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.

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 it can detect bases only.

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

Neutralisation: when an acid and a base are mixed in sufficient quantity the product is neither acidic nor basic. Acid + Base → Salt + Water + Heat.

“Let Us Enhance Our Learning” — NCERT Solutions

All questions below are reproduced verbatim from the NCERT Curiosity textbook; the answers are original and exam-ready.

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.

ANSWER Red rose extract gives a red shade in an acidic solution and a green shade in a basic solution, and shows no change in a neutral solution. So judge each figure by the strip’s colour: • A strip that turns a shade of red → the solution is acidic. • A strip that turns a shade of green → the solution is basic. • A strip that shows no colour change → the solution is neutral. Label each of Figs. 2.13, 2.14 and 2.15 accordingly by matching the strip colour shown in your textbook to the rule above.

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?

ANSWER Manya should use an olfactory indicator, such as onion (or vanilla/clove oil) prepared on cloth or filter-paper strips. Why: Litmus, red rose extract and turmeric all rely on colour change, which a blindfolded person cannot see. An olfactory indicator works by a change in smell in acidic and basic media, so Manya can tell the two solutions apart using only her sense of smell.

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.

ANSWER A red tint means grape juice is acidic. Baking-soda solution is basic, so adding it will gradually neutralise the acid; with enough baking soda the mixture stops being acidic and turns green (the colour red rose extract shows in a basic solution). Justification: Red rose extract is red in acids and green in bases. As baking soda makes the mixture basic, the indicator changes from red to green.

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.

ANSWER No, turmeric paper alone cannot identify all three. Turmeric turns red only in a base. So the baking soda solution (basic) will turn turmeric paper red and can be identified. But vinegar (acidic) and sugar solution (neutral) both leave turmeric unchanged, so turmeric paper cannot tell them apart. A second indicator such as litmus or red rose extract is needed to separate vinegar from the sugar solution.

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 ________________.

ANSWER • The soil can be acidic in nature — or — the soil can be basic in nature. • Indicator used to test the soil: litmus paper (or any natural acid-base indicator such as red rose extract / turmeric). • Acidic soil is treated with lime (a base). • Basic soil is treated with organic matter such as manure / composted leaves, which release acids that neutralise the base.

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.
  • Thinking turmeric detects acids — turmeric turns red only in a base; it cannot tell acids from neutral substances.
  • Swapping the red rose colours — it is red in acid and green in base.
  • Forgetting that neutral substances (tap water, sugar, salt solution) change no indicator.
  • 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

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.

ANSWERTap water and sugar solution (salt solution is also neutral).

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

ANSWERBases generally feel soapy or slippery to touch, as observed when baking-soda solution is rubbed between the fingers.

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”); do the heating/preparation under an adult’s supervision. Turmeric paper turns red in basic substances, so it can detect bases. However, it remains unchanged in both acidic and neutral substances, so it cannot differentiate between an acid and a neutral substance — that is its limitation.

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

ANSWER(i) Ant bite: an ant injects acidic formic acid causing pain; rubbing moist baking soda (a base) neutralises the acid and relieves the sting. (ii) Acidic soil: when soil becomes too acidic from excess chemical fertilisers, plants grow poorly; adding lime (a base) neutralises the acidity. If the soil is basic, organic matter like manure releases acids that neutralise it. (iii) Factory waste: if factory waste released into a lake is acidic and harms fish, basic substances are added to neutralise the waste before releasing it. In each case an acid and a base cancel each other to give a safer, neutral product.

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.

ANSWERTurning blue litmus red shows the solution is acidic; turmeric showing no change agrees, because turmeric reacts only with bases. To make it neutral, add a base such as baking-soda solution or lime water in sufficient quantity — the acid and base undergo neutralisation to form salt and water, and the solution stops affecting blue litmus.

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. An ant bite is relieved by applying moist baking soda because baking soda 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: Turmeric paper can 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 is basic 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-(C), 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 score full marks in this chapter

Memorise the colour-change rules in a table (litmus, red rose extract, turmeric for acid/base/neutral) — most marks come from these. 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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