Let Us Enhance Our Learning
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
Explanation:
The solution turns red litmus blue, so it is basic. Vinegar is acidic. Adding excess vinegar will neutralise the base and may make the solution acidic, thereby reversing the litmus colour change.
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
Explanation:
- Solution A turns red litmus blue → Basic
- Turmeric turns red in a basic solution → Basic
- Red rose extract turns green in a basic solution → Basic
Therefore, A, B and C are all 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:
The red rose indicator behaves as follows:
- Red colour → Acidic solution
- Green colour → Basic solution
- No significant colour change → Neutral solution
Therefore, label each container according to the colour shown in the corresponding figure.
Indicator guide:
RED ROSE INDICATOR
│
┌────────┼────────┐
↓ ↓ ↓
RED GREEN NO CHANGE
│ │ │
↓ ↓ ↓
ACIDIC BASIC NEUTRAL
The actual figures are not represented in the parsed text of the PDF, so the individual labels for Fig. 2.13, Fig. 2.14 and Fig. 2.15 cannot be determined reliably from the available text alone. The textbook establishes that red rose extract gives red in acidic solutions and green in basic solutions.
4. A liquid sample from the laboratory was tested using various indicators:
| Indicator | Observation |
| Red litmus | No change |
| Blue litmus | Turned red |
| Turmeric | No change in colour |
Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
Answer:
The liquid sample is acidic in nature.
Justification:
- The red litmus paper shows no change.
- The blue litmus paper turns red, which indicates an acid.
- Turmeric remains unchanged in acidic and neutral solutions.
Therefore, the liquid is acidic.
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 blue and red litmus papers or litmus solution.
Reason:
Litmus is an acid-base indicator that changes colour depending on the nature of a substance.
- Acidic solution turns blue litmus red.
- Basic solution turns red litmus blue.
Therefore, litmus can help Manya distinguish between acidic and basic solutions without tasting or directly identifying them.
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:
One possible method is to use soap solution for writing and turmeric solution as the spray.
Turmeric remains yellow in acidic and neutral solutions but changes to red in a basic solution. Soap solution is basic.
| Material used for writing | Liquid in spray bottle | Nature of writing material | Colour of writing |
| Soap solution | Turmeric solution | Basic | Red |
| Baking soda solution | Turmeric solution | Basic | Red |
| Lime water | Turmeric solution | Basic | Red |
| Washing powder solution | Turmeric solution | Basic | Red |
Example:
White paper
────────────────────────
Written with
soap solution
↓
Turmeric sprayed
↓
Basic substance
↓
Writing appears RED
────────────────────────
The chapter itself suggests that turmeric solution could be used for spraying and soap solution for writing.
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:
When baking soda is added, the red colour of the mixture will gradually change towards green.
Justification:
Grape juice is acidic, so the red rose extract becomes red. Baking soda is a base. When baking soda is added, it neutralises the acid. As the mixture becomes basic, the red rose extract changes to green.
Grape juice + Red rose extract
↓
RED colour
↓
+ Baking soda
↓
Neutralisation
↓
GREEN colour
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:
Use blue litmus solution/paper or another suitable acid-base indicator.
Orange juice is acidic. An acidic substance changes blue litmus to red. Therefore, applying blue litmus solution to the paper can reveal the message by producing a red colour where the orange juice was used.
Another suitable method: A natural indicator such as red rose extract could also be used, as it gives a red colour in an acidic medium.
9. How can natural indicators be prepared? Explain by giving an example.
Answer:
Natural indicators can be prepared by extracting colour-producing substances from natural materials such as flowers, vegetables or spices.
Example: Red rose indicator
- Collect some fallen red rose petals.
- Wash the petals with water.
- Crush them using a mortar and pestle.
- Put the crushed petals in a glass tumbler.
- Add hot water so that the petals are completely immersed.
- Cover the tumbler and wait for about 5–10 minutes.
- Filter the mixture.
- The coloured liquid obtained is the red rose extract, which can be used as a natural acid-base indicator.
Red rose petals
↓
Crush
↓
Add hot water
↓
Wait 5–10 min
↓
Filter
↓
Red rose extract
(Indicator)
The red rose extract gives red colour in acidic solutions and green colour in basic solutions.
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:
Yes, we can identify the baking soda solution, but we cannot distinguish vinegar from sugar solution using turmeric paper alone.
Explanation:
- Baking soda solution is basic and turns yellow turmeric paper red.
- Vinegar is acidic and does not change the colour of turmeric paper.
- Sugar solution is neutral and also does not change the colour of turmeric paper.
Therefore:
| Liquid | Nature | Turmeric paper |
| Vinegar | Acidic | No change |
| Baking soda solution | Basic | Turns red |
| Sugar solution | Neutral | No change |
Thus, turmeric paper can identify baking soda solution, but it cannot distinguish vinegar from 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 amla juice is added to liquid X?
Answer:
Liquid X is basic in nature.
Reason:
Red rose extract turns green in a basic solution.
Amla juice is acidic. When excess amla juice is added to liquid X, the acid will neutralise the base. With sufficient amla juice, the solution may become acidic, and the red rose indicator will change from green towards red.
Basic liquid X
↓
Red rose extract
↓
GREEN colour
↓
+ Excess amla juice
↓
Neutralisation → acidic medium
↓
RED colour
12. Observe and analyse the information given in the following flowchart. Complete the missing information.
Question:
Which indicator can be used to test the nature of the soil?
The basic soil can be treated with ________________________.
The acidic soil can be treated with ________________________.
The soil can be ______ in nature.
The soil can be ______ in nature.
Answer:
Which indicator can be used to test the nature of the soil?
Litmus indicator
The basic soil can be treated with:
Organic matter such as manure and composted leaves
The acidic soil can be treated with:
Lime
The soil can be:
Basic in nature.
Acidic in nature.
Completed flowchart
Garden with plants
showing poor health
│
↓
Test nature of the soil
│
↓
Litmus indicator
│
┌────────┴────────┐
↓ ↓
Acidic soil Basic soil
│ │
↓ ↓
Treat with Treat with
LIME MANURE / COMPOST
The chapter explains that acidic soil can be treated with lime, while basic soil can be treated with organic matter such as manure and composted leaves.
DIVE DEEPER
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:
Bubbles appeared when vinegar was poured on eggshell or marble because vinegar is acidic and eggshell/marble contains calcium carbonate.
The acid reacts with calcium carbonate and produces carbon dioxide gas, which appears as bubbles.
Soap solution is basic and does not react with calcium carbonate in the same way, so bubbles are not produced.
In simple form:
Vinegar (Acid)
+
Calcium carbonate
(eggshell / marble)
↓
Chemical reaction
↓
Carbon dioxide gas
↓
BUBBLES
Therefore, the bubbling occurs because of the release of carbon dioxide gas during the reaction between the acid in vinegar and calcium carbonate.
Important Questions from the Chapter Activities
1. Do you observe any change in the colour of the blue litmus pieces?
Answer:
Yes. Blue litmus turns red when a drop of an acidic substance is added to it. Acidic samples such as lemon juice, amla juice, tamarind water and vinegar turn blue litmus red.
2. What is the nature of lemon juice, amla juice, tamarind water and vinegar?
Answer:
They are acidic in nature because they turn blue litmus paper red.
3. What is the nature of soap solution, baking soda solution, lime water and washing powder solution?
Answer:
They are basic in nature because they turn red litmus paper blue.
4. What is the nature of tap water, sugar solution and salt solution?
Answer:
They are neutral because they do not change the colour of either red or blue litmus paper.
5. Find out and write the names of the most common acids present in the following substances:
Lemon: Citric acid
Curd: Lactic acid
Tamarind: Tartaric acid
Vinegar: Acetic acid
The chapter specifically illustrates common acids found in edible substances and asks for these four examples.
6. What do you observe when baking soda solution is rubbed between your fingers?
Answer:
It feels soapy or slippery. Basic substances are generally slippery to touch.
7. What happens when red rose extract is added to acidic and basic substances?
Answer:
- In an acidic solution, red rose extract becomes red.
- In a basic solution, red rose extract becomes green.
Therefore, red rose extract is a natural acid-base indicator.
8. Can turmeric paper be used as an indicator for acidic substances?
Answer:
No. Turmeric paper can be used to identify basic substances, because its yellow colour changes to red in a basic solution. However, it cannot distinguish between acidic and neutral substances, because both leave the turmeric colour unchanged.
9. Which solution can be used to write the message on Ashwin’s greeting card?
Answer:
A basic solution, such as soap solution, can be used.
Ashwin applied turmeric paste to the paper. Turmeric turns red when it comes into contact with a basic substance. Therefore, writing with soap solution would make the message appear red.
10. What are olfactory indicators?
Answer:
Some substances change their odour when acidic or basic substances are added to them. Such substances are called olfactory indicators.
Example: Onion can act as an olfactory indicator.
11. Why does the colour of the lemon juice and lime-water mixture change during the experiment?
Answer:
Lemon juice is acidic and initially turns blue litmus solution red. Lime water is basic. As lime water is gradually added, it neutralises the acid. When sufficient lime water is added, the solution is no longer acidic and the colour changes from red to blue.
12. What is a neutralisation reaction?
Answer:
When an acid and a base are mixed in sufficient quantities, they neutralise each other and form salt and water, with the release of heat. This is called a neutralisation reaction.
General reaction:
Acid + Base → Salt + Water + Heat
13. Why is baking soda applied to an ant bite?
Answer:
An ant bite injects formic acid into the skin. Baking soda is a base, so it neutralises the acid and helps relieve the effect of the bite.
14. How can acidic soil be treated?
Answer:
Acidic soil can be treated by adding lime, which is basic and neutralises the acidic nature of the soil.
15. How can basic soil be treated?
Answer:
Basic soil can be treated by adding organic matter such as manure and composted leaves. Organic matter releases acids that neutralise the basic nature of the soil.
16. How can acidic factory waste be treated before releasing it into a lake?
Answer:
If factory waste is acidic, it can be neutralised by adding a suitable basic substance before releasing it into the lake. This helps protect aquatic life.
Quick Revision Table
| Substance/Indicator | Acidic | Basic | Neutral |
| Blue litmus | Turns red | No change | No change |
| Red litmus | No change | Turns blue | No change |
| Red rose extract | Red | Green | No significant change |
| Turmeric | No change | Turns red | No change |
| Lemon juice | ✓ | — | — |
| Vinegar | ✓ | — | — |
| Amla juice | ✓ | — | — |
| Tamarind water | ✓ | — | — |
| Soap solution | — | ✓ | — |
| Baking soda solution | — | ✓ | — |
| Lime water | — | ✓ | — |
| Washing powder solution | — | ✓ | — |
| Sugar solution | — | — | ✓ |
| Salt solution | — | — | ✓ |
| Tap water | — | — | ✓ |
The chapter identifies these three broad groups based on their reactions with litmus indicators.
Key formula
Acid + Base → Salt + Water + Heat
Key facts
- Acids turn blue litmus red.
- Bases turn red litmus blue.
- Red rose extract → red in acid, green in base.
- Turmeric → red in base; unchanged in acid and neutral substances.
- Neutralisation produces salt, water and heat.