NCERT Solutions for Class 7 Science (Curiosity) Chapter 2: Exploring Substances: Acidic, Basic, and Neutral (NCERT 2026–27)

These Class 7 Science Curiosity Chapter 2 solutions cover Exploring Substances: Acidic, Basic, and Neutral from the new NCF-2023 textbook (2026–27). The chapter shows how everyday substances can be sorted into acidic, basic and neutral using acid-base indicators such as litmus, red rose extract and turmeric, and explains how an acid and a base cancel each other out in a neutralisation reaction. Every question of the “Let Us Enhance Our Learning” exercise is reproduced verbatim and solved below.

Class: 7 Subject: Science Book: Curiosity Chapter: 2 Topic: Acids, Bases, Indicators & Neutralisation Session: 2026–27
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Class 7 Science Curiosity Chapter 2 Solutions – Overview

Chapter 2 of Curiosity, Exploring Substances: Acidic, Basic, and Neutral, treats nature as a science laboratory. By testing common samples — lemon juice, vinegar, amla and tamarind water, soap and baking-soda solutions, lime water, tap water, sugar and salt solutions — we learn that substances are of three kinds: acidic (sour, turn blue litmus red), basic (bitter and slippery, turn red litmus blue) and neutral (affect neither litmus). Substances that show different colours in acids and bases are called acid-base indicators; the chapter prepares three of them — litmus (from lichens), red rose extract and turmeric paper — and even meets olfactory indicators such as onion. Finally it shows that when an acid and a base are mixed in the right amount they neutralise to give salt, water and heat, a process used to treat ant bites, acidic or basic soil and acidic factory waste.

Use teacher-approved dilute samples. Never identify substances by tasting, touching or directly sniffing unknown chemicals. The taste and touch descriptions below are properties, not instructions for testing.

Key Concepts & Definitions

Acidic substance: tastes sour and turns blue litmus paper red — e.g. lemon juice, amla juice, tamarind water, vinegar.

Basic substance: tastes bitter, feels soapy/slippery and turns red litmus paper blue — e.g. soap solution, baking-soda solution, lime water, washing-powder solution.

Neutral substance: neither acidic nor basic; does not change the colour of either litmus paper — e.g. tap water, sugar solution, salt solution.

Acid-base indicator: a substance that shows different colours in acidic and basic solutions. Litmus (obtained from lichens), red rose extract and turmeric are natural indicators.

Red rose extract: gives a red shade in acidic solutions and a green shade in basic solutions.

Turmeric: its yellow colour turns red in basic solutions but stays unchanged in acidic and neutral solutions, so fresh yellow paper detects bases but cannot directly distinguish acids from neutral solutions.

Olfactory indicator: a substance (e.g. onion) whose odour changes in an acidic or basic medium.

Neutralisation: an acid reacts with a base. The chapter’s simplified equation is Acid + Base → Salt + Water + Heat. Excess acid or base can leave the mixture acidic or basic; equal volumes are not necessarily the required amounts.

“Let Us Enhance Our Learning” — NCERT Solutions

Original explanations for Questions 1–12 on pages 20–21 of NCERT Curiosity, Reprint 2026–27. Diagram labels have been checked against the printed figures.

1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change? (i) Lime water (ii) Baking soda (iii) Vinegar (iv) Common salt solution

ANSWER (iii) Vinegar. A solution that turns red litmus blue is basic. To reverse the change the colour must go back towards red, which needs an acid. Among the options, only vinegar (acetic acid) is acidic; lime water and baking soda are themselves basic, and common salt solution is neutral, so they cannot reverse it.

2. You are provided with three unknown solutions labelled A, B, and C, but you do not know which of these are acidic, basic, or neutral. Upon adding a few drops of red litmus solution to solution A, it turns blue. When a few drops of turmeric solution are added to solution B, it turns red. Finally, after adding a few drops of red rose extract to solution C, it turns green. Based on the observations, which of the following is the correct sequence for the nature of solutions A, B, and C? (i) Acidic, acidic, and acidic (ii) Neutral, basic, and basic (iii) Basic, basic, and acidic (iv) Basic, basic, and basic

ANSWER (iv) Basic, basic, and basic. A turns red litmus blue → A is basic. Turmeric turns red only in a base, so B is basic. Red rose extract turns green in a base, so C is basic. Hence all three solutions are basic.

3. Observe and analyse Figs. 2.13, 2.14, and 2.15, in which red rose extract paper strips are used. Label the nature of solutions present in each of the containers.

ANSWERFig. 2.13: basic — the dipped part of the red-rose paper turns green.Fig. 2.14: neutral — the dipped part keeps the original colour.Fig. 2.15: acidic — the dipped part turns red. Compare the dipped tip with the unchanged part of each strip, not just the colour of the liquid.
Red-rose indicator strips: Fig. 2.13 has a green tip and is basic; Fig. 2.14 is unchanged and neutral; Fig. 2.15 has a red tip and is acidic.
Question 3: original schematic matching the three textbook figures. Colours are illustrative; compare with an untreated strip. Tap to view at full size.

4. A liquid sample from the laboratory was tested using various indicators:

IndicatorRed litmusBlue litmusTurmeric
ChangeNo changeTurned redNo change in colour

Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.

ANSWER The liquid is acidic. Justification: The blue litmus turned red, which is the test for an acid. The red litmus showed no change (acids do not affect red litmus), and turmeric showed no change too (turmeric turns red only in a base). All three observations agree that the liquid is acidic in nature.

5. Manya is blindfolded. She is given two unknown solutions to test and determine whether they are acidic or basic. Which indicator should Manya use to test the solutions and why?

ANSWERUse an olfactory indicator, such as onion-scented cloth or paper, because its odour behaves differently in acidic and basic media; a colour indicator needs sight. In the usual classroom demonstration, onion odour remains detectable in acid but becomes less detectable in a base.A teacher must select safe, dilute samples and handle them. Do not smell unknown laboratory chemicals directly or carry out a blindfolded chemical test alone.

6. Could you suggest various materials which can be used for writing the message on the white sheet of paper (given at the beginning of the chapter) and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.

ANSWER The trick is an indicator + an acid/base reaction. One simple method is to write with a base (e.g. soap solution) on paper and spray a turmeric solution, so the hidden words appear red. Other combinations:
Writing materialSpray (indicator)Colour of the writing
Soap / baking-soda solution (basic)Turmeric solutionRed
Soap / lime water (basic)Red rose extractGreen
Lemon juice / vinegar (acidic)Red rose extractRed
Soap solution (basic)Red litmus solutionBlue
Lemon juice (acidic)Blue litmus solutionRed
NOTEThe message stays invisible until the indicator spray reacts with the acidic/basic writing, revealing the words by a colour change. (Any reasonable indicator-based combination is accepted.)

7. Grape juice was mixed with red rose extract; the mixture got a tint of red colour. What will happen if baking soda is added to this mixture? Justify your answer.

ANSWERThe initial red tint shows that grape juice is acidic. Baking soda reacts with the acid, so the red colour becomes less pronounced. Only after sufficient excess makes the mixture basic is the expected red-rose indicator colour green; adding a little baking soda does not guarantee an immediate green colour.Baking soda is a hydrogen carbonate, so its reaction with acid also releases carbon dioxide. Bubbling may be observed. Use a small open container and do not seal the reaction.

8. Keerthi wrote a secret message to her grandmother on her birthday using orange juice. Can you assist her grandmother in revealing the message? Which indicator would you use to make it visible?

ANSWER Orange juice is acidic (it contains citric and ascorbic acid). Spray or gently apply red rose extract over the paper: the parts written with orange juice will turn a shade of red, making the hidden message visible. Blue litmus solution would also reveal it (the writing turns red), so any indicator that responds to an acid by a colour change can be used.

9. How can natural indicators be prepared? Explain by giving an example.

ANSWER Natural indicators are prepared from coloured parts of plants. Example — red rose extract: collect a fistful of fallen red rose petals and wash them; crush the petals using a mortar and pestle; place them in a glass tumbler and pour in some hot water (under an adult’s supervision) so the petals are fully immersed; cover and wait 5–10 minutes till the water becomes coloured, then filter it. The coloured filtrate is the indicator. Turmeric paper is another example: make a turmeric paste with a little water, dip filter paper in it till it is yellow, dry it and cut it into strips. Extracts of purple cabbage, beetroot, hibiscus (gudhal), etc. can be prepared in the same way.

10. Three liquids are given to you. One is vinegar, another is a baking soda solution, and the third is a sugar solution. Can you identify them only using turmeric paper? Explain.

ANSWERWith fresh yellow turmeric paper used directly: no. The baking-soda solution turns it reddish-brown. Vinegar and sugar solution both leave fresh paper yellow, so this direct test cannot distinguish them. This is the limitation explained on textbook page 15.Further investigation using the same indicator: first identify baking soda, then use a little of it to make two turmeric strips reddish-brown. Test the remaining liquids on separate prepared strips. Sufficient vinegar neutralises the base and restores yellow; sugar solution does not neutralise it and the strip stays reddish-brown. Avoid excess baking soda and compare with a control. Thus a two-stage test can distinguish these three known candidates; it is different from testing each with fresh yellow paper.

11. The extract of red rose turns the liquid X to green. What will the nature of liquid X be? What will happen when excess of amla juice is added to liquid X?

ANSWER Red rose extract is green in a base, so liquid X is basic in nature. Amla juice is acidic. Adding it neutralises the base; with an excess of amla juice the mixture becomes acidic, so the red rose extract changes from green back to red.

12. Observe and analyse the information given in the following flowchart. Complete the missing information. Imagine a garden with plants showing signs of poor health. The soil can be ______ in nature / The soil can be ______ in nature. Which indicator can be used to test the nature of the soil? _________________ The acidic soil can be treated with ________________. The basic soil can be treated with ________________.

ANSWERLeft branch: acidic; right branch: basic. Test a clear water extract of the soil with both red and blue litmus papers, or compare red-rose extract with controls. Fresh turmeric alone cannot distinguish acidic from neutral soil.Acidic soil: lime. Basic soil: organic matter such as manure or composted leaves. These are the textbook answers. The amount and type of treatment should follow a soil test and local gardening advice; different plants prefer different soil conditions.Poor growth does not itself prove an acid–base problem. Soil can be neutral but deficient in nutrients, as the textbook also notes.

Dive Deeper — Answer

Aman accidentally spilt vinegar on some pieces of an eggshell or marble and noticed bubbling. He then poured a soap solution on another piece of eggshell or marble, but no bubbles appeared. Why did bubbles occur with vinegar but not with soap solution?

ANSWER Eggshell and marble are made of a basic material (calcium carbonate). Vinegar is an acid, so it reacts with this material and releases a gas, which escapes as bubbles. Soap solution is basic, not acidic, so it does not react with the (already basic) eggshell or marble — hence no gas and no bubbles are produced.

Common Mistakes to Avoid

Watch out for these

  • Mixing up the litmus rule — remember Acid turns blue litmus red; base turns red litmus blue.
  • Using fresh yellow turmeric paper to tell an acid from a neutral sample: both leave it yellow. The two-stage method in Question 10 uses paper first made reddish-brown by a base.
  • Swapping the red rose colours — it is red in acid and green in base.
  • Treating tap water as always neutral. Its nature can vary; test it. Sugar and common-salt solutions in neutral water are classroom examples that change neither red nor blue litmus.
  • Confusing “lime water” (calcium hydroxide solution, a base) with the fruit “lime”.
  • Writing neutralisation products incompletely — it gives salt + water + heat, not just salt and water.
  • Tasting unknown substances to test them — never do this; use indicators instead.

Extra Practice Questions

These questions and MCQs are ClearStudy practice, separate from the NCERT exercise.

Short Answer Type Questions

Q1. From which living organism is litmus obtained?

ANSWERLitmus is a natural dye obtained from lichens.

Q2. Name two substances that are neutral in nature.

ANSWERSugar solution and common-salt (sodium chloride) solution made with neutral water. Tap water is treated as neutral in the introductory classroom example, but real tap-water samples can vary.

Q3. Why are basic substances said to feel different to touch?

ANSWERMany basic solutions feel soapy or slippery. This is a property, not a safe identification test: use indicators and do not touch or taste unknown chemicals.

Q4. What is an olfactory indicator? Give one example.

ANSWERA substance whose odour changes in an acidic or basic medium. Example: onion.

Q5. Write the general word equation for a neutralisation reaction.

ANSWERAcid + Base → Salt + Water + Heat.

Long Answer Type Questions

Q1. Describe how you would prepare turmeric paper and explain what it can and cannot test.

ANSWERTake a spoonful of turmeric in a dish and add a little water to make a paste. Dip a piece of filter paper in the paste until it turns yellow, take it out and let it dry, then cut it into thin strips (this is “turmeric paper”); ask an adult to supervise preparation and cutting; heating is not needed. Turmeric paper turns red in basic substances, so it can detect bases. However, it remains unchanged in both acidic and neutral substances, so fresh yellow paper cannot directly differentiate between an acid and a neutral substance. Question 10 explains a two-stage extension using paper first treated with a base.

Q2. Explain, with three examples, how neutralisation is useful in daily life.

ANSWER(i) Textbook ant-bite example: NCERT models acidic formic acid reacting with basic baking soda. This explains a chemical idea, not a general first-aid recommendation. For real bites, tell an adult and follow current first-aid guidance; the NHS advises against bicarbonate home remedies.(ii) Soil: lime can reduce excessive acidity; the textbook describes organic matter for basic soil. Use soil-test advice rather than adding materials blindly.(iii) Factory waste: controlled treatment can neutralise acidic waste before discharge. Neutralisation alone does not remove all pollutants or prove that water is safe.

Q3. A solution gives no colour change with turmeric paper but turns blue litmus red. Identify its nature and justify, and name one safe way to make it neutral.

ANSWERIt is acidic because blue litmus turns red. Fresh turmeric stays yellow in both acidic and neutral solutions, so the litmus result is decisive.For a teacher-selected dilute classroom acid, add a suitable dilute base slowly, mixing and testing after each addition. Check both litmus papers: a mixture that no longer changes blue litmus may already contain excess base. Do not try to neutralise unknown chemicals yourself.

MCQs & Assertion–Reason

1. A substance that turns blue litmus paper red is:

(a) basic    (b) neutral    (c) acidic    (d) an indicator

2. Litmus is obtained from:

(a) roses    (b) lichens    (c) turmeric    (d) onions

3. Red rose extract in a basic solution turns:

(a) red    (b) yellow    (c) green    (d) colourless

4. Turmeric paper turns red in:

(a) acidic solutions    (b) basic solutions    (c) neutral solutions    (d) all solutions

5. Which of the following is a neutral substance?

(a) lemon juice    (b) lime water    (c) sugar solution    (d) vinegar

6. The products of a neutralisation reaction are:

(a) only salt    (b) salt and water and heat    (c) only water    (d) acid and base

7. In the textbook’s ant-bite neutralisation model, baking soda represents a substance that is:

(a) acidic    (b) neutral    (c) basic    (d) an olfactory indicator

8. Onion can be used as an indicator because it is:

(a) acidic    (b) an olfactory indicator    (c) a synthetic indicator    (d) basic

9. Excessively acidic soil is best treated with:

(a) vinegar    (b) lime    (c) lemon juice    (d) common salt

10. Basic substances generally feel:

(a) rough    (b) sticky    (c) soapy/slippery    (d) gritty

Answer key: 1-(c), 2-(b), 3-(c), 4-(b), 5-(c), 6-(b), 7-(c), 8-(b), 9-(b), 10-(c).

For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.

A-R 1. Assertion: Lemon juice turns blue litmus paper red.

Reason: Lemon juice is acidic in nature.

A-R 2. Assertion: Fresh yellow turmeric paper can directly distinguish an acid from a neutral substance.

Reason: Turmeric turns red only in basic solutions.

A-R 3. Assertion: A neutralisation reaction releases heat.

Reason: In neutralisation an acid and a base react to form salt and water with the evolution of heat.

A-R 4. Assertion: Sugar solution turns red litmus blue.

Reason: Sugar solution in neutral water is neutral in nature.

A-R 5. Assertion: A blindfolded person can use an olfactory indicator to test solutions.

Reason: Olfactory indicators show a change in odour in acidic and basic media.

Answer key: 1-(A), 2-(D), 3-(A), 4-(D), 5-(A).

Quick Revision Summary

  • Substances are acidic, basic or neutral in nature.
  • Indicators show different colours in acids and bases — litmus (from lichens), red rose extract, turmeric, purple cabbage, hibiscus.
  • Acids turn blue litmus red; bases turn red litmus blue; neutral substances change neither.
  • Red rose extract: red in acid, green in base. Turmeric: red in base, unchanged in acid/neutral.
  • Olfactory indicators (e.g. onion) change odour in acidic/basic media.
  • Neutralisation: Acid + Base → Salt + Water + Heat — used for ant bites, treating acidic/basic soil and acidic factory waste.

How to write a clear answer

Memorise the colour-change rules in a table (litmus, red rose extract, turmeric for acid/base/neutral) and practise explaining each result. Always state the full neutralisation equation salt + water + heat. In “identify the nature” questions, justify each answer with the indicator result, and remember turmeric and an olfactory indicator have special uses (turmeric for bases only; onion for a blindfolded test). Mention real-life neutralisation examples — ant bite, soil, factory waste — to strengthen long answers.

Frequently Asked Questions

What is Class 7 Science Curiosity Chapter 2 about?

Chapter 2, Exploring Substances: Acidic, Basic, and Neutral, explains how to classify everyday substances as acidic, basic or neutral using acid-base indicators such as litmus, red rose extract and turmeric, and how an acid and a base neutralise each other to form salt, water and heat.

What is the difference between an acid and a base in this chapter?

Acids taste sour and turn blue litmus red (e.g. lemon juice, vinegar). Bases taste bitter, feel slippery and turn red litmus blue (e.g. soap solution, lime water). Neutral substances such as sugar and salt solutions affect neither litmus paper.

What is a neutralisation reaction?

When an acid and a base are mixed in sufficient quantity, the result is neither acidic nor basic. The reaction forms salt and water along with the release of heat: Acid + Base → Salt + Water + Heat.

Are these Class 7 Science Curiosity Chapter 2 solutions free?

Yes. All solutions are free and follow the official NCERT Curiosity textbook for session 2026–27.

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