Why can turmeric paper NOT be used to distinguish between acidic and neutral substances?

It does not change colour for either

What does the term 'neutralisation reaction' mean?

Acid and base react to form salt water and heat

Neutral substances react together

Ashwin's teacher's shirt had a turmeric stain. When soap was applied — the stain colour changed. Why?

Soap is basic and turmeric paper turns red in basic solutions

Soap is acidic and bleached the colour

Soap mixed with turmeric forms a dye

Hydrangea flowers grow blue in acidic soil and pink in basic soil. What does this tell us about the plant?

The plant is sensitive to light

The pigment in the flower acts as a natural acid-base indicator responding to soil pH

The plant absorbs colour from the soil

The colour is due to water quality

Organic matter like manure and composted leaves is added to basic soil. How does this help?

Organic matter releases acids that neutralise the basic nature of the soil

It improves only water retention

What is the advantage of using multiple natural indicators to test a substance rather than just one?

Using multiple indicators provides cross-verification and more confidence in determining the nature of a substance especially since some indicators like turmeric cannot distinguish between acidic and neutral

Multiple indicators always give conflicting results

Natural indicators are unreliable

Explain why bitter gourd (karela) tastes bitter but is NOT basic in nature.

Bitterness is a taste property — not all bitter substances are chemically basic — just as all acids taste sour but not all sour things are acidic — chemical indicators determine actual nature

Why is neutralisation important in agriculture when soil becomes too acidic?

Excessive acidity prevents plant growth and nutrient uptake — adding basic substances like lime neutralises the excess acid restoring the pH balance that plants require to thrive

Neutral soil is only needed for specific crops

What is the deeper significance of the neutralisation reaction formula — Acid + Base → Salt + Water + Heat — in environmental chemistry?

It is only a laboratory reaction

The heat released confirms an exothermic reaction and the fact that salts formed may be benign — this principle is applied industrially to treat acidic or basic waste before environmental release demonstrating that chemistry governs environmental remediation

Neutralisation only works in laboratories

The reaction always forms harmful products

Hydrangea flowers change colour based on soil pH. How does this phenomenon illustrate the concept of natural indicators at an ecological level?

Natural pigments in living organisms respond to pH in their environment — Hydrangea demonstrates that indicator chemistry is not limited to laboratories but is embedded in living systems — their colour IS a continuous pH measurement of their environment

A student writes a secret message using orange juice on white paper. To reveal it — a basic solution is sprayed. Which indicator paper should have been used as the writing surface and why?

Turmeric paper — because orange juice is acidic and would not change turmeric paper colour — but when a basic spray is applied the acidic juice neutralises and the remaining base turns the turmeric paper red revealing the message

How does the concept of neutralisation connect individual body biology (ant bite) environmental science (soil treatment) and industrial chemistry (factory waste)?

All three involve managing pH imbalances using neutralisation — the ant bite introduces acid treated by a base the acidic soil treated with lime and industrial acid waste treated with base — showing that neutralisation is a universal chemical principle across scales from human to industrial

Neutralisation only applies to soil

Biology and chemistry do not connect

NCERT Solutions Class 7 Science Chapter 2 – Exploring Substances: Acidic, Basic, and Neutral

Class 7 Science

Last Updated: 23 July 2026

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Chapter Introduction

This chapter explores substances as acidic, basic, or neutral using natural and synthetic indicators. Experiments with litmus, turmeric, and flower extracts show how acids and bases react and how neutralisation helps in everyday life.

हिंदी में (Hindi)

यह अध्याय पदार्थों को अम्लीय, क्षारीय और तटस्थ के रूप में प्राकृतिक और कृत्रिम संकेतकों से जानने पर आधारित है। नीला-लाल लिटमस, हल्दी और फूलों के अर्क से किए प्रयोग यह दिखाते हैं कि अम्ल और क्षार कैसे प्रतिक्रिया करते हैं और तटस्थकरण का दैनिक जीवन में उपयोग कैसे होता है।

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Important Points

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Practice MCQ Questions (33 questions)

Q1. What does blue litmus paper do when dipped in an acidic solution?

Q2. What does red litmus paper do when dipped in a basic solution?

Q3. Which of the following substances is acidic in nature?

Q4. Which of the following substances is basic in nature?

Q5. From which natural source is litmus obtained?

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Tough Words (Glossary)

Indicator

Meaning: A substance that shows a visible change (like color) to reveal the nature of another substance

अर्थ: सूचक – वह पदार्थ जो किसी दूसरी वस्तु का स्वभाव दिखाने के लिए रंग/आकृति में बदलाव दिखाए

"Red litmus is an indicator that turns blue in a base."

Litmus

Meaning: A natural dye from lichens used to test acids and bases

अर्थ: लिटमस – लाइकेन से प्राप्त प्राकृतिक रंजक, अम्ल‑क्षार की जाँच के लिए

"Litmus paper changed color when lemon juice was added."

Lichens

Meaning: Organisms made of a fungus and an alga living together

अर्थ: लाइकेन – कवक और शैवाल का सहजीवी संघटन

"Litmus is obtained from certain lichens."

Acidic

Meaning: Having the properties of an acid; turns blue litmus red

अर्थ: अम्लीय – अम्ल का स्वभाव रखने वाला; नीले लिटमस को लाल करता है

"Vinegar is acidic in nature."

Basic

Meaning: Having the properties of a base; turns red litmus blue

अर्थ: क्षारीय – क्षार का स्वभाव रखने वाला; लाल लिटमस को नीला करता है

"Soap solution is basic."

Neutral

Meaning: Neither acidic nor basic

अर्थ: उदासीन – न अम्लीय, न क्षारीय

"Pure water is neutral."

Lime water

Meaning: A solution of calcium hydroxide in water

अर्थ: चूने का पानी – कैल्शियम हाइड्रॉक्साइड का जलीय घोल

"Lime water neutralised the acid in the test tube."

Filtrate

Meaning: The liquid that passes through a filter during filtration

अर्थ: फिल्ट्रेट – छनन के बाद छननी/फ़िल्टर से होकर निकला द्रव

"Collect the filtrate in a clean beaker."

Extract

Meaning: A concentrated solution obtained by drawing out substances, often from plants

अर्थ: सार/निर्गत – किसी पदार्थ (अक्सर पौधों) से निकाला गया घना द्रव

"We used rose petal extract as an indicator."

Turmeric

Meaning: A yellow spice that can act as a natural indicator

अर्थ: हल्दी – पीला मसाला, प्राकृतिक सूचक का काम करता है

"Turmeric paper turned red with soap."

Olfactory

Meaning: Related to the sense of smell

अर्थ: घ्राण-संबंधी – सूँघने की इंद्रिय से जुड़ा

"Onion strips act as an olfactory indicator."

Neutralisation

Meaning: A reaction where an acid and a base react to form salt and water

अर्थ: उदासीनीकरण – अम्ल व क्षार की प्रतिक्रिया जिससे लवण व जल बनते हैं

"Baking soda gave relief by neutralisation of the ant acid."

Synthetic

Meaning: Made by chemical processes; artificial

अर्थ: संश्लिष्ट/कृत्रिम – रसायनिक विधि से बनाया गया

"Phenolphthalein is a synthetic indicator."

Edible

Meaning: Safe and suitable for eating

अर्थ: खाद्य – खाने योग्य

"Lemon and curd are edible acidic substances."

Slippery

Meaning: Smooth and difficult to hold due to low friction

अर्थ: फिसलनदार – पकड़ना कठिन, घर्षण कम होने के कारण

"Bases often feel slippery to touch."

Immersed

Meaning: Completely covered by a liquid

अर्थ: डुबोया हुआ/डूबा – द्रव में पूरी तरह ढँका

"Rose petals were immersed in hot water."

Mortar and pestle

Meaning: Tools used to crush or grind substances

अर्थ: ओखली और मूसल – पीसने‑कूटने के औज़ार

"Crush the petals with a mortar and pestle."

Compost

Meaning: Decayed organic material used to enrich soil

अर्थ: खाद/कम्पोस्ट – सड़ी‑गली जैविक सामग्री, मिट्टी को उपजाऊ बनाती है

"Compost is added to basic soil to improve it."

Deficiency

Meaning: A lack or shortage of something needed

अर्थ: कमी/अभाव – आवश्यक वस्तु का अभाव

"Poor plant health may be due to nutrient deficiency."

Abundant

Meaning: Existing in large quantities; plentiful

अर्थ: प्रचुर – बहुत मात्रा में उपलब्ध

"Lichens grow in areas with abundant clean air."

NCERT Solutions Class 7 Science Chapter 2 – Exploring Substances: Acidic, Basic, and Neutral

Chapter Introduction

Important Points

Practice MCQ Questions (33 questions)

Q1. What does blue litmus paper do when dipped in an acidic solution?

Q2. What does red litmus paper do when dipped in a basic solution?

Q3. Which of the following substances is acidic in nature?

Q4. Which of the following substances is basic in nature?

Q5. From which natural source is litmus obtained?

Q6. What happens to turmeric paper when a drop of basic solution is placed on it?

Q7. What is the general taste of acidic substances?

Q8. What is formed when an acid reacts with a base in a neutralisation reaction?

Q9. What is the nature of tap water and sugar solution as tested by litmus?

Q10. What acid does a red ant inject into the skin when it bites?

Q11. Which remedy is used to neutralise the acidic effect of an ant bite on skin?

Q12. What is the colour of red rose extract in an acidic solution?

Q13. What colour does red rose extract show in a basic solution?

Q14. What type of substances are baking soda and lime water?

Q15. Which substance is called an olfactory indicator because its odour changes in acidic or basic medium?

Q16. Why can turmeric paper NOT be used to distinguish between acidic and neutral substances?

Q17. What does the term 'neutralisation reaction' mean?

Q18. What is the characteristic texture of basic substances when rubbed between fingers?

Q19. If factory waste is acidic and is being released into a lake — what should be added to the waste before releasing it?

Q20. Ashwin's teacher's shirt had a turmeric stain. When soap was applied — the stain colour changed. Why?

Q21. Hydrangea flowers grow blue in acidic soil and pink in basic soil. What does this tell us about the plant?

Q22. A solution X turns red rose extract to green. What is the nature of solution X?

Q23. When lime water is added to an acidic lemon juice solution containing blue litmus indicator — what colour change occurs?

Q24. Organic matter like manure and composted leaves is added to basic soil. How does this help?

Q25. What is the advantage of using multiple natural indicators to test a substance rather than just one?

Q26. If an unknown liquid does not change red litmus but turns turmeric paper red — what is the nature of the liquid?

Q27. Explain why bitter gourd (karela) tastes bitter but is NOT basic in nature.

Q28. A student tests three solutions — P Q and R. P turns red litmus blue and blue litmus remains blue. Q does not change either litmus paper. R turns blue litmus red. Identify their natures.

Q29. Why is neutralisation important in agriculture when soil becomes too acidic?

Q30. What is the deeper significance of the neutralisation reaction formula — Acid + Base → Salt + Water + Heat — in environmental chemistry?

Q31. Hydrangea flowers change colour based on soil pH. How does this phenomenon illustrate the concept of natural indicators at an ecological level?

Q32. A student writes a secret message using orange juice on white paper. To reveal it — a basic solution is sprayed. Which indicator paper should have been used as the writing surface and why?

Q33. How does the concept of neutralisation connect individual body biology (ant bite) environmental science (soil treatment) and industrial chemistry (factory waste)?