Chapter 5: Changes Around Us — Physical & Chemical
Identify • Test • Compare • Fire Triangle • Reversible vs Irreversible • Natural Changes
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🗝️20 Most-Important Words (Meanings in Simple Hindi)
| Word | Meaning (Hindi) |
|---|---|
| Physical change | ऐसा परिवर्तन जिसमें केवल आकार, आकार-प्रकार या अवस्था बदले; नया पदार्थ नहीं बनता। |
| Chemical change | ऐसा परिवर्तन जिसमें एक या अधिक नए पदार्थ बनते हैं। |
| Chemical reaction | जब पदार्थ आपस में मिलकर नए पदार्थ बनाते हैं। |
| Combustion | ऑक्सीजन के साथ जलने की क्रिया जिसमें ऊष्मा/प्रकाश निकलता है। |
| Ignition temperature | वह न्यूनतम तापमान जिस पर कोई पदार्थ आग पकड़ लेता है। |
| Fuel | जो पदार्थ जलकर ऊष्मा/ऊर्जा देता है। |
| Oxygen | हवा का घटक जो जलने और श्वसन के लिए आवश्यक है। |
| Carbon dioxide | गैस जो श्वसन/जलने से बनती है; चूने के पानी को दूधिया करती है। |
| Lime water | कैल्शियम हाइड्रॉक्साइड का घोल; CO₂ की जाँच के लिए उपयोग। |
| Calcium carbonate | CO₂ + चूने के पानी से बनने वाला सफेद, पानी में अघुलनशील पदार्थ। |
| Rust | लोहे पर बनने वाली भूरी परत। |
| Rusting | नम हवा में लोहे का जंग लगना। |
| Corrosion | हवा/पानी से धातु की सतह का धीरे-धीरे खराब होना। |
| Evaporation | तरल का भाप/वाष्प में बदलना। |
| Condensation | भाप का ठंडा होकर तरल में बदलना। |
| Melting | ठोस का गर्म होकर तरल बनना। |
| Reversible change | जिसे उलटकर मूल अवस्था में लाया जा सके। |
| Irreversible change | जिसे वापस नहीं लाया जा सकता। |
| Weathering | चट्टानों का भौतिक/रासायनिक टूटना-बिखरना। |
| Erosion | हवा/पानी द्वारा कणों का घिसना और एक जगह से दूसरी जगह जाना। |
📚Detailed Notes — Changes Around Us: Physical & Chemical
👀Observing Changes All Around
- Daily examples: ice → water, cold bottle warms up, bud → flower, banana browning, chopped vegetables, boiling water, drying clothes, burning wood, making popcorn, rolling/flattening dough.
- We sense changes via sight, smell, touch, hearing, taste.
- Goal: Classify changes into physical and chemical; learn tests (like lime-water test for CO₂); understand combustion, ignition temperature, reversibility, and natural slow changes.
🧩5.1 Physical Changes — Appearance Changes, Substance Same
- Paper folding/origami: unfolds back to paper.
- Balloon: inflating/deflating is physical; pricking bursts it (irreversible physical change).
- Crushing chalk: powder formed, but still chalk (calcium carbonate).
- State changes of water: ice ⇄ water ⇄ vapour are physical; no new substance.
Definition: A physical change changes shape/size/state without forming a new substance.
🧪5.2 Chemical Changes — New Substance Forms
- Blowing into lime water: tap water shows bubbles only; lime water turns milky → CaCO₃ forms (new substance). Equation (word form): Calcium hydroxide + Carbon dioxide → Calcium carbonate (milky) + Water.
- Vinegar + baking soda: fizzing releases CO₂; passing gas into lime water makes it milky → confirms CO₂.
Definition: A chemical change (reaction) forms one/more new substances, often with clues like gas formation, colour/odour change, precipitate, energy change.
🧱5.3.1 Rusting (Chemical)
- Iron + moist air → rust (iron oxide), a new brown substance.
- Needs both air and water; dry air alone or water without air doesn’t rust.
- Corrosion ≠ rusting only; copper greens, silver blackens, etc.
🔥5.3.2 Combustion (Chemical)
- Magnesium ribbon burns → white magnesium oxide + heat + light (chemical change).
- Oxygen is essential: candle covered with glass goes out; CO₂ inside turns lime water milky.
Combustion: substance + oxygen → new oxide(s) + heat/light.
If clothes catch fire, wrap with a thick cloth/blanket to cut air (never synthetic blanket).
⚠️Is Air Alone Enough? — Ignition Temperature
- Paper burns with a match because match is hotter than its ignition temperature.
- Focusing sunrays with a magnifying glass heats paper until it reaches ignition temp → catches fire (no flame initially needed).
🔺The Fire Triangle
- Combustible fuel + oxygen + heat (to reach ignition temperature) → fire.
- Remove any one (cooling, smothering, removing fuel) to stop fire.
🕯️5.4 Candle — Physical & Chemical Together
- Physical: wax melts, flows, resolidifies; wax also evaporates.
- Chemical: wax vapour burns producing new substances with heat/light.
- Capillary action: wick draws liquid wax up to the flame.
- Faraday’s classic lectures used a candle to explain these ideas.
🔁5.5 Reversible vs Irreversible Changes
- Reversible: melting/freezing of ice; evaporation/condensation of water; folding a cloth/mat; twisting a string (often).
- Irreversible: chopping vegetables; making popcorn; burning wood; rusting; curdling milk; cooking food.
✅/⛔5.6 Desirable vs Undesirable Changes
- Desirable: milk → curd, ripening fruits (to a point), cooking, drying clothes.
- Undesirable: rusting, food decay; but decomposition can be desirable for composting.
- Environmental note: more fuel burning → more CO₂; drying paints release vapours → air pollution.
⛰️5.7.1 Weathering (Physical + Chemical)
- Physical: temperature change, root growth, freezing water in cracks → rocks break into smaller pieces (sediments).
- Chemical: rock minerals react with air/water; e.g., iron in basalt → red iron oxide layer.
- Weathering over long time forms soil.
🌬️🌊5.7.2 Erosion & Deposition
- Wind and flowing water carry away rock/soil particles → erosion (physical change).
- Slowing water deposits sediments; over time, layers harden into rocks (very slow, effectively irreversible).
⚖️Quick Compare — Physical vs Chemical Changes
Physical
- No new substance formed.
- Often reversible.
- Changes in state/shape/size (melting, cutting, dissolving).
Chemical
- New substance(s) formed.
- Usually irreversible.
- Clues: gas/precipitate, colour/odour change, heat/light.
🪲Fascinating Fact — Bioluminescence
Fireflies glow via a chemical change inside their bodies that produces light without heat — called bioluminescence.
🧠In a Nutshell (Quick Revision)
- Physical change: no new substance; examples — melting, evaporation, folding, crushing.
- Chemical change: new substance(s) form; examples — rusting, burning, curdling, vinegar + baking soda (CO₂ test with lime water).
- Combustion: needs fuel + oxygen + heat (ignition temp). Gives heat/light; stop by removing any one.
- Reversible vs irreversible and desirable vs undesirable depend on context.
- Natural changes: weathering (physical & chemical), erosion (physical), soil formation (very slow, irreversible).
Practice Pack — Changes Around Us: Physical & Chemical
One-word • Fill in the Blanks • True/False • Very Short • Short Answers
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🎯Section 1 — One-Word Answers (10)
1) Change in which no new substance is formed:
Physical change
2) Change in which one or more new substances are formed:
Chemical change
3) Gas that turns lime water milky:
Carbon dioxide
4) Minimum temperature at which a substance catches fire:
Ignition temperature
5) Chemical process with oxygen giving heat/light:
Combustion
6) Brown coating formed on iron in moist air:
Rust
7) Solution used to test CO₂ in class activities:
Lime water
8) Substance that burns to give energy:
Fuel
9) Slow breaking and alteration of rocks at Earth’s surface:
Weathering
10) Movement of weathered particles by wind/water:
Erosion
🧩Section 2 — Fill in the Blanks (10)
| Question | Answer |
|---|---|
| 1) Melting of ice into water is a ________ change. | physical |
| 2) Lime water turns ________ in the presence of carbon dioxide. | milky |
| 3) Vinegar reacts with baking soda to produce ________ gas. | carbon dioxide |
| 4) A substance will catch fire only after it reaches its ________ temperature. | ignition |
| 5) The three essentials for fire are fuel, oxygen and ________. | heat |
| 6) Rusting of iron requires the presence of both ________ and ________. | air and water (moist air) |
| 7) Burning of magnesium ribbon forms a white powder called ________ ________. | magnesium oxide |
| 8) Breaking rocks into smaller pieces by temperature/root action is called ________. | weathering |
| 9) Carrying away of sediments by rivers/wind is known as ________. | erosion |
| 10) Changes that can be undone to get the original substance back are called ________ changes. | reversible |
✅Section 3 — True / False with Explanations (10)
| # | Statement | Answer | Explanation |
|---|---|---|---|
| 1 | Melting of wax is a chemical change. | False | No new substance is formed; only state changes (solid → liquid). |
| 2 | Lime water turning milky indicates presence of CO₂. | True | CO₂ reacts with calcium hydroxide to form insoluble calcium carbonate. |
| 3 | Combustion can continue without oxygen. | False | Oxygen is essential; cutting air supply extinguishes flame. |
| 4 | Rusting of iron needs only water but no air. | False | Rusting requires both air (oxygen) and water (moisture). |
| 5 | Making popcorn from corn is an irreversible change. | True | New substances/structure form; you cannot get raw corn back. |
| 6 | Candle burning involves only physical changes. | False | It has physical (melting/evaporation) and chemical (burning vapour) changes. |
| 7 | Erosion is a physical change. | True | It involves movement/wearing away of particles, not new substances. |
| 8 | Curdling of milk is a chemical change. | True | New substances (curd proteins network) form; cannot be reversed to milk. |
| 9 | Evaporation followed by condensation of water is reversible. | True | It’s a physical cycle: liquid ⇄ vapour with no new substance formed. |
| 10 | Paper will catch fire in air even if it is below its ignition temperature. | False | Fuel must first reach its ignition temperature for burning to start. |
✍️Section 4 — Very Short Answers (10) • 2–3 lines each
1) Define a physical change with one daily example.
A physical change alters shape/size/state without forming a new substance. Example: ice melting into water; it can be refrozen.
2) What is a chemical change? Give one clue to identify it.
In a chemical change, new substance(s) form. Clues include gas formation, colour/odour change, precipitate, or heat/light release.
3) Why does lime water turn milky when you blow air into it?
Exhaled air contains CO₂. It reacts with calcium hydroxide (lime water) to form insoluble calcium carbonate, making it look milky.
4) State the “fire triangle”.
Fire needs three things: fuel (combustible substance), oxygen (air), and heat (to reach ignition temperature).
5) Why does a candle covered with a glass tumbler go out?
Oxygen gets used up inside the tumbler and cannot be replenished. Without oxygen, combustion stops and the flame extinguishes.
6) Explain ignition temperature briefly.
It is the minimum temperature at which a substance catches fire. A match or focused sunrays can raise the fuel to this temperature.
7) List one reversible and one irreversible change from your home.
Reversible: water boiling and then condensing. Irreversible: cooking food or burning paper.
8) What changes occur when a candle burns?
Wax melts/solidifies/evaporates (physical). Wax vapour burns to form new substances with heat/light (chemical).
9) Differentiate weathering and erosion in one line each.
Weathering: breaking/altering rocks in place. Erosion: transporting those particles by wind/water to other places.
10) Is rusting desirable? Give a reason.
No. Rust weakens iron structures and spoils appearance; it is a slow chemical change needing moist air.
📝Section 5 — Short Answers (10) • 3–4 lines each
1) Describe the lime-water test using your breath and state the inference.
Blow exhaled air through a straw into lime water. It turns milky due to formation of insoluble calcium carbonate. Hence, exhaled air contains CO₂.
2) Explain the vinegar and baking soda activity with the key observation.
Adding baking soda to vinegar produces fizzing bubbles of CO₂. Passing this gas into lime water makes it milky, confirming a chemical change.
3) State the conditions necessary for rusting and one way to slow it down.
Rusting requires both air (oxygen) and water (moisture). Keeping iron dry or coating it (paint/oil) reduces contact with moist air and slows rusting.
4) How does the “magnifying glass on paper” activity prove ignition temperature?
Focused sunrays heat paper gradually. When paper reaches its ignition temperature, it starts burning even without a flame, proving the concept.
5) Distinguish physical and chemical changes by features and examples.
Physical: no new substance; often reversible (melting, dissolving). Chemical: new substance(s) form; usually irreversible (rusting, burning, curdling).
6) Explain why clothes catch fire are wrapped in a blanket to extinguish.
Covering cuts off air (oxygen), one side of the fire triangle. Without oxygen, combustion stops and the fire goes out (avoid synthetic blankets).
7) What changes happen to wax in a burning candle? Classify them.
Wax near the flame melts and flows (physical), some evaporates (physical), vapour burns producing heat/light and new substances (chemical).
8) Give two environmental impacts of everyday chemical/physical changes.
Increased fuel combustion adds CO₂ (climate impact). Drying paints release vapours causing air pollution. Both affect air quality over time.
9) How do weathering and erosion together lead to soil and new rocks?
Weathering breaks rocks into sediments; erosion transports them. When water/wind slows, sediments deposit and, over long time, form soils and sedimentary rocks.
10) Classify: boiling water, chopping vegetables, ripening fruit, drying clothes.
Boiling water—physical & reversible (with condensation). Chopping—physical but irreversible shape. Ripening—chemical. Drying clothes—physical and reversible (by wetting).