Class 7 Science · Chapter 5
Short answer: Chapter 5 'Changes Around Us: Physical and Chemical' Class 7 Curiosity book ka woh chapter hai jo sikhata hai ki changes ko physical (koi naya substance nahi, reversible) aur chemical (naya substance banta hai, irreversible) me kaise classify karte hain — rusting, crystallisation, aur exothermic/endothermic changes jaise real-life examples ke saath.
Chapter 5 'Changes Around Us: Physical and Chemical' Class 7 Science Curiosity book ka woh chapter hai jo humare around ho rahe har roz ke changes ko scientifically samajhne ki neev rakhta hai. Chahe woh paani ka barf banna ho, mombatti ka jalna ho, ya loha me jung lagna — is chapter me hum seekhte hain ki kaise har change ko do broad categories, physical aur chemical, me classify kiya jaa sakta hai. Yeh chapter sirf definitions tak limited nahi hai — reversibility, energy changes (exothermic/endothermic), aur real-life applications jaise crystallisation aur rust-prevention jaise practical concepts bhi cover karta hai, jo aage Class 8-10 ki Chemistry (acids-bases, chemical reactions, metals-nonmetals) ki neev banate hain.
Chapter 5 Summary — 5 Minute Revision
Is chapter me hum seekhte hain ki changes do types ki hoti hain — physical changes, jinme sirf shape/size/state badalta hai aur koi naya substance nahi banta (jaise barf pighalna, kaanch tootna), aur chemical changes, jinme ek ya zyada naye substances form hote hain jinki properties original substance se alag hoti hain (jaise lakdi jalna, dahi jamna, rusting). Physical changes usually reversible hoti hain jabki chemical changes generally irreversible hoti hain. Changes ke saath energy bhi involve hoti hai — exothermic changes me heat release hoti hai aur endothermic changes me heat absorb hoti hai. Rusting (loha + oxygen + moisture → iron oxide) ek important real-life chemical change hai jise painting, oiling ya galvanisation se roka ja sakta hai. Crystallisation jaisi technique se saturated solutions se pure crystals prapt kiye jaate hain, jo purification ka ek useful method hai. In sab concepts ka strong foundation Class 8 se aage ki Chemistry ke liye zaroori hai.In-Text Questions — Solutions
Page ke shuru me diya gaya intext question: Kya sabhi changes reversible hote hain?
Nahi, sabhi changes reversible nahi hote. Physical changes jaise paani ka barf banna ya mombatti ka pighalna generally reversible hote hain — inhe wapas original form me laaya ja sakta hai (heat/cool karke). Lekin chemical changes jaise lakdi ka jalna ya doodh ka dahi banna irreversible hote hain — naya substance banne ke baad use wapas original substance me convert karna asaan nahi hota.Kya dahi jamna (curdling of milk) ek reversible process hai?
Nahi, dahi jamna irreversible chemical change hai. Doodh me present bacteria lactic acid banate hain jo milk protein (casein) ko coagulate kar deta hai. Ek baar dahi ban jaane ke baad use wapas doodh me convert nahi kiya ja sakta — yeh confirm karta hai ki yeh chemical change hai, physical nahi.Mombatti jalne me physical aur chemical dono changes kyun hoti hain?
Mombatti jalne par wax pehle pighalta hai (solid se liquid) — yeh physical change hai jo reversible hai. Fir wick ke through pighla hua wax vapourise hokar jalta hai aur carbon dioxide + paani vapour banata hai — yeh chemical change hai jo irreversible hai. Isliye ek hi process me dono types ke changes ek saath ho sakte hain.Exercise Questions — Solutions (Q1–Q6)
1. Neeche diye gaye changes ko physical ya chemical me classify karo: (a) Paani ka barf banna (b) Lakdi ka jalna (c) Kaanch tootna (d) Doodh ka dahi banna (e) Mombatti ka pighalna (f) Loha me jung lagna
Physical: (a) paani ka barf banna, (c) kaanch tootna, (e) mombatti ka pighalna — inme sirf state/shape change hoti hai, koi nayi substance nahi banti aur ye reversible hain. Chemical: (b) lakdi ka jalna, (d) doodh ka dahi banna, (f) loha me jung lagna — inme naye substances (ash+gases, lactic acid, iron oxide) banate hain aur ye irreversible hain.2. Physical aur chemical change me koi teen antar likho.
(i) Physical change me koi nayi substance nahi banti, chemical change me naya substance (different composition wala) banta hai. (ii) Physical change aksar reversible hoti hai (jaise paani ↔ barf), chemical change generally irreversible hoti hai (jaise jala hua kagaz wapas kagaz nahi banta). (iii) Physical change me sirf physical properties (shape, size, state) badalti hain, chemical change me chemical properties bhi badalti hain aur energy (heat/light) release ya absorb hoti hai.3. Rusting kya hai aur ise rokne ke do tareeke likho.
Rusting ek chemical change hai jisme loha (Fe), hawa ki oxygen aur moisture ke saath react karke iron oxide (Fe₂O₃·xH₂O) banata hai, jise jung kehte hain. Rokne ke tareeke: (i) Painting ya oiling — dhaatu ki surface ko hawa/nami ke direct contact se bachana. (ii) Galvanisation — loha ki surface par zinc ki thin layer chadhana, jo barrier ka kaam karti hai.4. Crystallisation kya hai? Ek example ke saath samjhao.
Crystallisation ek technique hai jisme kisi saturated solution ko dheere-dheere thanda karke ya evaporate karke pure substance ke crystals prapt kiye jaate hain. Example: Copper sulphate ke saturated solution ko dheere-dheere cool karne par neele rang ke sundar copper sulphate crystals ban jaate hain. Yeh solubility badalne ka result hai — cooling se solubility kam hoti hai, isliye extra solute crystal ki form me bahar aa jaata hai.5. Anushka ne ek prayog kiya: usne baking soda me lemon juice mila diya aur bubbles nikalne lage. Kya yeh physical ya chemical change hai? Reason do.
Yeh ek chemical change hai. Baking soda (sodium bicarbonate) aur lemon juice (citric acid) aapas me react karke carbon dioxide gas banate hain — jo bubbles ki form me nikalti hai. Naya gas banna is baat ka proof hai ki naya substance form hua hai, isliye yeh reaction irreversible aur chemical change hai.6. Do exothermic aur do endothermic changes ke examples do.
Exothermic changes (heat release hoti hai): (i) Lakdi/kagaz ka jalna (combustion), (ii) Paani ka barf me badalna (freezing/solidification). Endothermic changes (heat absorb hoti hai): (i) Barf ka pighalna (melting), (ii) Paani ka evaporate hokar water vapour banna.Important Equations — Ek Nazar Me
| Concept | Rule / Key Point |
|---|---|
| Physical Change | Sirf shape, size, ya state badalti hai — koi naya substance nahi banta; usually reversible |
| Chemical Change | Naya substance (naye properties ke saath) banta hai; usually irreversible; energy change (heat/light/gas) hoti hai |
| Rusting (chemical eqn concept) | Iron + Oxygen + Water (moisture) → Iron oxide (jung) — Fe₂O₃·xH₂O |
| Crystallisation | Saturated solution → slow cooling/evaporation → pure crystals (solubility temperature-dependent hoti hai) |
| Exothermic Change | Reaction/change jisme heat release hoti hai (e.g. burning, freezing) |
| Endothermic Change | Reaction/change jisme heat absorb hoti hai (e.g. melting, evaporation) |
| Test for Chemical Change | Gas evolution, colour change, precipitate formation, temperature change, irreversibility — inme se koi bhi sign present ho to usually chemical change hai |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Colour badalne wale har change ko chemical change samajh lena — jabki kuch colour changes (jaise mixing paints) physical bhi ho sakte hain agar koi naya substance na bane.
- Rusting ko physical change samajhna kyunki 'sirf colour badla hai' — yeh galat hai, rusting me naya substance (iron oxide) banta hai isliye yeh chemical change hai.
- Reversibility ko hi eksaath physical/chemical ka final test maan lena — kuch chemical changes bhi conditions me reversible ho sakte hain (rare cases), isliye reversibility ke saath naya substance banna bhi check karna chahiye.
- State change (jaise melting, boiling) ko chemical change samajhna — state change sirf physical change hai kyunki substance ki chemical composition same rehti hai.
- Crystallisation aur simple evaporation ko same process samajh lena — dono alag hain, crystallisation slow cooling/evaporation se pure crystals deta hai jabki simple evaporation fast hoti hai aur crystals impure ho sakte hain.
- Exothermic aur endothermic terms ko sirf 'temperature badalne' tak limit kar dena — inka sahi matlab hai heat release (exothermic) ya heat absorb (endothermic) hona, jo chemical ya physical dono tarah ke changes me ho sakta hai.
Board-Style Important Questions
- School/board-pattern practice question: 'Rusting of iron' ko chemical change kyun mana jaata hai, do reasons ke saath samjhao.
- School/board-pattern practice question: Physical aur chemical change me antar batate hue kisi bhi do-do examples do.
- School/board-pattern practice question: Crystallisation process ko diagram ki madad se samjhao aur iska ek real-life use likho.
- School/board-pattern practice question: Exothermic aur endothermic changes me antar spasht karo, examples sahit.
- School/board-pattern practice question: Kya doodh ka dahi banna reversible hai? Apne answer ko justify karo.
Aksar Poochhe Jaane Wale Sawaal
Physical aur chemical change me sabse simple antar kya hai?
Physical change me substance ka sirf form (shape, size, state) badalta hai lekin uski chemical composition same rehti hai — jaise barf ka pighalna. Chemical change me ek ya zyada naye substances banate hain jinki chemical composition original substance se alag hoti hai — jaise lakdi ka jalna. Physical change usually reversible hoti hai, chemical change generally irreversible.
Kya har chemical change ke saath physical change bhi hoti hai?
Haan, aksar aisa hota hai. Jab koi chemical reaction hoti hai, to usually state, colour, ya temperature bhi change hoti hai — jo physical changes hain. Jaise mombatti jalne par wax pighalta bhi hai (physical) aur burn bhi hota hai (chemical, CO₂ aur paani banate hue). Isliye dono changes aksar saath-saath hoti hain.
Rusting ko slow karne ke practical tareeke kya hain ghar par?
Ghar par loha ke saaman ko rust se bachane ke liye: use dry rakho, paint ya grease/oil ki thin layer lagao, aur agar possible ho to stainless steel ya galvanised (zinc-coated) items use karo. Nami (moisture) rusting ka main trigger hai, isliye humid areas me items ko wrap ya cover karke rakhna helpful hota hai.
Crystallisation aur simple evaporation me kya farak hai?
Simple evaporation me saara paani ud jaane tak solution ko heat karte hain, jisse crystals chhote aur impure ban sakte hain (kyunki fast heating me impurities bhi saath trap ho jaati hain). Crystallisation me solution ko slowly evaporate ya cool karte hain, jisse bade aur pure crystals form hote hain — is wajah se salt jaise substances ko purify karne ke liye crystallisation better technique mani jaati hai.
Class 7 Curiosity book me total kitne chapters hain aur yeh chapter kaunse number pe hai?
NCERT Class 7 Science ki 'Curiosity' book (2026-27 rationalised session) me total 12 chapters hain. 'Changes Around Us: Physical and Chemical' iska Chapter 5 hai.
Kya rusting ek physical change hai kyunki loha ka colour hi to badalta hai?
Nahi, yeh galat soch hai. Rusting sirf colour change nahi hai — ismein loha (Fe) chemically react karke ek bilkul naya substance, iron oxide (Fe₂O₃·xH₂O), banata hai jiski chemical composition original loha se alag hai. Yeh process irreversible hai (jung lage loha ko wapas pure loha me convert nahi kar sakte simple tareeke se), isliye rusting ek chemical change hai, physical nahi.
Class 7 Science — Saare Chapters
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