Chapter 13: Changes – Physical and Chemical
1) 20 Important Words & Meanings (Meanings in simple Hindi only)
Physical Changeआकार/आयतन/अवस्था बदले, पर पदार्थ वही रहे; नया पदार्थ नहीं बनता।
Chemical Changeपदार्थ का स्वरूप बदलकर नया पदार्थ बन जाए, गुण भी बदलें।
Natural Changeजो अपने-आप प्रकृति में होते हैं, जैसे बारिश, फल पकना।
Man-made Changeजो इंसान की क्रिया से होते हैं, जैसे खाना पकाना, लकड़ी काटना।
Useful Changeजो हमारे काम का हो, लाभ दे; जैसे रोटी पकना, दूध का दही बनना।
Harmful Changeजो हानि पहुंचाए; जैसे दूध खराब होना, लोहे में जंग लगना।
Fast Changeजो बहुत जल्दी हो; जैसे गुब्बारा फटना।
Slow Changeजो धीरे-धीरे समय लेकर हो; जैसे लोहे में जंग लगना।
Reversible Changeजिसे उल्टा कर मूल अवस्था में लौटाया जा सके; जैसे मोम पिघलना-जमना।
Irreversible Changeजिसे उल्टा नहीं किया जा सके; जैसे कागज़ जलना।
Periodic Changeनिश्चित अंतराल पर बार-बार होने वाला परिवर्तन; जैसे दिन-रात।
Non-periodic Changeअनिश्चित समय पर होने वाला; जैसे भूकंप, बिजली कड़कना।
Evaporationतरल का वाष्प में बदलना; जैसे कपड़े सूखना।
Condensationवाष्प का तरल में बदलना; जैसे ठंडे गिलास पर पानी की बूंदें।
Meltingठोस का तरल बनना; जैसे बर्फ पिघलना।
Freezingतरल का ठोस बनना; जैसे पानी जमना।
Dissolvingघुलना—घुलनशील पदार्थ का विलायक में मिलना; जैसे चीनी पानी में।
Corrosionधातुओं का धीरे-धीरे खराब होना; लोहे में जंग, तांबे पर हरापन।
Galvanizationलोहे पर जस्ता-परत चढ़ाकर जंग से बचाना।
Tinningतांबा/पीतल पर टिन-परत चढ़ाकर खाना सुरक्षित रखना।
2) Important Notes (Quick Revision)
- Classification by cause: Natural (rain, ripening) vs Man-made (cooking, cutting).
- By effect: Useful (baking bread) vs Harmful (spoilage, rusting).
- By speed: Fast (bursting balloon, lightning) vs Slow (rusting, fruit ripening).
- By repeatability: Periodic (day–night, seasons) vs Non-periodic (earthquake).
- By reversibility: Reversible (ice ⇄ water; dissolving → crystallizing) vs Irreversible (burning, souring milk).
- Physical change: No new substance; properties mainly unchanged (state/shape/size). Examples: melting, boiling, evaporating, dissolving, breaking glass, stretching spring (elastic).
- Chemical change: New substance(s) with new properties; often energy change, colour change, gas/precipitate, usually irreversible. Examples: rusting, burning, cooking food, milk → yoghurt.
- Corrosion: Surface reaction with $\\mathrm{O_2}$, moisture, chemicals—iron rusts (reddish-brown), copper greens. Prevention: painting, oil/grease, galvanization (zinc coat), tinning (tin coat on copper/brass), powder coating, proper storage.
- Daily science: Check expiry dates—chemical changes can make food unsafe. Dry cloth gets wet quickly (capillary action) but dries slowly (evaporation depends on temperature, airflow, humidity).
3) 20 Important One-Word/Line Questions (Answers 1–2 lines)
- Define physical change.Change in state/shape/size without forming a new substance.
- Define chemical change.Formation of a new substance with new properties.
- Give one natural change.Rainfall / fruit ripening.
- Give one man-made change.Cooking food / sharpening a pencil.
- Example of a useful change.Baking bread.
- Example of a harmful change.Milk turning sour/spoiled.
- One fast change.Balloon bursting.
- One slow change.Rusting of iron.
- One reversible change.Melting and solidifying wax.
- One irreversible change.Burning paper.
- One periodic change.Day and night / seasons.
- One non-periodic change.Earthquake / lightning strike.
- Liquid $\to$ gas is called?Evaporation/boiling.
- Gas $\to$ liquid is called?Condensation.
- Solid $\to$ liquid is called?Melting.
- Liquid $\to$ solid is called?Freezing/solidification.
- What is dissolving?Mixing a solute uniformly into a solvent (e.g., sugar in water).
- What is corrosion?Slow degradation of metals due to air/moisture/chemicals.
- How can iron be protected from corrosion?Painting, oiling, or galvanization (zinc coating).
- Why is tinning done on copper/brass utensils?To prevent reaction with acidic food and avoid green corrosion.
4) 20 Very Short Answer Questions (1–2 lines)
- Differentiate natural and man-made changes in one line.Natural occur by themselves; man-made are caused by human actions.
- State one criterion to decide useful vs harmful.Whether the change benefits us (useful) or causes loss/risk (harmful).
- Is chopping wood physical or chemical?Physical—composition remains wood.
- Is burning wood physical or chemical?Chemical—new substances like ash, gases form.
- Give a reversible change from daily life.Ice $\leftrightarrow$ water.
- Give an irreversible change from daily life.Cooking chapati.
- Name one sign of a chemical change.Gas evolution / colour change / heat or light produced.
- What happens in evaporation?Surface molecules escape as vapour—cooling effect.
- Why does a copper article turn green?Corrosion forms basic copper carbonate (green layer).
- Why check expiry date?Chemical changes can spoil food—unsafe after expiry.
- Is souring of milk reversible?No; it’s a chemical, irreversible change.
- Stretching a spring within limit—type of change?Physical and reversible (elastic).
- Making curd from milk—type?Chemical; slow; mostly man-aided; irreversible.
- Lighting an electric bulb—type?Physical change (no new substance; energy conversion).
- Rusting faster in coastal areas—why?More moisture and salts accelerate corrosion.
- Is rain periodic or non-periodic?Individual showers are non-periodic; seasons are periodic.
- Define powder coating in one line.Dry coloured polymer layer baked onto metals to prevent corrosion.
- Why does cloth dry slowly on humid days?Low evaporation rate due to high moisture in air.
- Give an example of fast non-periodic change.Lightning flash.
- Why polish wooden articles?To block moisture/air and slow decay; improve appearance.
5) 20 Short Answer Questions (about 2–3 lines)
- Give two differences between physical and chemical changes.Physical: no new substance; often reversible. Chemical: new substance(s) with new properties; mostly irreversible.
- Classify: breaking a glass vs rusting iron.Breaking glass—physical (no new substance). Rusting—chemical (iron oxide forms).
- Why is cooking considered a chemical change?New flavours, colours, aromas form; composition and properties change; irreversible.
- Explain reversible vs irreversible with daily examples.Ice $\leftrightarrow$ water (reversible). Paper $\to$ ash on burning (irreversible).
- How does galvanization protect iron?A zinc coat isolates iron from air/moisture and sacrificially corrodes first.
- Why is tinning applied to copper/brass utensils?Tin is less reactive with acidic foods; prevents green corrosion and makes utensils food-safe.
- Differentiate periodic and non-periodic changes with examples.Periodic repeat at fixed intervals (day–night); non-periodic do not (earthquake).
- Explain fast vs slow changes with one each.Fast: cracker bursting in seconds; Slow: fruit ripening over days.
- Why do we paint or powder-coat metal gates?To create a barrier against oxygen/moisture and slow corrosion; also aesthetic.
- Is dissolving salt in water chemical or physical? Explain.Physical; salt can be recovered by evaporation; no new substance formed.
- State two signs that a chemical change has occurred.Gas/odour produced; colour change; temperature/light change; precipitate formation.
- Why does a dry handkerchief get wet instantly but dries slowly?Absorption is quick via capillaries; drying depends on evaporation—slower and weather-dependent.
- Is making dough from flour and water physical or chemical?Mostly physical (hydration/mixing). Cooking that dough later is chemical.
- Give one example each: useful chemical change and harmful chemical change.Useful: milk $\to$ yoghurt. Harmful: food spoilage/rancidity.
- What is corrosion of copper called visually and how to reduce it?Green patina; reduce by tinning, lacquering, or keeping dry/clean.
- Is lighting an electric bulb a chemical change? Why/why not?No new substance forms; it’s mainly an energy change (physical phenomenon).
- How does humidity affect evaporation?Higher humidity lowers the rate of evaporation; air already moist cannot accept much more vapour.
- Explain why stretching a spring can be reversible.Within elastic limit, spring regains shape; beyond limit, permanent (irreversible) deformation.
- Why is checking expiry date a safety measure?Prevents ingestion of food altered by chemical changes/microbial toxins.
- Give two ways to slow rusting at home.Keep metals dry and clean; apply paint/oil; use silica gel in storage boxes.
6) Textbook Exercise – Perfect Answers (Chapter 13)
Q1) Distinguish between the following.
| (a) Physical change | (a) Chemical change |
|---|---|
| No new substance formed; composition same. | New substance(s) formed; composition changes. |
| Often reversible (e.g., melting, dissolving). | Usually irreversible (e.g., rusting, burning). |
| Examples: cutting, melting wax. | Examples: milk $\to$ yoghurt, cooking. |
| (b) Periodic change | (b) Non-periodic change |
|---|---|
| Occurs at fixed intervals; predictable. | No fixed interval; unpredictable timing. |
| Examples: day–night, seasons, clock hands. | Examples: earthquake, lightning, accidents. |
| (c) Natural change | (c) Man-made change |
|---|---|
| Happens on its own in nature. | Caused by human activity. |
| Examples: raining, fruit ripening. | Examples: baking, cutting, painting. |
Q2) Classify the following changes under different types.
- Milk $\to$ yoghurt: Chemical, mostly man-made (curd culture), useful, slow, irreversible.
- Bursting of a cracker: Chemical, man-made, fast, non-periodic, mostly harmful (noise/air).
- Occurrence of an earthquake: Natural, fast, non-periodic, harmful; mainly physical changes on earth’s surface.
- Revolution of the earth around the sun: Natural, periodic, slow; no change in substance (physical motion).
- Stretching of a spring: Physical; reversible if within elastic limit; fast; man-made action.
Q3) Give reasons.
- Check expiry date for canned food: Chemical changes/spoilage may occur over time; beyond expiry, food can be unsafe.
- Iron article should be painted: Paint isolates iron from oxygen and moisture, preventing corrosion (rusting).
- Wooden article should be polished: Polish forms a barrier against air/moisture, slows decay/warping and improves look.
- Copper/brass utensils should be tinned: Tin prevents reaction with acidic foods and stops green corrosion—safe for cooking.
- Dry handkerchief wets quickly but dries slowly: Capillary absorption is quick; drying depends on slower evaporation controlled by temperature, airflow, and humidity.
Q4) What will you take into account while identifying…
(a) a physical change: No new substance; change in state/shape/size; often reversible; properties largely unchanged.
(b) a chemical change: New substance formed; colour/odour/temperature/gas/precipitate change; usually irreversible.
Q5) Read the paragraph and note down the various types of changes mentioned.
- Near six o’clock; sun setting: Natural, periodic (day–night cycle), slow visual change.
- Breeze blowing; leaves shaking: Natural, generally non-periodic gusts; physical movement; fast and reversible.
- Rolling wet soil into toys: Physical (shaping), reversible by reshaping; drying of wet soil—physical (evaporation).
- Making dough from wheat flour: Mostly physical (mixing/hydration).
- Frying puris: Chemical, fast, irreversible, useful.
- Raining; lightning: Natural; rain—non-periodic event (though season is periodic); lightning—fast, non-periodic.
- Dim light while eating: Physical change in illumination; linked to periodic sunset.
✅ Tip: To prevent corrosion at home—keep metals dry, apply paint/oil, and store with desiccants.