NCERT Solutions for Class 7 science Chapter 2: Exploring Substances Acidic Basic and Neutral Class 7

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 made from paper soaked in an extract of lichens. They change colour when they come in contact with acidic or basic substances. Acidic substances turn blue litmus to red, and basic substances turn red litmus to blue.

Do you find lichens on trees in your neighbourhood? (Page 10)
Answer:
Yes, lichens can be found growing on trees, especially in areas with clean air and good rainfall.

If litmus is not available, are there some other natural substances that can serve as acid- base indicators? (Page 11)
Answer:
Yes, there are other natural substances that can be used as acid-base indicators, such as red cabbage juice, turmeric, and rose extract. These substances also change colour in the presence of acids or bases.

Are there any substances whose odours change on adding acidic or basic substances? (Page 16)
Answer:
Yes, there are substances like onion and vanilla extract whose odours change when exposed to acidic or basic substances are known as olfactory indicators.

What remedies do people use to treat ant bites in your region? (Page 18)
Answer:
People often use natural remedies like applying a paste of baking soda and water, using a cold compress to reduce pain and swelling caused by ant bites.

If the factory waste is acidic in nature, what could be done to save the fish in the lake? (Page 19)
Answer:
To save the fish in the lake, neutralising the acidic waste by adding a base like lime before releasing it into the water could help restore the neutral nature of water, making it safer for the fish.

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, turmeric solution can be used as an indicator for spraying, as it changes colour from yellow to red in basic solutions, while a soap solution could be used for writing on the paper, since soap is basic in nature.

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) The solution turns from red to blue, it means it is basic. To reverse this, an acid is needed. Lime water and baking soda are basic, and the common salt solution is neutral, so they will not reverse the change. Vinegar is acidic, so it will change the colour back to red.

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 turns red litmus to blue – Basic in nature Solution B turns turmeric solution to red – Basic in nature Solution C turns rose extract to green – Basic nature Thus, the correct sequence for the nature of solutions A, B and C is basic, basic and basic.

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 extract turns red in acidic solutions and green in basic solutions. In Fig. 2.13, the red rose extract paper strip turns green, indicating that the solution in this container is basic in nature. In Fig. 2.14, there is no change in the colour of the strip, which shows that the solution is neutral. However, in Fig. 2.15, the strip turns red, indicating that the solution is acidic in nature.

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

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. Olfactory indicators change their smell in acidic or basic solutions. For example, substances like onion produce a strong smell in a basic solution and a mild smell in an acidic solution.

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 of the writing will depend on the nature of the writing material (acidic or basic) and the solution in the spray bottle (acidic or basic). Below is the table of possible combinations.

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 (a base) is added to the mixture of grape juice and red rose extract, the basicity increases. In basic conditions, red rose extract turns green. So, after adding baking soda, the mixture will turn green.

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 reveal the secret message by using acid-base indicators, as orange juice is acidic in nature. She can use rose extract or beetroot juice (which will turn red) or Indian blackberry extract (which will turn pink) to make the message visible.

How can natural indicators be prepared? Explain by giving an example.
Answer:
Natural indicators can be made using plant materials with pigments that change colour with acid or base. For example, to make red rose extract:

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:
Using turmeric paper, we can identify a basic solution but not an acidic or neutral solution. When baking soda solution (basic) is tested, the turmeric paper will turn red. However, both vinegar (acidic) and sugar solution (neutral) will leave the paper yellow, as turmeric paper does not change colour in acidic or neutral solutions. Therefore, turmeric paper can only identify a basic solution.

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 the extract of red rose turns the liquid X to green, it indicates that liquid X is basic. Red rose extract acts as an indicator, turning red in acidic solutions and green in basic solutions. When excess amla juice (which is acidic) is added to liquid X, it will neutralise the base. The green colour of the liquid will turn to red as the solution becomes more acidic due to the addition of the acidic amla juice.

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

Answer:
The complete flowchart is as follows: