This MCQ module is based on: Exploring Substances
TOPIC 6 OF 46
Exploring Substances
🎓 Class 7
Science
CBSE
Theory
Ch 2 — Exploring Substances: Acidic, Basic and Neutral
⏱ ~8 min
🌐 Language: [gtranslate]
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In a Nutshell — Chapter 2 Summary
Key Concepts from Chapter 2:
- Substances can be grouped as acidic, basic (alkaline), or neutral.
- An acid-base indicator is a substance that shows different colours (or odour changes) in the presence of acids and bases.
- Litmus is the most common indicator, obtained from lichens. It comes as blue and red litmus paper.
- Acidic substances taste sour and turn blue litmus paper red.
- Basic substances taste bitter, feel soapy/slippery, and turn red litmus paper blue.
- Neutral substances do not change the colour of either litmus paper.
- Red rose extract is a natural indicator — it stays red in acids, turns green in bases, and shows no change in neutral substances.
- Turmeric is a natural indicator that turns reddish-brown only in basic solutions. It shows no change with acids or neutral substances.
- Olfactory indicators (e.g., onion, clove oil) change their smell in acidic or basic solutions. They are useful for visually impaired individuals.
- Neutralisation is the reaction between an acid and a base: Acid + Base → Salt + Water + Heat.
- Neutralisation in daily life: treating ant bites (formic acid + baking soda), improving acidic soil (adding lime), treating factory waste.
Keywords
Acid
Base
Neutral
Indicator
Litmus
Lichen
Red Rose Extract
Turmeric
Olfactory Indicator
Neutralisation
Salt
Formic Acid
Citric Acid
Acetic Acid
Lime Water
Let Us Enhance Our Learning
Q1. Blue litmus paper was dipped in a solution and it turned red. Which of the following solutions can be used to reverse this change (turn it back to blue)?
(a) Lime water (b) Baking soda solution (c) Vinegar (d) Common salt solution
Answer: (a) Lime water and (b) Baking soda solution.
The blue litmus turned red, which means the original solution was acidic. To reverse the change (turn it back to blue), we need a basic substance. Among the options:
• Lime water — basic (will turn red litmus blue) ✔
• Baking soda solution — basic (will turn red litmus blue) ✔
• Vinegar — acidic (will not help)
• Common salt — neutral (will not change litmus colour)
The blue litmus turned red, which means the original solution was acidic. To reverse the change (turn it back to blue), we need a basic substance. Among the options:
• Lime water — basic (will turn red litmus blue) ✔
• Baking soda solution — basic (will turn red litmus blue) ✔
• Vinegar — acidic (will not help)
• Common salt — neutral (will not change litmus colour)
Q2. Three unknown solutions A, B, and C were tested with litmus paper and turmeric paper. The results are shown below. Identify the nature of each solution (acidic, basic, or neutral).
| Solution | Blue Litmus | Red Litmus | Turmeric Paper |
|---|---|---|---|
| A | Turns Red | No change | No change |
| B | No change | Turns Blue | Turns Red |
| C | No change | No change | No change |
Solution A — Acidic: It turns blue litmus red (confirms acid). Red litmus and turmeric show no change (both expected for acids).
Solution B — Basic: It turns red litmus blue and turmeric paper turns reddish (both confirm a base). Blue litmus shows no change (expected for bases).
Solution C — Neutral: No change in blue litmus, red litmus, or turmeric paper. This is characteristic of a neutral substance.
Solution B — Basic: It turns red litmus blue and turmeric paper turns reddish (both confirm a base). Blue litmus shows no change (expected for bases).
Solution C — Neutral: No change in blue litmus, red litmus, or turmeric paper. This is characteristic of a neutral substance.
Q3. Observe the figures below. Each shows strips of paper dipped in red rose extract and then placed in different solutions. Based on the colour of the strip, identify whether the solution is acidic, basic, or neutral.
Fig 2.13 (Solution X) — Acidic: The red rose extract strip remains red/dark pink in acidic solutions.
Fig 2.14 (Solution Y) — Basic: The strip turns green, which is the characteristic colour change of red rose extract in a basic solution.
Fig 2.15 (Solution Z) — Neutral: The strip shows no significant colour change, remaining its original reddish colour. This indicates a neutral substance.
Fig 2.14 (Solution Y) — Basic: The strip turns green, which is the characteristic colour change of red rose extract in a basic solution.
Fig 2.15 (Solution Z) — Neutral: The strip shows no significant colour change, remaining its original reddish colour. This indicates a neutral substance.
Q4. A liquid sample from a lab was tested with three indicators. The results are given below:
| Indicator | Observation |
|---|---|
| Red litmus paper | Turns Blue |
| Blue litmus paper | No change |
| Turmeric paper | Turns Red |
What is the nature of the liquid? Justify your answer.
Answer: The liquid is basic.
Evidence: (1) Red litmus turns blue — this only happens with bases. (2) Blue litmus shows no change — consistent with a base (bases do not affect blue litmus). (3) Turmeric turns reddish-brown — turmeric only changes colour with bases. All three indicators confirm the liquid is basic in nature.
Evidence: (1) Red litmus turns blue — this only happens with bases. (2) Blue litmus shows no change — consistent with a base (bases do not affect blue litmus). (3) Turmeric turns reddish-brown — turmeric only changes colour with bases. All three indicators confirm the liquid is basic in nature.
Q5. Manya is blindfolded during a science game. She needs to determine whether a given solution is acidic or basic without seeing it. Which type of indicator should she use, and why?
Answer: Manya should use an olfactory indicator such as onion. Since she is blindfolded and cannot see colour changes, visual indicators like litmus, turmeric, or red rose extract would be useless. Olfactory indicators change their smell in acidic or basic solutions. For example, the characteristic odour of onion diminishes in a basic solution but remains strong in an acidic solution. By smelling the difference, Manya can identify the nature of the solution without seeing it.
Q6. Suggest two materials that could be used to create an "invisible writing" effect, where a hidden message appears when a special liquid is sprayed on it. Explain the science behind it.
Answer: Use turmeric solution as the invisible ink and baking soda (sodium bicarbonate) solution as the revealing spray.
Method: Write the message on white paper using turmeric solution. When dry, the writing is nearly invisible (pale yellow on white). Spray baking soda solution over the paper. Since baking soda is a base, the turmeric indicator reacts and turns reddish-brown, making the message visible.
Science: This works because turmeric is a natural indicator that changes colour in the presence of a base.
Method: Write the message on white paper using turmeric solution. When dry, the writing is nearly invisible (pale yellow on white). Spray baking soda solution over the paper. Since baking soda is a base, the turmeric indicator reacts and turns reddish-brown, making the message visible.
Science: This works because turmeric is a natural indicator that changes colour in the presence of a base.
Q7. Grape juice was mixed with red rose extract, and the resulting mixture showed a red colour. What will happen if baking soda solution is now added to this mixture? Justify your prediction.
Answer: When baking soda solution is added, the colour will change from red to green.
Reasoning: The red colour of the mixture indicates that grape juice is acidic (since red rose extract remains red in acidic solutions). Baking soda is a basic substance. When added, two things happen: (1) Neutralisation occurs — the base neutralises the acid. (2) If enough baking soda is added to make the solution basic, the red rose extract turns green (its characteristic colour in basic solutions).
Reasoning: The red colour of the mixture indicates that grape juice is acidic (since red rose extract remains red in acidic solutions). Baking soda is a basic substance. When added, two things happen: (1) Neutralisation occurs — the base neutralises the acid. (2) If enough baking soda is added to make the solution basic, the red rose extract turns green (its characteristic colour in basic solutions).
Q8. Keerthi wanted to write a secret birthday message for her friend. She used orange juice as invisible ink and wrote a message on white paper. Suggest how her friend could reveal the message and explain the underlying concept.
Answer: The friend could hold the paper near a gentle heat source (like an iron or a lamp). When heated, orange juice (which contains citric acid) oxidises and turns brown, making the message visible.
Alternatively, the concept of indicators could be used: since orange juice is acidic, applying a substance that changes colour with acids (like blue litmus solution carefully applied as a wash) could also help reveal the message. The key concept is that orange juice is an acidic substance that can undergo visible changes with heat or indicators.
Alternatively, the concept of indicators could be used: since orange juice is acidic, applying a substance that changes colour with acids (like blue litmus solution carefully applied as a wash) could also help reveal the message. The key concept is that orange juice is an acidic substance that can undergo visible changes with heat or indicators.
Q9. How can natural indicators be prepared at home? Give two examples and describe the preparation method for each.
Example 1 — Red Rose Extract: Collect a handful of red rose petals. Crush them in a mortar. Add a small amount of hot water to the crushed petals and mix well. Filter the mixture through a clean cloth to obtain a deep red liquid. This liquid is the red rose extract indicator, which turns green in bases and stays red in acids.
Example 2 — Turmeric Paper: Mix turmeric powder with a little water to form a smooth paste. Spread the paste evenly on filter paper and let it dry completely. Cut the dried paper into thin strips. These turmeric indicator strips turn reddish-brown when dipped in a basic solution and show no change with acidic or neutral substances.
Example 2 — Turmeric Paper: Mix turmeric powder with a little water to form a smooth paste. Spread the paste evenly on filter paper and let it dry completely. Cut the dried paper into thin strips. These turmeric indicator strips turn reddish-brown when dipped in a basic solution and show no change with acidic or neutral substances.
Q10. You are given three liquids: vinegar, baking soda solution, and sugar solution. Using only turmeric paper, how would you identify each one? Describe your procedure step by step.
Step 1: Place a drop of each liquid on separate turmeric paper strips.
Step 2: Observe which strip changes colour.
Result: Only one strip will turn reddish-brown — the liquid on this strip is baking soda solution (basic). The other two strips (vinegar and sugar solution) will show no change.
Step 3: To distinguish between vinegar and sugar solution, turmeric alone is insufficient (both show no change). However, you can use a simple taste test since both are safe household items — vinegar is sour (acidic) while sugar solution is sweet (neutral).
Limitation noted: Turmeric can only confirm bases (turns red/brown). It cannot distinguish between acidic and neutral substances, so an additional method is needed for full identification.
Step 2: Observe which strip changes colour.
Result: Only one strip will turn reddish-brown — the liquid on this strip is baking soda solution (basic). The other two strips (vinegar and sugar solution) will show no change.
Step 3: To distinguish between vinegar and sugar solution, turmeric alone is insufficient (both show no change). However, you can use a simple taste test since both are safe household items — vinegar is sour (acidic) while sugar solution is sweet (neutral).
Limitation noted: Turmeric can only confirm bases (turns red/brown). It cannot distinguish between acidic and neutral substances, so an additional method is needed for full identification.
Q11. Red rose extract was added to an unknown liquid X, and the solution turned green. (a) What is the nature of liquid X? (b) What would happen if amla juice (acidic) is now added to this green solution?
(a) Liquid X is basic. Red rose extract turns green only in the presence of a base.
(b) When amla juice (acidic) is added to the green solution, neutralisation begins. The acid (amla juice) will react with the base (liquid X). As the base is neutralised, the green colour will gradually change. If enough amla juice is added to completely neutralise the base and make the solution acidic, the colour will change from green back to red (since red rose extract is red in acidic solutions). If only partial neutralisation occurs, the colour may reach an intermediate shade.
(b) When amla juice (acidic) is added to the green solution, neutralisation begins. The acid (amla juice) will react with the base (liquid X). As the base is neutralised, the green colour will gradually change. If enough amla juice is added to completely neutralise the base and make the solution acidic, the colour will change from green back to red (since red rose extract is red in acidic solutions). If only partial neutralisation occurs, the colour may reach an intermediate shade.
Q12. Complete the following flowchart about treating soil for proper plant growth:
The completed flowchart:
• Decision box: Blue litmus turns RED → proceed to left branch.
• Left box: Soil is ACIDIC.
• Treatment box: Add LIME (base) to neutralise the acidity.
• Result: Neutral soil → healthy crops grow well!
• Right branch (No change): If blue litmus shows no change, the soil is not acidic. It could be neutral or basic, and may not need lime treatment.
• Decision box: Blue litmus turns RED → proceed to left branch.
• Left box: Soil is ACIDIC.
• Treatment box: Add LIME (base) to neutralise the acidity.
• Result: Neutral soil → healthy crops grow well!
• Right branch (No change): If blue litmus shows no change, the soil is not acidic. It could be neutral or basic, and may not need lime treatment.
Dive Deeper
Exploration: Take two clean bowls. In Bowl A, place a piece of eggshell and add a spoonful of vinegar. In Bowl B, place another piece of eggshell and add some soap solution. Observe both bowls over 15–20 minutes. What differences do you notice? Which bowl shows bubbles? Can you explain why?
Expected observations:
• Bowl A (Eggshell + Vinegar): You will see bubbles forming on the eggshell surface. The eggshell may slowly start to dissolve. The bubbles are carbon dioxide gas, produced because eggshell (calcium carbonate) reacts with vinegar (acetic acid). This is an acid-carbonate reaction.
• Bowl B (Eggshell + Soap solution): No significant reaction is observed. The eggshell remains unchanged because soap is a base, and calcium carbonate does not react vigorously with bases.
Conclusion: Acids react with carbonates to produce carbon dioxide gas (bubbles), while bases generally do not cause such a reaction with eggshells.
• Bowl A (Eggshell + Vinegar): You will see bubbles forming on the eggshell surface. The eggshell may slowly start to dissolve. The bubbles are carbon dioxide gas, produced because eggshell (calcium carbonate) reacts with vinegar (acetic acid). This is an acid-carbonate reaction.
• Bowl B (Eggshell + Soap solution): No significant reaction is observed. The eggshell remains unchanged because soap is a base, and calcium carbonate does not react vigorously with bases.
Conclusion: Acids react with carbonates to produce carbon dioxide gas (bubbles), while bases generally do not cause such a reaction with eggshells.
Exploratory Projects
Project Ideas for Further Exploration:
- Indicator Garden: Collect petals from different flowers (hibiscus, marigold, bougainvillea, petunia) and prepare extracts from each. Test them with acidic and basic solutions. Make a chart showing which flowers work as natural indicators.
- Soil Survey: Collect soil samples from different locations (garden, roadside, near a pond, farmland). Test each sample using turmeric paper and litmus paper. Classify each soil as acidic, basic, or neutral and discuss which crops might grow best in each type.
- Kitchen Chemistry: Test at least 10 kitchen items (different juices, sauces, cleaning products) using homemade red rose and turmeric indicators. Create a colourful poster showing your findings.
Frequently Asked Questions — Exploring Substances — Chapter 2 Exercises
What does the topic 'Exploring Substances — Chapter 2 Exercises' cover in Class 7 Science?
The topic 'Exploring Substances — Chapter 2 Exercises' is part of NCERT Class 7 Science Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral. It covers the key ideas of acids, bases, indicators, neutralisation, NCERT exercises, worksheet, MCQ, short answer, explained through everyday examples, labelled diagrams and hands-on activities drawn from the NCERT Curiosity textbook. Students learn not just definitions but also the reasoning behind each concept so they can answer competency-based questions and assertion–reason items. The lesson helps Class 7 students build a strong base for higher classes by linking each idea to real observations at home, school and in nature, and by preparing them for CBSE school assessments and Olympiads.
Why is 'Exploring Substances — Chapter 2 Exercises' important for Class 7 NCERT Science?
'Exploring Substances — Chapter 2 Exercises' is important because it builds core scientific thinking that Class 7 students will use throughout middle and secondary school. NCERT Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — introduces acids and related ideas that appear again in Class 8, 9 and 10 Science. Mastering this subtopic helps students read labels and safety signs, understand news about science and technology, and perform better in CBSE school exams. The chapter also encourages curiosity and evidence-based thinking — skills that support the National Education Policy (NEP) 2020 focus on conceptual understanding and competency-based learning.
What are the key concepts students should remember from Exploring Substances — Chapter 2 Exercises?
The key concepts in 'Exploring Substances — Chapter 2 Exercises' for Class 7 Science are: acids, bases, indicators, neutralisation, NCERT exercises, worksheet, MCQ, short answer. Students should be able to define each term in their own words, give at least one everyday example, and explain how the concept connects to other chapters in NCERT Class 7 Science. For example, linking the idea to daily life — in the kitchen, classroom or outdoors — makes revision easier. Writing short notes, drawing labelled diagrams and solving the NCERT in-text and exercise questions for Chapter 2 will help students retain these concepts for unit tests and the annual CBSE examination.
How is Exploring Substances — Chapter 2 Exercises taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Exploring Substances — Chapter 2 Exercises' using an inquiry-based approach with Predict–Observe–Explain activities. Students are asked to make a guess first, then perform a simple experiment with safe, easily available materials, and finally explain what they observed. This matches the NEP 2020 focus on learning by doing. For Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — the textbook includes hands-on tasks, labelled diagrams and questions that build Bloom's Taxonomy skills from Remember (L1) to Create (L6). Teachers use these activities, along with competency-based questions (CBQs) and assertion–reason items, to check real understanding rather than rote memorisation.
How should Class 7 students prepare for the Chapter 2 exercises?
To prepare for the Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — exercises in NCERT Class 7 Science, students should first revise the theory in Parts 1–3 and make a short list of definitions and diagrams for acids, bases, indicators, neutralisation, NCERT exercises, worksheet, MCQ, short answer. Next, attempt each exercise question on their own before checking the solution. Pay extra attention to MCQs, assertion–reason questions and short-answer items, as these appear in CBSE competency-based tests. Practising with the NCERT Curiosity textbook, the exemplar questions, and the MyAiSchool practice bank helps Class 7 students score better in unit tests and the annual examination.
How does 'Exploring Substances — Chapter 2 Exercises' connect to other chapters of Class 7 Science?
'Exploring Substances — Chapter 2 Exercises' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of acids appear again when students study related topics like heat, light, changes, life processes and Earth-Sun-Moon. For example, understanding this subtopic helps in building mental models for later chapters and for Class 8, 9 and 10 Science. Teachers often use cross-chapter questions in CBSE examinations to test whether students can apply what they learned in Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — to new situations. This integrated approach matches the NEP 2020 and NCF 2023 focus on holistic, competency-based learning.
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