NCERT Solutions for Class 7 Science Curiosity Chapter 2: Exploring Substances: Acidic, Basic, and Neutral by PW

Identifying whether a substance is acidic, basic or neutral can be difficult when different indicators show different colour or smell changes. Questions in the chapter also involve natural indicators, neutralisation, secret messages and real-life situations such as ant bites, factory waste and soil treatment.

The Class 7 Science Curiosity Chapter 2 Solutions provide answers and explanations for the textbook questions from Exploring Substances: Acidic, Basic, and Neutral. They explain how litmus, turmeric, flower extracts and olfactory indicators help identify substances and apply the concepts of acids, bases and neutralisation to different situations.

Class 7 Science Chapter 2 NCERT Solutions

The following solutions cover questions on litmus, natural indicators, olfactory indicators, neutralisation, secret messages, classification of substances and indicator-based activities.

Q1. What are the red and blue litmus paper strips made of? Why do they change colour when drops of some samples are put on them?

Answer:
Red and blue litmus papers are made by treating paper with a solution obtained from lichens. They act as indicators and help identify whether a substance is acidic or basic.

• Acids turn blue litmus red.

• Bases turn red litmus blue.

The colour changes occur because litmus reacts differently with acidic and basic substances.

Q2. Do you find lichens on trees in your neighbourhood?

Answer:
Yes, lichens can often be seen growing on tree trunks. They are commonly found in areas with relatively clean air and suitable moisture conditions.

Q3. If litmus is not available, are there some other natural substances that can serve as acid-base indicators?

Answer:
Yes, several natural substances can act as acid-base indicators. Examples include:

• China rose extract

• Turmeric

• Rose extract

These substances show characteristic colour changes when added to acidic or basic solutions.

Q4. Are there any substances whose odours change on adding acidic or basic substances?

Answer:
Yes, some substances change their smell when acidic or basic substances are added to them. These are called olfactory indicators.

Examples include:

• Onion

• Vanilla essence

• Clove oil

For example, the smell of onion, vanilla essence and clove oil may remain in an acidic medium but become difficult to detect in a basic medium.

Q5. What remedies do people use to treat ant bites in your region?

Answer:
A paste of baking soda and water can be applied to the affected area. A cold compress can also help reduce pain and swelling.

Ant bites are associated with formic acid. Baking soda is basic, so it can help neutralise the acid and provide relief from irritation.

Q6. If the factory waste is acidic in nature, what could be done to save the fish in the lake?

Answer:
The acidic factory waste should be treated before it is released into the lake. A suitable base, such as lime, can be added to neutralise the acidic waste.

This helps reduce the harmful effect of the waste on fish and other aquatic organisms.

Principle:
Acid + Base → Neutralisation

Q7. 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?

Answer:
Yes, this method can work.

• Turmeric acts as a natural indicator.

• Soap solution is basic in nature.

• Turmeric turns reddish-brown in a basic medium.

Therefore, when turmeric solution is sprayed on paper containing a hidden message written with soap solution, the writing becomes visible as reddish-brown marks.

Indicator-Based Questions

Q8(a). A solution turns the red litmus paper to blue. Excess addition of which of the following solutions would reverse the change?

Options:

1. Lime water

2. Baking soda

3. Vinegar

4. Common salt solution

Solution:
A solution that turns red litmus blue is basic. To reverse this change, an acidic substance is required. Among the given options, vinegar is acidic.

Answer: 3. Vinegar

Q8(b). Three unknown solutions A, B and C are given. Identify their nature.

• Red litmus added to A turns blue.

• Turmeric added to B turns red.

• Red rose extract added to C turns green.

What is the correct sequence for A, B and C?

Options:

1. Acidic, acidic, acidic

2. Neutral, basic, basic

3. Basic, basic, acidic

4. Basic, basic, basic

Solution:

• A turns red litmus blue → A is basic.

• B turns turmeric reddish-brown/red → B is basic.

• C turns red rose extract green → C is basic.

Answer: 4. Basic, basic and basic

Q9. 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 container.

Answer:

Thus, the colour shown by the red rose extract paper helps identify whether each solution is acidic, basic or neutral.

Q10. A liquid sample from the laboratory was tested using various indicators. The observations are:

Identify the acidic or basic nature of the liquid and justify your answer.

Answer:
The liquid is acidic.

Reason:

• Red litmus shows no change.

• Blue litmus turns red, indicating an acidic substance.

• Turmeric does not show a characteristic change in an acidic solution.

Therefore, the liquid is acidic in nature.

Q11. 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, such as onion, vanilla essence or clove oil.

Since she is blindfolded, she cannot observe colour changes. An olfactory indicator can help because its smell changes in different types of solutions.

Thus, smell-based indicators allow Manya to test the solutions without seeing them.

Q12. Could you suggest various materials which can be used for writing the message on the white sheet of paper and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.

Answer:

The hidden writing becomes visible because the writing material and spray solution interact through their acid-base properties and the indicator produces a colour change.

Q13. 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:
Grape juice is acidic, while baking soda is basic. When baking soda is added, neutralisation can occur.

• With sufficient baking soda, the acidity of the mixture may be reduced.

• If excess baking soda is added, the mixture can become basic.

• In a basic medium, red rose extract turns green.

Therefore, with excess baking soda, the mixture may turn green.

Q14. 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:
Orange juice is acidic in nature. China rose (hibiscus) indicator can be used to reveal the hidden message.

When the indicator is applied, the areas containing orange juice can show a dark pink/red colour, making the message visible.

Answer: China rose indicator (hibiscus indicator).

Q15. How can natural indicators be prepared? Explain by giving an example.

Answer:
Natural indicators can be prepared from substances such as turmeric and flower petals by extracting their colour-producing substances.

Class 7 Science Curiosity Chapter 2 PDF

Keep the Class 7 Science Curiosity Chapter 2 Solutions PDF available while practising the textbook questions and activities. The PDF can help you revise concepts related to acids, bases, indicators, neutralisation and their everyday applications.

What Can Children Learn from the NCERT Class 7 Science Chapter 2 Solutions by PW?

The solutions for Exploring Substances: Acidic, Basic, and Neutral help children apply the concepts of acids, bases and indicators to textbook questions and everyday situations.

• Identify acids and bases: Understand how litmus and other indicators help determine the nature of substances.

• Understand natural indicators: Learn about turmeric, rose and China rose as natural indicators.

• Learn about olfactory indicators: Understand how onion, vanilla essence and clove oil can indicate acidic or basic conditions through changes in smell.

• Understand neutralisation: Apply the reaction between acids and bases to situations such as ant bites and acidic factory waste.

• Interpret colour changes: Connect indicator colours with the acidic, basic or neutral nature of solutions.

• Apply concepts to real-life situations: Use scientific reasoning to understand soil treatment, secret messages and other activities.

• Compare substances: Distinguish between acidic, basic and neutral substances using suitable tests.

• Solve activity-based questions: Work through questions involving observations, combinations and experimental situations.

Working through the questions in Exploring Substances: Acidic, Basic, and Neutral allows children to connect indicator-based observations with the concepts of acids, bases and neutralisation. The Class 7 Science Chapter 2 Solutions can be used to check answers, understand the reasoning behind each response and revise important concepts from the chapter.

Related Resources

• CBSE Class 7 Science Syllabus

• CBSE Class 7 Science NCERT Solutions

• CBSE Class 7 Science Sample Papers

• CBSE Class 7 Science Notes