This MCQ module is based on: Natural Indicators — Turmeric, China Rose, Red Cabbage
TOPIC 4 OF 46
Natural Indicators — Turmeric, China Rose, Red Cabbage
🎓 Class 7
Science
CBSE
Theory
Ch 2 — Exploring Substances: Acidic, Basic and Neutral
⏱ ~14 min
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Common Acids and Bases Around Us
In the previous part, we learned how to classify substances as acidic, basic, or neutral using litmus paper. Now let us look more closely at the acids and bases we encounter in daily life.
Activity 2.2 — Identifying Acids in Everyday Foods
L2 Understand
Think about the foods you eat daily. Which ones taste sour? Can you guess what acid each might contain?
- Collect samples of lemon, orange, amla (Indian gooseberry), tamarind, curd, and vinegar.
- Taste each one carefully (only these safe, edible items!).
- Notice the common sour taste — this sourness comes from the acid present in each food.
- Match each food to the acid it contains using the information below.
Acids found in common foods:
• Citric acid — Lemon, orange, and other citrus fruits
• Ascorbic acid (Vitamin C) — Amla, guava, citrus fruits
• Tartaric acid — Tamarind, grapes
• Lactic acid — Curd, buttermilk
• Acetic acid — Vinegar
• Oxalic acid — Spinach, tomato
Key observation: All acidic substances share a common sour taste. Bases, on the other hand, taste bitter and feel soapy or slippery when touched.
• Citric acid — Lemon, orange, and other citrus fruits
• Ascorbic acid (Vitamin C) — Amla, guava, citrus fruits
• Tartaric acid — Tamarind, grapes
• Lactic acid — Curd, buttermilk
• Acetic acid — Vinegar
• Oxalic acid — Spinach, tomato
Key observation: All acidic substances share a common sour taste. Bases, on the other hand, taste bitter and feel soapy or slippery when touched.
Safety Warning: Although we described acids as sour-tasting and bases as bitter-tasting, you should never taste any unknown substance in a laboratory. Always use indicators like litmus, turmeric, or red rose to test the nature of a substance safely.
2.1.2 Red Rose as an Indicator
Besides litmus, there are several natural indicators that can tell us whether a substance is acidic or basic. One of the simplest to prepare at home is an extract made from red rose petals.
Activity 2.3 — Preparing Red Rose Petal Extract
L3 Apply
What colour do you think the extract will be when you crush red rose petals in water? Will it remain the same colour when you add lemon juice or soap water to it?
- Collect a handful of fresh red rose petals.
- Place the petals in a mortar and crush them gently.
- Add a small amount of hot water to the crushed petals and mix well.
- Filter the mixture using a piece of clean cloth or filter paper.
- Collect the deep red-coloured filtrate in a clean beaker — this is your red rose petal extract.
What you get: A deep red/dark pink liquid that serves as a natural acid-base indicator. This extract will change colour differently when mixed with acidic or basic substances, as we will discover in Activity 2.4.
Activity 2.4 — Testing Substances with Red Rose Extract
L3 Apply
You have your red rose extract ready. If you add a few drops to lemon juice, what colour change do you expect? What about adding it to soap solution?
- Take several test tubes and add small amounts of different substances: lemon juice, soap solution, salt solution, baking soda solution, vinegar, lime water, sugar solution, and tap water.
- Add 2–3 drops of the red rose petal extract to each test tube.
- Observe the colour change in each test tube and note your results in Table 2.3.
What you should observe:
• In acidic substances (lemon juice, vinegar): the extract remains red or turns a deeper red/dark pink.
• In basic substances (soap solution, baking soda, lime water): the extract turns green.
• In neutral substances (salt solution, sugar solution, tap water): no significant colour change — remains the original reddish colour.
• In acidic substances (lemon juice, vinegar): the extract remains red or turns a deeper red/dark pink.
• In basic substances (soap solution, baking soda, lime water): the extract turns green.
• In neutral substances (salt solution, sugar solution, tap water): no significant colour change — remains the original reddish colour.
Table 2.3: Colour Changes with Red Rose Extract
| S. No. | Substance Tested | Colour with Red Rose Extract | Nature |
|---|---|---|---|
| 1. | Lemon juice | Red (dark pink) | Acidic |
| 2. | Soap solution | Green | Basic |
| 3. | Salt solution | No change (remains reddish) | Neutral |
| 4. | Baking soda solution | Green | Basic |
| 5. | Vinegar | Red (dark pink) | Acidic |
| 6. | Lime water | Green | Basic |
| 7. | Sugar solution | No change (remains reddish) | Neutral |
| 8. | Tap water | No change (remains reddish) | Neutral |
Summary — Red Rose as an Indicator:
• In acidic solutions → red rose extract stays red / dark pink
• In basic solutions → red rose extract turns green
• In neutral solutions → no significant change in colour
• In acidic solutions → red rose extract stays red / dark pink
• In basic solutions → red rose extract turns green
• In neutral solutions → no significant change in colour
Fascinating Fact — Colour-Changing Flowers!
The hydrangea plant produces flowers that change colour depending on the nature of the soil. In acidic soil, its flowers bloom in shades of blue. In basic (alkaline) soil, the same plant produces pink or red flowers. This is a beautiful example of nature acting as its own indicator!
The hydrangea plant produces flowers that change colour depending on the nature of the soil. In acidic soil, its flowers bloom in shades of blue. In basic (alkaline) soil, the same plant produces pink or red flowers. This is a beautiful example of nature acting as its own indicator!
2.1.3 Turmeric as an Indicator
Turmeric is another easily available natural indicator. You might have noticed that when you accidentally spill curry (which contains turmeric) on a white cloth and wash it with soap, a reddish-brown stain appears. This colour change happens because soap is basic!
Activity 2.5 — Preparing Turmeric Indicator Paper
L3 Apply
Do you think turmeric paper will behave the same way as litmus paper? Will it change colour with both acids and bases, or only with one of them?
- Take a teaspoon of turmeric powder and mix it with a little water to make a smooth paste.
- Spread this paste evenly on a sheet of filter paper (or blotting paper) and allow it to dry completely.
- Once dry, cut the paper into thin strips — these are your turmeric indicator strips.
- Now test different substances: place a drop of lemon juice, soap solution, baking soda solution, vinegar, salt solution, and lime water on separate strips.
- Observe and record the colour changes in Table 2.4.
What you should observe:
• Acidic substances (lemon juice, vinegar): Turmeric paper shows no change — remains yellow.
• Basic substances (soap solution, baking soda, lime water): Turmeric paper turns red / reddish-brown.
• Neutral substances (salt solution, tap water): No change — remains yellow.
Important: Unlike litmus, turmeric only changes colour with bases (not with acids). So turmeric can confirm a substance is basic, but it cannot distinguish between acidic and neutral substances on its own.
• Acidic substances (lemon juice, vinegar): Turmeric paper shows no change — remains yellow.
• Basic substances (soap solution, baking soda, lime water): Turmeric paper turns red / reddish-brown.
• Neutral substances (salt solution, tap water): No change — remains yellow.
Important: Unlike litmus, turmeric only changes colour with bases (not with acids). So turmeric can confirm a substance is basic, but it cannot distinguish between acidic and neutral substances on its own.
Table 2.4: Testing with Turmeric Paper
| S. No. | Substance Tested | Colour Change on Turmeric Paper | Nature |
|---|---|---|---|
| 1. | Lemon juice | No change (yellow) | Acidic |
| 2. | Soap solution | Turns Red / Brown | Basic |
| 3. | Baking soda solution | Turns Red / Brown | Basic |
| 4. | Vinegar | No change (yellow) | Acidic |
| 5. | Salt solution | No change (yellow) | Neutral |
| 6. | Lime water | Turns Red / Brown | Basic |
| 7. | Tap water | No change (yellow) | Neutral |
Summary — Turmeric as an Indicator:
• Turmeric paper turns red/reddish-brown with bases only.
• It shows no colour change with acids or neutral substances.
• Limitation: Turmeric cannot distinguish between acidic and neutral substances (both leave it yellow).
• Turmeric paper turns red/reddish-brown with bases only.
• It shows no colour change with acids or neutral substances.
• Limitation: Turmeric cannot distinguish between acidic and neutral substances (both leave it yellow).
Fascinating Fact — The Golden Spice
Turmeric belongs to the ginger family and is sometimes called the "golden spice". It has been used for centuries in Indian cooking and in Ayurvedic medicine for its anti-inflammatory and healing properties. The compound that gives turmeric its yellow colour is called curcumin.
Turmeric belongs to the ginger family and is sometimes called the "golden spice". It has been used for centuries in Indian cooking and in Ayurvedic medicine for its anti-inflammatory and healing properties. The compound that gives turmeric its yellow colour is called curcumin.
Ashwin's Clever Greeting Card
Ashwin wanted to surprise his friend with a "magic message" birthday card. He dissolved turmeric in water and used it as invisible ink to write a message on white paper. The writing was almost invisible because the pale yellow dried nearly clear. When his friend sprayed a solution of baking soda (a base) on the card, the hidden message turned reddish-brown and appeared like magic! This works because turmeric is an indicator that changes colour with bases.
Ashwin wanted to surprise his friend with a "magic message" birthday card. He dissolved turmeric in water and used it as invisible ink to write a message on white paper. The writing was almost invisible because the pale yellow dried nearly clear. When his friend sprayed a solution of baking soda (a base) on the card, the hidden message turned reddish-brown and appeared like magic! This works because turmeric is an indicator that changes colour with bases.
Olfactory Indicators
So far we have used indicators that show colour changes. But did you know that some substances change their smell (odour) when mixed with acids or bases? Such substances are called olfactory indicators.
Activity 2.6 — Testing Olfactory Indicators
L4 Analyse
If you chop onions and add them to tamarind water (acidic) and baking soda solution (basic), do you think the characteristic onion smell will change? Which solution might reduce the smell?
- Finely chop some onion pieces.
- Divide them into two portions.
- Add one portion to a beaker containing tamarind water (acidic solution).
- Add the other portion to a beaker containing baking soda solution (basic solution).
- After a few minutes, carefully smell both beakers (wave your hand over the beaker to waft the smell — do not bring your nose too close).
- Compare: in which beaker can you still smell the onion strongly? In which has the odour reduced or changed?
Observations:
• In tamarind water (acidic): The onion smell remains strong.
• In baking soda solution (basic): The onion smell is significantly reduced or changes.
This shows that onion acts as an olfactory indicator — its characteristic odour changes in a basic medium. Olfactory indicators are especially useful for visually impaired individuals who may not be able to observe colour changes but can detect changes in smell.
• In tamarind water (acidic): The onion smell remains strong.
• In baking soda solution (basic): The onion smell is significantly reduced or changes.
This shows that onion acts as an olfactory indicator — its characteristic odour changes in a basic medium. Olfactory indicators are especially useful for visually impaired individuals who may not be able to observe colour changes but can detect changes in smell.
Key Point: Olfactory indicators change their smell (not colour) in the presence of acids or bases. Examples include onion and clove oil. They are particularly helpful for people who cannot see colour changes.
Know a Scientist — Prafulla Chandra Ray (1861–1944)
P.C. Ray is regarded as the Father of Modern Indian Chemistry. Born in 1861 in present-day Bangladesh, he founded India's first pharmaceutical company, Bengal Chemicals & Pharmaceuticals, in 1901. He made significant contributions to the study of mercurous nitrite and other chemical compounds. Beyond chemistry, he was a dedicated teacher, social reformer, and philanthropist who donated most of his earnings to support education and the poor.
P.C. Ray is regarded as the Father of Modern Indian Chemistry. Born in 1861 in present-day Bangladesh, he founded India's first pharmaceutical company, Bengal Chemicals & Pharmaceuticals, in 1901. He made significant contributions to the study of mercurous nitrite and other chemical compounds. Beyond chemistry, he was a dedicated teacher, social reformer, and philanthropist who donated most of his earnings to support education and the poor.
Competency-Based Questions
Meera prepared red rose petal extract and turmeric paper at home. She tested four kitchen samples — vinegar, baking soda dissolved in water, common salt solution, and lime water — using both indicators.
Q1. L1 Remember Which natural indicator turns green in the presence of a basic substance?
Answer: C. Red rose petal extract turns green when mixed with a basic substance. Litmus turns blue (not green) with bases, turmeric turns reddish-brown, and onion changes odour (not colour).
Q2. L2 Understand Fill in the blank: Turmeric paper turns __________ when a drop of soap solution is placed on it, because soap is a __________ substance.
Answer: Turmeric paper turns red (reddish-brown) when a drop of soap solution is placed on it, because soap is a basic substance.
Q3. L4 Analyse Meera found that her turmeric paper showed no colour change with both vinegar and salt solution. Can she conclude that both substances have the same nature? Explain your reasoning. (Short Answer — 2 marks)
Answer: No, Meera cannot conclude that both have the same nature. Turmeric paper only changes colour with bases (turns reddish-brown). It shows no change with both acidic and neutral substances. Vinegar is acidic while salt solution is neutral — turmeric cannot distinguish between them. She would need to use litmus paper or red rose extract as an additional indicator to tell them apart.
Q4. L5 Evaluate True or False: "Red rose extract is a better indicator than litmus because it can show three different colours." Justify. (3 marks)
Answer: The statement has some validity but is an oversimplification. Red rose extract does show distinct responses — red in acids, green in bases, and no change in neutral solutions. This makes it useful for distinguishing all three types of substances. However, litmus paper is more widely used because it is standardised, easy to store, and available as convenient paper strips. Each indicator has its own advantages. The "best" indicator depends on the situation — for example, olfactory indicators are better for visually impaired users.
Q5. L6 Create HOT: A visually impaired student needs to determine whether a solution is acidic or basic. Which type of indicator should they use? Design a simple procedure for them. (3 marks)
Hint: Use an olfactory indicator like onion. Procedure: (1) Chop onion pieces and divide into two samples, (2) Add one sample to the unknown solution, (3) Add the other to plain water as a control, (4) After a few minutes, waft and smell both. If the onion odour in the unknown solution is significantly reduced compared to the control, the solution is basic. If the odour remains similar to the control, the solution is likely acidic or neutral.
Assertion–Reason Questions
Assertion (A): Turmeric is not a suitable indicator for identifying acidic substances.
Reason (R): Turmeric paper does not change colour when treated with acids — it only turns reddish-brown with bases.
Answer: A. Both statements are true. Turmeric cannot distinguish acids from neutral substances because it only responds to bases (turns reddish-brown). The Reason correctly explains why turmeric is not suitable for identifying acids.
Assertion (A): Onion can be used as an olfactory indicator.
Reason (R): The smell of onion changes when it is mixed with a basic substance.
Answer: A. Both are true. An olfactory indicator is defined as one whose smell changes in acidic or basic conditions. Onion's characteristic odour diminishes or changes in a basic medium, which is exactly why it qualifies as an olfactory indicator. The Reason correctly explains the Assertion.
Assertion (A): Hydrangea flowers always bloom in blue colour.
Reason (R): The colour of hydrangea flowers depends on the pH of the soil — acidic soil gives blue flowers and basic soil gives pink flowers.
Answer: D. The Assertion is false — hydrangea flowers do NOT always bloom in blue. Their colour varies with the soil's nature. The Reason is true — acidic soil produces blue flowers while basic soil produces pink/red flowers.
Frequently Asked Questions — Natural Indicators — Turmeric, China Rose, Red Cabbage
What does the topic 'Natural Indicators — Turmeric, China Rose, Red Cabbage' cover in Class 7 Science?
The topic 'Natural Indicators — Turmeric, China Rose, Red Cabbage' is part of NCERT Class 7 Science Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral. It covers the key ideas of natural indicators, turmeric, china rose, red cabbage, acid-base testing, pH indicators, extract preparation, 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 'Natural Indicators — Turmeric, China Rose, Red Cabbage' important for Class 7 NCERT Science?
'Natural Indicators — Turmeric, China Rose, Red Cabbage' 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 natural indicators 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 Natural Indicators — Turmeric, China Rose, Red Cabbage?
The key concepts in 'Natural Indicators — Turmeric, China Rose, Red Cabbage' for Class 7 Science are: natural indicators, turmeric, china rose, red cabbage, acid-base testing, pH indicators, extract preparation. 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 Natural Indicators — Turmeric, China Rose, Red Cabbage taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Natural Indicators — Turmeric, China Rose, Red Cabbage' 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 natural indicators can Class 7 students observe at home?
Class 7 students can observe natural indicators at home in many simple ways linked to 'Natural Indicators — Turmeric, China Rose, Red Cabbage'. 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 'Natural Indicators — Turmeric, China Rose, Red Cabbage' connect to other chapters of Class 7 Science?
'Natural Indicators — Turmeric, China Rose, Red Cabbage' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of natural indicators 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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