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

Chapter 2 of the Class 7 NCERT Curiosity textbook is titled “Exploring Substances: Acidic, Basic and Neutral”. It covers testing substances with litmus paper and natural indicators (turmeric, red rose extract), classifying substances as acidic, basic or neutral, and the idea of neutralization. These Class 7 Science Chapter 2 solutions are also useful as quick revision notes before exams.

Below are original, independently checked solutions to the chapter’s in-text questions, exercise, and key activities.

In-Text Questions

Q1. What are litmus paper strips made of, and why do they change colour?
Solution: Litmus paper is made from paper soaked in an extract obtained from lichens. It changes colour on contact with acidic or basic substances: acidic substances turn blue litmus red, and basic substances turn red litmus blue. Neutral substances cause no colour change to either strip.

Q2. If litmus isn’t available, what other natural substances can act as acid-base indicators?
Solution: Several natural extracts work as indicators, including turmeric and red rose extract (both tested directly in this chapter’s activities), as well as other commonly cited natural indicators like red cabbage juice.

Q3. Are there substances whose smell changes when acid or base is added?
Solution: Yes — substances like onion and vanilla extract change smell depending on whether an acid or base is added; these are called olfactory indicators (indicators detected by smell rather than colour).

Q4. If factory waste released into a lake is acidic, how could the fish be protected?
Solution: The acidic waste could be neutralised by adding a base (such as lime) before it’s released into the lake, bringing its pH closer to neutral and reducing harm to aquatic life.

Exercise — Let Us Enhance Our Learning

Q1. A solution turns red litmus paper blue. Excess addition of which substance would reverse this change — (i) Lime water (ii) Baking soda (iii) Vinegar (iv) Common salt solution?
Solution: (iii) Vinegar. The original solution is basic (it turned red litmus blue). Lime water and baking soda are themselves basic (wouldn’t reverse the change), and common salt solution is neutral (also wouldn’t reverse it). Vinegar is acidic, so adding it in excess would neutralise the base and turn the litmus back to red.

Q2. Solution A turns red litmus blue; Solution B turns turmeric paper red; Solution C turns red rose extract green. What is the nature of A, B and C respectively?
Solution: All three are basic. (Red litmus → blue = basic; turmeric → red = basic; rose extract → green = basic, based on this chapter’s own indicator-colour rules.)

Q3. A liquid sample shows: no change with red litmus, red litmus turns… [table: red litmus — no change; blue litmus — turned red; turmeric — no change]. What is the nature of the liquid?
Solution: The liquid is acidic. Reasoning: an acid doesn’t affect red litmus (stays red, matching “no change”), turns blue litmus red (matches the table), and doesn’t change turmeric paper’s colour (matches “no change” — turmeric only reacts to bases).

Q4. Manya is blindfolded and must identify whether a solution is acidic or basic without seeing colour. Which type of indicator should she use?
Solution: Manya should use an olfactory indicator (e.g. onion extract), since these can be identified by smell rather than sight — onion produces a stronger smell in a basic solution and a milder smell in an acidic one.

Q5. If red rose extract turns a liquid X green, what is the nature of X? What happens if excess amla juice (acidic) is then added?
Solution: Liquid X is basic (rose extract turns green in basic solutions). Adding excess amla juice (acidic) would neutralise the base, and as the solution becomes more acidic, the green colour would gradually turn back towards red (the acidic colour for rose extract).

Q6. Three unlabelled liquids are vinegar (acidic), baking soda solution (basic), and sugar solution (neutral). Can turmeric paper alone identify all three?
Solution: Not fully. Turmeric paper turns red only in a basic solution (baking soda) and shows no change in either an acidic solution (vinegar) or a neutral one (sugar solution) — so turmeric paper can positively identify the basic sample, but cannot by itself distinguish the acidic sample from the neutral one (a second indicator, like litmus, would be needed for that).

Activities & Key Data

Activity: Testing common household substances with litmus (Let Us Explore). Testing lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, salt solution, and orange juice with both litmus colours led to three groups:
• Group A (Acidic) — turns blue litmus red, no effect on (already) red litmus: lemon juice, amla juice, tamarind water, vinegar, orange juice.
• Group B (Basic) — turns red litmus blue, no effect on (already) blue litmus: soap solution, baking soda solution, lime water, washing powder solution.
• Group C (Neutral) — no colour change to either litmus: tap water, sugar solution, salt solution.

Activity: Acids in common foods (Let Us Relate and Explore). Group A substances taste sour because they contain natural acids: lemon juice — citric acid; tamarind — tartaric acid; vinegar — acetic acid; orange juice — citric acid (like lemon). Other sour foods not in this chapter’s own test group also contain acids — for example, curd contains lactic acid.

Activity: Testing with red rose extract (Let Us Find Out). Lemon juice, amla juice and vinegar (all acidic) turned the extract red; soap solution and baking soda solution (both basic) turned it green — consistent with the litmus-based groupings above.

Activity: Testing with turmeric paper (Let Us Prepare). Lemon juice, amla juice and sugar solution left turmeric paper yellow (no change), while soap solution and baking soda solution turned it red — confirming that turmeric paper only reacts (turns red) in the presence of a base.

Why This Chapter Matters

Understanding how to classify substances as acidic, basic or neutral using simple indicators (litmus, turmeric, natural extracts) is the foundation for chemistry topics in later grades, including pH, neutralisation reactions, and real-world applications like treating acidic soil, antacids, and industrial waste management.

More on This Chapter

See also: Class 7 Science NCERT Book (Curiosity) and the Class 7 Science Formulas Handbook.

Related pages: Extra Questions for Class 7 Science Chapter 2 | Revision Notes for Class 7 Science Chapter 2

Frequently Asked Questions

Q: What’s the difference between litmus paper and a natural indicator like turmeric?
A: Both change colour in response to acids/bases, but litmus is a manufactured/prepared indicator paper (from lichen extract), while turmeric, rose extract, and similar substances are readily available natural indicators you can prepare yourself at home from plant extracts.

Q: Why does turmeric paper only change colour for bases, not acids?
A: Turmeric’s natural pigment (curcumin) is chemically sensitive specifically to basic (alkaline) conditions, which shift it to a reddish colour; it doesn’t undergo the same colour-changing reaction in acidic or neutral conditions, so it stays yellow in those cases.

Written by Satish

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