Class 7 Science Chapter 2: Stepwise Answers, Diagrams & Revision Tips

Question 1.
What are these red and blue litmus paper strips made of? Why do they change colour when drops of some samples are put on them? (Page 10)

Answer:

Red and blue litmus paper strips are prepared by soaking paper in an extract obtained from lichens. These strips change colour when they come into contact with acidic or basic substances. Acids turn blue litmus paper red, while bases change red litmus paper to blue.

Question 2.

Do you find lichens on trees in your neighbourhood? (Page 10)

Answer: Yes, lichens commonly grow on trees, particularly in regions with clean air and sufficient rainfall.

Question 3.

If litmus is not available, are there some other natural substances that can serve as acid- base indicators? (Page 11)

Answer: Yes, several natural substances can act as acid–base indicators, including red cabbage juice, turmeric, and rose extract. These materials show colour changes when they come into contact with acidic or basic substances.

Question 4.

Are there any substances whose odours change on adding acidic or basic substances? (Page 16)

Answer: Yes, substances such as onion and vanilla extract, whose smells change when they are exposed to acidic or basic substances, are called olfactory indicators.

Question 5.

What remedies do people use to treat ant bites in your region? (Page 18)

Answer: People commonly use natural remedies, such as applying a paste made of baking soda and water or placing a cold compress, to relieve pain and reduce swelling caused by ant bites.

Question 6.

If the factory waste is acidic in nature, what could be done to save the fish in the lake? (Page 19)

Answer: To protect the fish in the lake, the acidic waste can be neutralised by adding a base such as lime before it is released into the water. This helps bring the water back to a neutral state, making it safer for aquatic life.

Question 7.

Do you think that one possibility could be using a turmeric solution for the spraying liquid and a soap solution for writing on the paper? (Page 19)

Answer: Yes, a turmeric solution can be used as a spraying indicator because it changes from yellow to red in the presence of a basic solution. A soap solution, being basic in nature, can be used for writing on the paper.

Let Us Enhance Our Learning

Question 1.
A solution turns the red litmus paper to blue. Excess addition of which of the following solutions would reverse the change?

(i) Lime water
(ii) Baking soda
(iii) Vinegar
(iv) Common salt solution

Answer:

(iii) If the solution changes from red to blue, it indicates that it is basic. To reverse this change, an acidic substance is required. Lime water and baking soda are bases, and a common salt solution is neutral, so they cannot reverse the colour change. Vinegar, being acidic, will turn the solution back to red.

Question 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) Solution A changes red litmus paper to blue, showing that it is basic in nature. Solution B turns the turmeric solution red, which also indicates a basic nature. Solution C changes rose extract to green, confirming that it is basic as well. Therefore, the correct nature of solutions A, B, and C is basic, basic, and basic.

Question 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:

Red rose extract appears red in acidic solutions and turns green in basic solutions. In Fig. 2.13, the rose extract paper strip changes to green, showing that the solution is basic. In Fig. 2.14, the strip shows no colour change, which indicates a neutral solution. In Fig. 2.15, the strip turns red, confirming that the solution is acidic in nature.

Question 4.
A liquid sample from the laboratory was tested using various indicators:

Answer:

The given liquid sample is acidic in nature. It does not cause any change in red litmus paper but turns blue litmus paper red, which indicates acidity. Additionally, there is no colour change in the turmeric indicator, further confirming that the sample is acidic.

Question 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 an olfactory indicator to test the solutions, as these indicators change their smell in acidic or basic conditions. For instance, substances like onion give a strong smell in a basic solution and a faint smell in an acidic solution.

Question 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: 

The colour that appears in the writing depends on whether the writing substance is acidic or basic and on the nature of the solution used for spraying. Different combinations produce different colour changes, as shown in the table below.

Question 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, which is a base, is added to the mixture of grape juice and red rose extract, it increases the basic nature of the mixture. In a basic medium, red rose extract changes its colour to green. Therefore, after adding baking soda, the mixture will turn green.

Question 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: Keerthi’s grandmother can uncover the hidden message by using acid–base indicators, since orange juice is acidic in nature. She may use rose extract or beetroot juice, which will turn red, or Indian blackberry extract, which will turn pink, to make the message visible.

Question 9.

How can natural indicators be prepared? Explain by giving an example.

Answer: Natural indicators can be prepared from plant materials that contain pigments which change colour in acidic or basic conditions. For instance, red rose extract can be made as follows:

• Wash and crush fresh rose petals.
  
  

• Put them in a glass tumbler and pour hot water over them.
  
  

• Allow the mixture to stand for 5–10 minutes, then filter it.
  
  

The obtained liquid acts as a natural indicator, turning red in acidic solutions and green in basic solutions.

Question 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: Turmeric paper can be used to detect a basic solution, but it cannot distinguish between acidic and neutral solutions. When a baking soda solution, which is basic, is tested, the turmeric paper changes from yellow to red. However, vinegar (acidic) and sugar solution (neutral) do not cause any colour change and the paper remains yellow. Hence, turmeric paper is useful only for identifying basic solutions.

Question 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 of amla juice is added to liquid X?

Answer:

If red rose extract changes the colour of liquid X to green, it shows that liquid X is basic in nature. Red rose extract works as an indicator by turning red in acidic solutions and green in basic ones. When an excess amount of amla juice, which is acidic, is added to liquid X, it neutralises the base. As the solution becomes acidic, the green colour changes to red.

Question 12.

Observe and analyse the information given in the following flowchart. Complete the missing information

Answer:

The complete flowchart is as follows:

Activity 2.1: Let Us Explore (Page 8-9)

Table 2.1: Testing the nature of samples with blue and red litmus papers

Table 2.2: Grouping of samples tested in Table 2.1

Observation and conclusion: Substances in Group A, like lemon juice and vinegar, are acidic as they turn blue litmus paper red. Group B substances, like soap and baking soda, are basic as they turn red litmus paper blue. Group C substances, like tap water and sugar solution, are neutral because they do not change the litmus paper colour.

Activity 2.2: Let Us Relate and explore (Page 11)

All the substances in Group A – lemon juice, amla juice, tamarind water, vinegar and orange juice are edible. They all have a sour taste because they contain acids and are acidic in nature. The common acids found in these substances are:

1. Lemon: Citric acid

2. Curd: Lactic acid

3. Tamarind: Tartaric acid

4. Vinegar: Acetic acid

Activity 2.3: Let Us Prepare (Page 12)

Answer:
Students should do it by themselves.

Activity 2.4: Let Us Find out (Pages 12-13)

Table 2.3: Testing the nature of samples with red rose extract

Observation and conclusion: The rose extract gives a red colour in an acidic solution and a green colour in a basic solution.

Activity 2.5: Let Us Prepare (Pages 14-15)

Table 2.4: Testing the nature of samples with turmeric paper

Observation and conclusion: Turmeric paper changes colour from yellow to red in a basic solution but shows no change in acidic or neutral solutions.

Activity 2.6: Let Us Investigate (Page 16)

Answer:
Students should do it by themselves.

Activity 2.7: Let Us Experiment (Page 17)

Answer:
Students should do it by themselves.

Understanding Acids, Bases, and Neutrals: Class 7 Science Chapter 2

Dive into Class 7 Science Chapter 2: Exploring Substances: Acidic, Basic, and Neutral to discover how indicators like turmeric and litmus paper can make chemistry fun and relatable in real life. Observing colour changes brings science concepts alive!

Learning about acid, base, and neutral substances prepares you for practical situations—like neutralising ant bites or treating acidic soils. Practice the chapter’s activities to build strong conceptual understanding and confidence for exams.

Revise the NCERT questions regularly and use examples from daily life to remember key reactions. Consistent review and hands-on experiments are great strategies to excel in Class 7 Science Chapter 2 for 2025-26.