Class 7 Science · Chapter 2
Short answer:
Is chapter me hum seekhte hain substances ko acidic, basic, ya neutral classify karna using taste, indicators (litmus, turmeric, china rose), aur olfactory indicators — aur samajhte hain neutralization reaction (Acid + Base → Salt + Water) ke real-life applications jaise antacids, ant sting treatment, aur acidic soil ka lime se treatment.
Chapter 2 poore Class 10 Chemistry ke Acids-Bases-Salts unit ki neev hai. NCERT Class 7 Science 'Curiosity' book (2026-27 rationalised syllabus) me ye chapter — "Exploring Substances: Acidic, Basic and Neutral" — humein sikhata hai ki daily life ke substances ko unke taste, feel, aur reaction ke basis pe kaise classify karte hain: acidic, basic, ya neutral.
Is chapter me hum seekhte hain indicators ka use — litmus paper, turmeric, china rose, aur olfactory indicators (smell-based) — jinse hum bina taste kiye (kyunki lab me kabhi taste nahi karte!) pata laga sakte hain ki koi substance acidic hai ya basic. Chapter ka climax hai neutralization reaction — jab ek acid aur base react karke salt aur water banate hain — jiske real-life applications hain jaise antacid tablets, ant sting treatment, aur acidic soil ka lime se treatment.
Ye topic sirf exam ke liye nahi, balki daily life me bhi useful hai — chahe khane ki cheezon ka taste samjhna ho ya kitchen/garden ke chhote problems solve karne ho.
Chapter 2 Summary — 5 Minute Revision
Chapter 2 "Exploring Substances: Acidic, Basic and Neutral" NCERT Class 7 Science Curiosity book ka foundational chemistry chapter hai jo students ko everyday substances ki nature samajhna sikhata hai — bina unhe taste kiye, safely indicators ka use karke.
Key takeaways: (1) Acidic substances khatte hote hain aur blue litmus ko red karte hain — jaise lemon, vinegar, curd, tamarind. (2) Basic substances kadwe aur soapy hote hain, red litmus ko blue karte hain — jaise soap, baking soda, sodium hydroxide. (3) Neutral substances kisi bhi litmus ko change nahi karte — jaise distilled water, common salt solution. (4) Natural indicators (litmus, turmeric, china rose) aur olfactory indicators (onion, vanilla) hume acid-base identify karne me help karte hain bina taste kiye. (5) Neutralization reaction (Acid + Base → Salt + Water) ka use hum daily life me karte hain — antacid tablets se acidity treat karna, bee/ant sting me formic acid ko baking soda se neutralize karna, aur acidic soil ko lime se treat karna crop yield improve karne ke liye.
Ye concepts aage Class 10 Chemistry ke Acids, Bases and Salts unit ki strong foundation banate hain, isliye inhe achhe se practice karna important hai — especially indicator-based activities aur neutralization ke real-life examples.
In-Text Questions — Solutions
In-text: Why does turmeric-stained cloth turn red when soap water touches it?
Turmeric ek natural indicator hai. Soap solution basic hoti hai, aur jab basic substance turmeric ke contact me aata hai to turmeric ka yellow colour reddish-brown/maroon ho jaata hai. Isiliye jab hum turmeric-stained cloth ko soap se wash karte hain, wo daag temporarily red-brown dikhne lagta hai — pani se dhone ke baad wapas yellow ho jaata hai jab soap wash ho jaata hai.
In-text: Classify the following as acidic, basic or neutral — tamarind, sodium hydroxide solution, sugar solution, gastric juice.
Tamarind (imli): Acidic — tartaric acid present hota hai, khatta taste.
Sodium hydroxide solution: Basic — strong base hai, red litmus ko blue karta hai.
Sugar solution: Neutral — sugar na acid hai na base, litmus unchanged rehta hai.
Gastric juice: Acidic — isme hydrochloric acid hota hai jo digestion me help karta hai.
In-text: Why do farmers add lime (chuna) to acidic soil before sowing crops?
Acidic soil me crops properly grow nahi kar paate kyunki plant roots ke liye nutrients ka absorption disturb ho jaata hai. Lime (calcium oxide/hydroxide) ek basic substance hai. Jab isse acidic soil me mix karte hain, to neutralization reaction hoti hai jisse soil ka pH neutral ke close aa jaata hai, jo crop growth ke liye favourable hota hai. Isiliye farmers sowing se pehle acidic soil me lime treat karte hain.
Exercise Questions — Solutions (Q1–Q5)
Ex 1: Classify the following into acidic, basic and neutral substances: vinegar, baking soda solution, common salt solution, curd, soap solution, lemon juice.
Solution (classification with reasoning):
| Substance | Nature | Reason |
|---|---|---|
| Vinegar | Acidic | Contains acetic acid — khatta taste, blue litmus ko red karta hai |
| Baking soda solution | Basic | Sodium bicarbonate ek base hai — red litmus ko blue karta hai |
| Common salt solution | Neutral | NaCl ek neutral salt hai — kisi bhi litmus ko change nahi karta |
| Curd | Acidic | Lactic acid ki wajah se khatta hota hai |
| Soap solution | Basic | Soapy/slippery feel + red litmus ko blue karta hai |
| Lemon juice | Acidic | Citric acid present hai |
↔ Table ko side me swipe karein
Ex 2: A farmer found ki uski soil bahut acidic ho gayi hai jisse crop yield kam ho raha hai. Usse kya treatment suggest karoge aur kyun?
Solution:
Farmer ko apni soil me quicklime (calcium oxide) ya slaked lime (calcium hydroxide) ya chalk (calcium carbonate) milani chahiye.
Reasoning: Ye sab basic substances hain. Jab ye acidic soil me milaye jaate hain, to ek neutralization reaction hoti hai jisme soil ki excess acidity neutralize ho jaati hai aur soil ka pH almost neutral (~7) ho jaata hai, jo crop growth ke liye ideal hota hai.
Acid (soil) + Base (lime) → Salt + Water
Ex 3: Explain why bee sting ko rubbing alcohol se treat nahi karte balki baking soda paste se treat karte hain. Ant sting ke case me ye kaise different hai?
Solution:
Bee sting me formic acid (acidic in nature) inject hota hai. Isko neutralize karne ke liye ek mild base chahiye — baking soda (sodium bicarbonate) ek mild base hai jo formic acid ko neutralize karke jalan/dard kam karta hai.
Ant sting bhi formic acid hi release karta hai (isiliye ant ko "formica" family se naam mila), isliye ant sting pe bhi baking soda ya moist soil (jo halki basic hoti hai) laga sakte hain — same neutralization principle kaam karta hai.
Rubbing alcohol iska treatment nahi hai kyunki wo neutralization nahi karta, sirf sterilize karta hai.
Ex 4: Turmeric paper ka use karke aap kaise pata karoge ki ek unknown solution acidic hai ya basic? Ek example ke saath samjhao.
Solution (activity-based answer):
Turmeric ek natural indicator hai jiska yellow colour basic solutions ke sath red-brown/maroon ho jaata hai, jabki acidic ya neutral solutions ke sath yellow hi rehta hai (koi change nahi).
Method: Unknown solution ki ek boond turmeric-soaked paper (ya turmeric paste wale paper) par dalo.
- Agar colour yellow se red-brown ho jaaye → solution basic hai (jaise soap solution)
- Agar colour yellow hi rahe → solution acidic ya neutral hai
Example: Soap solution turmeric paper ko red-brown kar dega, kyunki soap basic hota hai. Lemon juice koi change nahi karega kyunki wo acidic hai.
Ex 5: Neutralization reaction ka ek daily-life example do (khane ka context) aur balanced word equation likho.
Solution:
Example: Jab hume acidity (stomach me excess hydrochloric acid) hoti hai, hum antacid tablet (jaise Milk of Magnesia — magnesium hydroxide) lete hain.
Hydrochloric acid + Magnesium hydroxide → Magnesium chloride + Water
Ye ek classic neutralization reaction hai jisme stomach ka acid (base ke sath react karke) neutral ho jaata hai, aur jalan/pain kam ho jaata hai.
Important Equations — Ek Nazar Me
| Concept | Rule / Definition |
|---|---|
| Acid | Sour taste, blue litmus → red, pH < 7 (jaise lemon, vinegar, curd) |
| Base | Bitter taste + soapy feel, red litmus → blue, pH > 7 (jaise soap, baking soda) |
| Neutral substance | Koi litmus change nahi, pH = 7 (jaise distilled water, common salt solution) |
| Indicator | Substance jiska colour acid/base ke contact me change hota hai (litmus, turmeric, china rose) |
| Natural indicator | Plants se mile indicators — turmeric, china rose, red cabbage extract, litmus (lichen se) |
| Olfactory indicator | Smell-based indicator — onion, vanilla essence (basic solution me smell change/reduce) |
| Neutralization reaction | Acid + Base → Salt + Water |
| Ant/Bee sting treatment | Formic acid (acidic sting) → neutralize with mild base (baking soda) |
| Antacid | Mild base jo stomach ki excess HCl acidity neutralize karta hai (jaise Milk of Magnesia) |
↔ Table ko side me swipe karein
Common Mistakes — Yahan Marks Kat te Hain
- Students often galti se sochte hain ki sirf khatta taste hi acid pehchanne ka tareeka hai — lekin lab me kabhi bhi kisi unknown substance ko taste nahi karna chahiye, safety ke liye hamesha indicators use karo.
- Confusion hoti hai turmeric ke colour change me — students bhool jaate hain ki turmeric SIRF basic substances ke sath colour change karta hai (yellow → red-brown), acid/neutral ke sath NAHI change hota — dono directions yaad rakhna zaroori hai.
- Litmus paper ka colour reverse likh dena — yaad rakho: acid 'blue ko red' karta hai, base 'red ko blue' karta hai — order mat ulta karo.
- Neutral substance ko 'no reaction hone wala' samajhna galat hai — neutral substances (jaise NaCl solution, distilled water) apna alag properties rakhte hain, sirf litmus color change nahi karte.
- Neutralization reaction likhte waqt sirf 'acid + base = salt' likh dena — water (H2O) product likhna bhool jaate hain, jo reaction ka zaroori hissa hai.
- Olfactory indicators (onion, vanilla) ko litmus jaisa 'colour change' wala samajh lena — ye actually SMELL change/reduce karte hain, colour nahi — is difference ko clear rakho.
Board-Style Important Questions
- School-level unit test me common question: 'Classify the following into acidic, basic and neutral: lemon juice, soap solution, sugar solution' — direct classification-based question jo NCERT examples se banta hai.
- Frequently asked short-answer question: 'What is an indicator? Name two natural indicators used to test acids and bases.' — turmeric aur litmus ka naam expected hota hai.
- Common assertion-reasoning style question on neutralization: 'Why is baking soda used to treat bee stings?' — formic acid ka neutralization concept test hota hai.
- Diagram/activity-based question often asked in school FA: 'Describe an activity to test whether a given solution is acidic or basic using red and blue litmus paper.'
- Value-based/application question seen in practice papers: 'A farmer's field has become acidic. Suggest a remedy and explain the chemistry behind it.' — lime treatment aur neutralization dono cover karta hai.
Aksar Poochhe Jaane Wale Sawaal
NCERT Class 7 Science Curiosity book me total kitne chapters hain 2026-27 ke liye?
Current rationalised NCERT Class 7 Science 'Curiosity' book (2026-27 session) me total 12 chapters hain — pehle wale 13-chapter structure se reduce kiya gaya hai. Purane chapters jaise Acids, Bases and Salts (separate chapter), Nutrition in Plants, Nutrition in Animals, Respiration in Organisms, Transportation in Animals and Plants, Reproduction in Plants, Motion and Time, Electric Current and its Effects, Forests: Our Lifeline, aur Wastewater Story ab syllabus me nahi hain — inka content naye combined chapters me merge/rework ho gaya hai.
Litmus paper kya hota hai aur kaise kaam karta hai?
Litmus ek natural dye hai jo lichen naam ke plant se milta hai. Ye do form me aata hai — red litmus paper aur blue litmus paper. Acidic substances blue litmus ko red kar dete hain, aur basic substances red litmus ko blue kar dete hain. Neutral substances dono me se kisi ka colour change nahi karte. Ye sabse common indicator hai jo Class 7 me use hota hai.
Olfactory indicators kya hote hain?
Olfactory indicators wo substances hain jinki smell (gandh) acidic ya basic solution ke sath change ho jaati hai. Example: onion (pyaaz) aur vanilla essence ki smell basic solution me change ho jaati hai ya reduce ho jaati hai, jabki acidic/neutral solution me original smell rehti hai. Clove oil bhi ek olfactory indicator ki tarah use hota hai.
Kya distilled water acidic hai, basic hai, ya neutral?
Distilled water neutral hota hai — iska pH exactly 7 hota hai. Ye na to red litmus ko blue karta hai na blue litmus ko red — dono litmus papers unchanged rehte hain, isliye ye pure water neutral substance ka classic example hai.
China rose indicator kya hai aur ye kaise use hota hai?
China rose (Gudhal) ke petals ek natural indicator ki tarah kaam karte hain. China rose extract acidic solution me dark pink/magenta ho jaata hai aur basic solution me green ho jaata hai. Ye litmus se alag isliye useful hai kyunki ye do clearly different colours deta hai jo dono type ki solutions ke liye distinguish karna easy banata hai.
Class 7 Curiosity book me is chapter ke alawa kaunse important chapters hain jinki notes chahiye?
Curiosity book ke important chapters me shamil hain: Chapter 1 (Curiosity — introduction), Electricity: Circuits and their Components, Adolescence: Stage of Growth and Change, Heat Transfer in Nature, Measurement of Time and Motion, Life Processes in Animals, Life Processes in Plants, Light: Shadows and Reflections, Earth, Moon and Sun, World of Metals and Non-metals, aur Changes Around Us: Physical and Chemical. In sabki NCERT solutions aur notes hinglish me alag-alag prepare karni chahiye taaki poora syllabus cover ho.
Class 7 Science — Saare Chapters
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