This MCQ module is based on: Litmus as an Indicator of Acids and Bases
TOPIC 3 OF 46
Litmus as an Indicator of Acids and Bases
🎓 Class 7
Science
CBSE
Theory
Ch 2 — Exploring Substances: Acidic, Basic and Neutral
⏱ ~14 min
🌐 Language: [gtranslate]
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Exploring Substances: Acidic, Basic, and Neutral
On 28 February, the school hosted a science fair to celebrate National Science Day. At the entry gate, siblings Ashwin and Keerthi were greeted with a white sheet of paper. A volunteer was spraying a liquid on these sheets. To their surprise, the words "Welcome to the Wonderful World of Science" appeared on the papers the moment the liquid was sprayed!
Their curiosity was partly satisfied at the "Colourful World of Substances" stall, where they saw many activities showing colour changes on mixing different substances. They decided to explore these changes further. Let us join them on this learning adventure!
🎉 National Science Day is celebrated on 28 February every year in India to commemorate the discovery of the Raman Effect by Sir C.V. Raman in 1928, for which he received the Nobel Prize in Physics.
2.1 Nature — Our Science Laboratory
2.1.1 Litmus as an Indicator
Litmus is a natural substance obtained from lichens. It is available both as a solution and in the form of paper strips, known as litmus paper. The litmus paper comes in two colours — blue and red.
📖 How Litmus Works:
🔵→🔴 Acidic substances turn blue litmus paper red.
🔴→🔵 Basic substances turn red litmus paper blue.
⚪ Neutral substances do not change the colour of either litmus paper.
Since litmus shows different colours in acidic and basic solutions, it is called an acid-base indicator.
🔵→🔴 Acidic substances turn blue litmus paper red.
🔴→🔵 Basic substances turn red litmus paper blue.
⚪ Neutral substances do not change the colour of either litmus paper.
Since litmus shows different colours in acidic and basic solutions, it is called an acid-base indicator.
🧪 Activity 2.1 — Let Us Explore
L3 Apply
🤔 Predict first: If you put a drop of lemon juice on blue litmus paper, what do you think will happen to the colour? What about soap solution?
- Collect samples of lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, and salt solution.
- Take a strip of blue litmus paper and cut it into small pieces. Spread them on a clean, dry white tile.
- Using a dropper, put one drop of each sample, one by one, on these litmus paper pieces.
- Observe any change in colour and record your observations in Table 2.1.
- Repeat with pieces of red litmus paper and record your observations.
What you should observe:
• Group A (turn blue litmus red): Lemon juice, amla juice, tamarind water, vinegar — these are acidic substances.
• Group B (turn red litmus blue): Soap solution, baking soda solution, lime water, washing powder solution — these are basic substances.
• Group C (no change to either litmus): Tap water, sugar solution, salt solution — these are neutral substances.
• Group A (turn blue litmus red): Lemon juice, amla juice, tamarind water, vinegar — these are acidic substances.
• Group B (turn red litmus blue): Soap solution, baking soda solution, lime water, washing powder solution — these are basic substances.
• Group C (no change to either litmus): Tap water, sugar solution, salt solution — these are neutral substances.
Table 2.1: Testing Samples with Blue and Red Litmus Paper
| S. No. | Name of Sample | Blue Litmus → ? | Red Litmus → ? | Nature |
|---|---|---|---|---|
| 1. | Lemon juice | Turns RED | No change | Acidic |
| 2. | Soap solution | No change | Turns BLUE | Basic |
| 3. | Amla juice | Turns RED | No change | Acidic |
| 4. | Tamarind water | Turns RED | No change | Acidic |
| 5. | Vinegar | Turns RED | No change | Acidic |
| 6. | Baking soda solution | No change | Turns BLUE | Basic |
| 7. | Lime water | No change | Turns BLUE | Basic |
| 8. | Tap water | No change | No change | Neutral |
| 9. | Washing powder solution | No change | Turns BLUE | Basic |
| 10. | Sugar solution | No change | No change | Neutral |
| 11. | Salt solution | No change | No change | Neutral |
Table 2.2: Grouping of Substances
| Group A — Acidic | Group B — Basic | Group C — Neutral |
|---|---|---|
| Lemon juice, Amla juice, Tamarind water, Vinegar | Soap solution, Baking soda, Lime water, Washing powder | Tap water, Sugar solution, Salt solution |
| Turn blue litmus → red | Turn red litmus → blue | No change to either litmus |
📖 Summary:
🍋 Acids — Substances that turn blue litmus paper red. They generally taste sour. Common in: lemon, vinegar, amla, tamarind.
🧼 Bases — Substances that turn red litmus paper blue. They generally feel soapy/slippery and taste bitter. Common in: soap, baking soda, lime water.
💧 Neutral substances — Neither acidic nor basic. They do not change the colour of either litmus paper. Common in: pure water, sugar solution, salt solution.
🍋 Acids — Substances that turn blue litmus paper red. They generally taste sour. Common in: lemon, vinegar, amla, tamarind.
🧼 Bases — Substances that turn red litmus paper blue. They generally feel soapy/slippery and taste bitter. Common in: soap, baking soda, lime water.
💧 Neutral substances — Neither acidic nor basic. They do not change the colour of either litmus paper. Common in: pure water, sugar solution, salt solution.
🌿 Holistic Link — Lichens: Lichens are formed by the association of two living organisms — a fungus and an alga. They grow on rocks and trees in regions that have abundant rainfall and clean air. Do you find lichens on trees in your neighbourhood?
🧪 Interactive: Virtual Litmus Test L3 Apply
Select a substance and a litmus paper colour to see what happens:
Choose a substance:
Choose litmus paper:
📋 Competency-Based Questions
During a science fair, Priya tested an unknown liquid using blue and red litmus paper. She found that the blue litmus paper turned red, but the red litmus paper showed no change.
Q1. L3 Apply Based on Priya's observations, what is the nature of the unknown liquid?
Answer: C. The liquid is acidic. Acidic substances turn blue litmus paper red but do not affect red litmus paper (since red litmus is already red and acids don't change it further).
Q2. L1 Remember Fill in the blank: A substance that shows different colours in acidic and basic solutions is called an __________.
Answer: acid-base indicator
Q3. L4 Analyse A student tested sugar solution with both blue and red litmus paper and found no colour change in either. She concluded the solution is neutral. Is her conclusion correct? Explain. (Short Answer — 2 marks)
Answer: Yes, her conclusion is correct. A neutral substance does not change the colour of either blue or red litmus paper. Since sugar solution is neither acidic nor basic, it has no effect on litmus — confirming it is neutral.
Q4. L5 Evaluate True or False: "All substances that taste sour are acidic in nature." Justify your answer with examples. (3 marks)
Answer: True (generally). Substances that taste sour — like lemon juice (citric acid), vinegar (acetic acid), amla (ascorbic acid), tamarind (tartaric acid) — are acidic in nature. They all turn blue litmus paper red. However, Caution: we must never taste unknown substances to determine their nature — always use indicators like litmus paper instead.
Q5. L6 Create HOT: Design a simple experiment using litmus paper to determine whether your local tap water is acidic, basic, or neutral. List the materials needed and steps. (3 marks)
Hint: You need: blue litmus paper, red litmus paper, a dropper, a clean white tile, and tap water. Steps: (1) Place blue and red litmus strips on the tile, (2) Put a drop of tap water on each, (3) Observe colour changes, (4) If blue turns red → acidic; if red turns blue → basic; if neither changes → neutral.
🔗 Assertion–Reason Questions
Assertion (A): Litmus paper is obtained from a natural source.
Reason (R): Litmus is extracted from lichens, which are organisms formed by a fungus and an alga living together.
Answer: A. Both are true. Litmus is indeed a natural substance (Assertion), and it is specifically obtained from lichens — symbiotic organisms of fungus and alga (Reason). The Reason correctly explains the Assertion.
Assertion (A): Vinegar turns red litmus paper blue.
Reason (R): Vinegar contains acetic acid, which is acidic in nature.
Answer: D. The Assertion is false — vinegar does NOT turn red litmus blue. Being acidic, vinegar turns blue litmus red (not red litmus blue). The Reason is true — vinegar does contain acetic acid.
Assertion (A): Salt solution is a neutral substance.
Reason (R): Neutral substances do not change the colour of either blue or red litmus paper.
Answer: A. Both are true. Salt solution is neutral (Assertion), and the Reason correctly explains what "neutral" means — it does not affect either litmus paper. When we test salt solution with both litmus papers, neither changes colour.
Frequently Asked Questions — Litmus as an Indicator of Acids and Bases
What does the topic 'Litmus as an Indicator of Acids and Bases' cover in Class 7 Science?
The topic 'Litmus as an Indicator of Acids and Bases' is part of NCERT Class 7 Science Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral. It covers the key ideas of acids, bases, indicators, litmus paper, red litmus, blue litmus, acidic solution, basic solution, 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 'Litmus as an Indicator of Acids and Bases' important for Class 7 NCERT Science?
'Litmus as an Indicator of Acids and Bases' 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 Litmus as an Indicator of Acids and Bases?
The key concepts in 'Litmus as an Indicator of Acids and Bases' for Class 7 Science are: acids, bases, indicators, litmus paper, red litmus, blue litmus, acidic solution, basic solution. 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 Litmus as an Indicator of Acids and Bases taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Litmus as an Indicator of Acids and Bases' 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.
What real-life examples of acids can Class 7 students observe at home?
Class 7 students can observe acids at home in many simple ways linked to 'Litmus as an Indicator of Acids and Bases'. Kitchens, school bags, playgrounds and the night sky are full of examples that connect to NCERT Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral. For instance, students can check labels on food and cleaning products, watch changes while cooking, or observe the Sun and Moon across a week. Keeping a small science diary — noting the date, what was observed and a quick sketch — turns everyday life into a science lab. These real-life connections make concepts stick and prepare students well for competency-based questions in CBSE Class 7 Science.
How does 'Litmus as an Indicator of Acids and Bases' connect to other chapters of Class 7 Science?
'Litmus as an Indicator of Acids and Bases' 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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