove dust. Cover lightly and let it cool undisturbed for 24 hours.
Predict: What will appear at the bottom of the beaker the next day?
Beautiful bright-blue crystals of copper sulphate grow at the bottom and on the sides! As the solution cools, it can no longer hold all the copper sulphate, and the extra separates out as pure, shiny crystals. The dirt stays back in the filtrate — that's how crystallisation purifies.
At the seashore, sea water is allowed to collect in shallow pans. Sunlight evaporates the water slowly, and shiny common salt (NaCl) crystals are left behind — the same process that fills our food shelves!
Why is crystallisation a physical change? No new substance forms — the copper sulphate dissolved in water is the same copper sulphate that crystallises out. The crystals can be re-dissolved to get back the solution. So crystallisation is a reversible, physical change.
5.5 Combustion — Fire, Heat & Light
Combustion is a chemical change in which a substance reacts with oxygen from the air to produce heat and light. The substance that burns is called the fuel (wood, LPG, kerosene, wax, paper …).
Activity 5.7 — Starting a Fire with Sunlight L3 Apply
On a bright sunny day, hold a magnifying glass so that it focuses sunlight into a tiny bright dot on a dry piece of newspaper. Keep it steady (ask your teacher or a parent to supervise).
Predict: What will happen to the paper after a minute?
The paper first smokes, then browns, and finally catches fire! The magnifying glass concentrates sunlight, raising the paper's temperature beyond its ignition temperature. With oxygen all around, combustion starts.
The Fire Triangle
For any fire to start and keep burning, three things must be present together — miss even one and the fire dies!
- Combustible substance (fuel) — something that can burn (wood, petrol, paper, cloth)
- Oxygen (supporter of combustion) — usually from the air
- Heat / ignition temperature — enough heat to start the burning
The minimum temperature at which a substance catches fire is called its ignition temperature. Different materials have very different ignition temperatures — that is why kerosene lights up easily but a wet log needs long heating.
Fire Prevention & Safety
Understanding the fire triangle gives us the secret to stopping fires — we simply remove one corner of the triangle.
- Water cools the fuel below its ignition temperature (removes the "heat" side). Water is the most common fire extinguisher for paper, wood and cloth fires.
- Water should NEVER be used on electrical fires — water conducts electricity and the person can get a fatal shock. It is also wrong for oil fires — oil floats on water and the flaming oil spreads further!
- Carbon dioxide (CO₂) extinguishers cut off the oxygen supply by covering the burning material with CO₂. Best for electrical and oil fires.
- Foam extinguishers coat burning oil with a blanket that keeps air out — perfect for petrol and kitchen oil fires.
- Sand or a fire blanket can smother small home fires by blocking the air.
Emergency help: In India, dial 101 for the fire brigade, or 112 for any emergency. Never try to fight a large fire alone — raise an alarm and evacuate first.
Interactive: Fire Safety Quiz
Pick the correct extinguisher for each fire. Green = right, red = wrong. Explanations appear below each question.
Score: 0 / 4
Competency-Based Questions L3 Apply
During monsoon, Dev's cycle chain develops a reddish-brown coating. His mother asks him to oil it every week. Meanwhile, in the school lab, his teacher shows two demos: (i) slow cooling of hot copper sulphate solution that forms sparkling blue crystals, and (ii) a candle placed inside a jar of limited air that slowly dies out.
1. The reddish-brown coating on Dev's chain is:
(b) — Rust is hydrated iron(III) oxide, Fe₂O₃·nH₂O.
2. Why does oiling prevent rust?
The oil forms a thin barrier between the iron surface and the air, cutting off contact with oxygen and moisture — both of which are needed for rusting.
3. Is the crystallisation of copper sulphate a physical or a chemical change? Justify. L4
It is a physical change. The crystals are still copper sulphate — no new substance is formed. The process is reversible (crystals can be re-dissolved).
4. The candle in the closed jar dies out because:
(b) — No oxygen means no combustion (one corner of the fire triangle is missing).
5. Dev's grandmother pours water on a frying pan that suddenly catches fire in the kitchen. Why is this dangerous? What should she do instead?
Water with burning oil is dangerous — oil floats and the flaming oil spreads outward, and the sudden steam can spray hot oil everywhere. She should switch off the gas and cover the pan with a lid or a damp cloth to cut off oxygen, or use a foam/CO₂ extinguisher.
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: Iron gates are painted every few years.
R: Paint forms a protective layer that keeps oxygen and moisture away from the iron.
(A) — Paint is a barrier method to prevent rusting.
A: Water is an excellent extinguisher for electrical fires.
R: Water is a poor conductor of electricity.
(D) — A is false: water must NEVER be used on electrical fires. R is also false — ordinary water (with impurities) does conduct electricity.
A: Salt obtained from sea water by evaporation is an example of crystallisation.
R: Crystallisation separates pure crystals from a saturated solution.
(A) — R correctly explains A; the salt pans work exactly this way.
Frequently Asked Questions — Rusting, Crystallisation and Combustion
What does the topic 'Rusting, Crystallisation and Combustion' cover in Class 7 Science?
The topic 'Rusting, Crystallisation and Combustion' is part of NCERT Class 7 Science Chapter 5 — Changes Around Us: Physical and Chemical. It covers the key ideas of rusting, crystallisation, combustion, chemical change, copper sulphate, iron oxide, burning, 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 'Rusting, Crystallisation and Combustion' important for Class 7 NCERT Science?
'Rusting, Crystallisation and Combustion' 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 rusting 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 Rusting, Crystallisation and Combustion?
The key concepts in 'Rusting, Crystallisation and Combustion' for Class 7 Science are: rusting, crystallisation, combustion, chemical change, copper sulphate, iron oxide, burning. 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 Rusting, Crystallisation and Combustion taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Rusting, Crystallisation and Combustion' 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 rusting can Class 7 students observe at home?
Class 7 students can observe rusting at home in many simple ways linked to 'Rusting, Crystallisation and Combustion'. 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 'Rusting, Crystallisation and Combustion' connect to other chapters of Class 7 Science?
'Rusting, Crystallisation and Combustion' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of rusting 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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