Chapter 2: Exploring Substances: Acidic, Basic and Neutral
🎯 Exam-Focused Questions • Latest Pattern • With Solutions
Class 7 ScienceEnglishNCERT Curiosity2–5 Marks
📝 What about 1-Mark Questions?
The 1-mark MCQs are covered separately in the Chapter-wise MCQ Test.
So, this page focuses on 2, 3, 4 and 5-mark exam questions, while the 1-mark preparation is done through the MCQ test.
🎯 Go to 1-Mark MCQ Test →
🟡 Section A — 2 Marks Questions
2 Marks1. What is an acid-base indicator? Give one example.
Answer: An acid-base indicator is a substance that shows whether a solution is acidic, basic or neutral by a change in colour or smell. Example: litmus.
2 Marks2. How can you identify an acidic and a basic substance using litmus?
Answer: An acidic substance turns blue litmus red, while a basic substance turns red litmus blue.
2 Marks3. Why are sugar solution and salt solution considered neutral?
Answer: Sugar solution and common salt solution do not show acidic or basic behaviour with the indicators used in the chapter. Hence, they are classified as neutral.
2 Marks4. What are olfactory indicators? Give one example.
Answer: Substances whose smell changes in acidic or basic conditions are called olfactory indicators. Onion is one example.
2 Marks5. What happens when turmeric paper is treated with a basic substance?
Answer: Turmeric paper changes its colour to red when treated with a basic substance.
2 Marks6. What is a neutralisation reaction? Write its general form.
Answer: The reaction between an acid and a base to form salt and water is called a neutralisation reaction.
Acid + Base → Salt + Water
2 Marks7. How does the nature of soil affect the colour of hydrangea flowers?
Answer: The colour of hydrangea flowers can vary with the nature of the soil. The same plant may produce flowers of different colours in different soils.
2 Marks8. Why is moist baking soda applied after an ant bite?
Answer: An ant bite injects an acidic substance into the skin. Baking soda is basic and helps neutralise the acid, giving relief from the effect of the bite.
2 Marks9. What are red and blue litmus paper strips made from? Why do they change colour?
Answer: Red and blue litmus paper strips are made from paper soaked in an extract of lichens. They change colour when they come in contact with acidic or basic substances. Acids turn blue litmus red, while bases turn red litmus blue.
2 Marks10. Aman is a visually impaired student. How can he identify whether a solution is acidic or basic without seeing the colour change?
Answer: Aman can use an olfactory indicator. The smell of such substances changes in acidic and basic conditions. Onion is an example of an olfactory indicator. Therefore, he can identify the nature of the solution by observing the change in smell.
🟢 Section B — 3 Marks Questions
3 Marks1. Explain how litmus can be used to classify substances as acidic, basic or neutral.
Answer:- An acid turns blue litmus red.
- A base turns red litmus blue.
- A neutral substance does not produce these colour changes.
3 Marks2. How is red rose extract prepared and used as an indicator?
Answer: Red rose petals are crushed with water to prepare an extract. The extract is added to different substances. It shows different colour changes in acidic and basic conditions and therefore acts as a natural indicator.
3 Marks3. Why can turmeric identify a basic substance but cannot by itself distinguish an acidic substance from a neutral one?
Answer: Turmeric changes to red in a basic substance. It does not show a distinct colour change for acidic and neutral substances. Therefore, turmeric alone cannot distinguish an acidic substance from a neutral substance.
3 Marks4. A student has lemon juice, soap solution and sugar solution. How can the student identify them using litmus?
Answer:- Lemon juice is acidic and turns blue litmus red.
- Soap solution is basic and turns red litmus blue.
- Sugar solution is neutral and does not show the acidic/basic litmus colour change.
3 Marks5. What happens when lime water is slowly added to lemon juice containing a suitable indicator? Explain the change.
Answer: Lemon juice is acidic and lime water is basic. As lime water is added slowly, the acid is gradually neutralised by the base. Near the neutral point, the indicator shows a corresponding colour change.
3 Marks6. How does the nature of soil affect hydrangea flower colour?
Answer: Hydrangea plants can produce flowers of different colours depending on the nature of the soil. Thus, soil conditions can affect the colour of flowers produced by the same type of plant.
3 Marks7. How can acidic and basic soil be treated?
Answer:- Acidic soil can be treated by adding suitable basic substances such as lime.
- Basic soil can be treated using suitable organic materials.
- The treatment helps bring the soil towards a suitable condition for plant growth.
3 Marks8. A factory releases acidic waste into a lake. What can be done to protect aquatic life?
Answer: The acidic waste should be treated before it is released into the lake. A suitable basic substance can be used to neutralise the acidic waste. This reduces its harmful effect on aquatic organisms.
3 Marks9. How can turmeric solution and soap solution be used to write and reveal a secret message on paper?
Answer: Turmeric solution acts as an indicator because it changes from yellow to red in a basic solution. A soap solution is basic, so it can be used for writing on paper. When a suitable indicator is applied, the hidden writing becomes visible because of the colour change in the basic areas.
3 Marks10. Grape juice is mixed with red rose extract and then baking soda is added. Predict the colour change and explain why.
Answer: Red rose extract shows a red colour in acidic conditions. Grape juice with the extract therefore gives a reddish mixture. When baking soda, which is a base, is added, the basicity of the mixture increases. The red rose extract changes towards green in basic conditions, so the mixture turns green.
🟠 Section C — 4 Marks Case-Based Questions
Case 1: A student has three unknown solutions A, B and C. Solution A turns red litmus blue. Solution B turns turmeric paper red. Solution C changes the colour of red rose extract in a way characteristic of a basic substance.
Answer the following:
(a) What is the nature of solution A? (b) What is the nature of solution B? (c) Which natural indicator is used in B? (d) What does the observation with solution C indicate?
Answer:
(a) A is basic because it turns red litmus blue.
(b) B is basic because turmeric turns red in a basic substance.
(c) Turmeric is used.
(d) C is also basic because the red rose extract shows the characteristic colour change of a basic substance.
Case 2: Riya was bitten by an ant. Her skin became uncomfortable at the place of the bite. Her teacher suggested applying moist baking soda to the affected area.
Answer the following:
(a) What type of substance is associated with an ant bite? (b) What is the nature of baking soda? (c) Name the reaction involved when an acid reacts with a base. (d) How does baking soda help?
Answer:
(a) An acidic substance is associated with an ant bite.
(b) Baking soda is basic.
(c) It is a neutralisation reaction.
(d) Baking soda helps by neutralising the acidic effect of the bite.
Case 3: A farmer finds that the soil of a field is too acidic. In another place, a factory is producing acidic waste that may enter a nearby water body.
Answer the following:
(a) What can be added to acidic soil to reduce its acidity? (b) Why is neutralisation useful in soil treatment? (c) What should the factory do before releasing acidic waste? (d) Why is this treatment important?
Answer:
(a) A suitable basic substance such as lime can be added.
(b) It helps reduce the effect of excess acidity.
(c) The acidic waste should be treated with a suitable basic substance before release.
(d) It reduces the harmful effect of acidic waste on the environment and aquatic life.
4 Marks4. Keerthi writes a secret message using orange juice. How can the hidden message be revealed? Name a suitable indicator and explain the observation.
Answer: The hidden message can be revealed using an acid-base indicator such as red rose extract or Indian blackberry extract. Orange juice is acidic. When the indicator is applied to the paper, the areas containing the writing react differently and the hidden message becomes visible because of the indicator's colour change.
🔴 Section D — 5 Marks Questions
5 Marks1. Explain the neutralisation activity using lemon juice and lime water. What conclusion can be drawn from the activity?
Answer: Lemon juice is acidic, while lime water is basic. Lime water is added slowly to lemon juice while observing the indicator. As more lime water is added, the acidic nature is gradually reduced because the acid reacts with the base. The indicator shows a colour change when the solution approaches a neutral condition. This demonstrates that an acid and a base react with each other in a neutralisation reaction to form salt and water.
General form: Acid + Base → Salt + Water.
5 Marks2. Describe how different indicators help us identify acidic and basic substances. Include litmus, turmeric, red rose extract and an olfactory indicator in your answer.
Answer:- Litmus: Acid turns blue litmus red and base turns red litmus blue.
- Turmeric: Turmeric changes to red in a basic substance.
- Red rose extract: It is a natural indicator and shows different colour changes with acidic and basic substances.
- Olfactory indicator: Substances such as onion can show a change in smell in different conditions.
- Thus, indicators help us identify the nature of substances without directly tasting or otherwise testing them.
5 Marks3. Explain three important applications of neutralisation in daily life or the environment.
Answer:- Ant bite: Baking soda, a basic substance, helps neutralise the acidic effect associated with an ant bite.
- Acidic soil: A suitable basic substance such as lime can be added to acidic soil to reduce its acidity.
- Industrial waste: Acidic or basic industrial waste can be treated appropriately before disposal so that its harmful effect on the environment is reduced.
These examples show that neutralisation is useful in daily life as well as in protecting the environment.
5 Marks4. Red rose extract turns liquid X green. What is the nature of liquid X? What happens when excess amla juice is added to liquid X? Explain.
Answer: Liquid X is basic because red rose extract turns green in a basic solution. Amla juice is acidic. When excess amla juice is added to liquid X, it neutralises the basic nature of the solution. As the solution becomes more acidic, the colour shown by the red rose extract changes from green to red.
5 Marks5. Fill in the blanks with suitable words: (5 × 1 = 5)
- An __________ is a substance that helps identify whether a substance is acidic, basic or neutral by showing a change in colour or smell.
- Red rose extract turns __________ in a basic substance.
- __________ is an example of an olfactory indicator because its smell can change in acidic or basic conditions.
- The acidic substance associated with an ant bite can be neutralised by applying moist __________.
- __________ can be added to acidic soil to reduce its acidity.
Answer: (a) acid-base indicator (b) green (c) onion (d) baking soda (e) lime
5 Marks6. Match Column A with the correct option from Column B: (5 × 1 = 5)
| Column A | Column B |
|---|
| 1. Blue litmus | A. Turns red in a basic substance |
| 2. Turmeric | B. Used to treat acidic industrial waste |
| 3. Red rose extract | C. Turns red in an acidic substance |
| 4. Acidic industrial waste | D. Shows a colour change in acidic and basic conditions |
| 5. Neutralisation | E. Reaction between an acid and a base forming salt and water |
Answer: 1 → C 2 → A 3 → D 4 → B 5 → E