Class 7 Science

Chapter 2 Solutions — Exploring Substances: Acidic, Basic and Neutral

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Step-by-step NCERT solutions for Exploring Substances: Acidic, Basic and Neutral (Chapter 2, NCERT Class 7 Science) — every question and answer worked out in full, not just the final result. You can also read the Exploring Substances: Acidic, Basic and Neutral textbook chapter.

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All 12 questions in Exploring Substances: Acidic, Basic and Neutral are solved in the PDF. Here's what's inside, exercise by exercise:

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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
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…
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.
A liquid sample from the laboratory was tested using various indicators: Indicator: Red litmus — Change: No change Indicator: Blue litmus — Change: Turned red Indicator: Turmeric — Change: No change in colour Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
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?
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.
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.
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?
How can natural indicators be prepared? Explain by giving an example.
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.
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?
Observe and analyse the information given in the following flowchart. Complete the missing information. Imagine a garden with plants showing signs of poor health. → The soil can be ______ in nature. → The soil can be ______ in nature. → Which indicator can be used to test the nature of the soil? _________________ → The acidic soil can be treated with ________________________. → The basic soil…

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3 questions from Exploring Substances: Acidic, Basic and Neutral, solved step by step.

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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

A solution that turns red litmus paper to blue is basic in nature (bases turn red litmus to blue).

To reverse this — i.e., to turn the litmus back from blue to red — an acidic substance must be added in excess to neutralise the base.

Among the options: lime water and baking soda are basic; common salt solution is neutral. Vinegar is acidic.

Adding excess vinegar (an acid) would neutralise the base and eventually make the solution acidic, turning the litmus back to red.

Answer: (iii) Vinegar — it is acidic and will neutralise the base, reversing the blue colour back to red.

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A liquid sample from the laboratory was tested using various indicators: Indicator: Red litmus — Change: No change Indicator: Blue litmus — Change: Turned red Indicator: Turmeric — Change: No change in colour Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.

Let us analyse each indicator result:

Red litmus shows no change: bases turn red litmus to blue, so if there is no change, the liquid is NOT basic.

Blue litmus turns red: acids turn blue litmus to red, which means the liquid is acidic.

Turmeric shows no change in colour: turmeric paper changes colour (turns red) only in basic solutions; it remains unchanged in acidic and neutral solutions. Since there is no change and blue litmus turned red, this is consistent with the liquid being acidic (not basic, not neutral).

All three observations together confirm the liquid is acidic: it turns blue litmus red, does not affect red litmus, and does not change turmeric colour.

Answer: The liquid is acidic. It turns blue litmus red (characteristic of acids), does not change red litmus (rules out basic nature), and does not change turmeric paper (consistent with acidic or neutral, but blue litmus result confirms acidic).

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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?

Since Manya is blindfolded, she cannot observe colour changes in indicators like litmus paper, red rose extract, or turmeric paper.

She should use an olfactory indicator — a substance whose odour changes in acidic or basic solutions.

From the chapter, onion is described as an olfactory indicator: its odour changes when it comes into contact with acidic or basic substances.

By smelling whether the odour of the onion-soaked strip changes, Manya can determine the nature of the solution without relying on sight.

Answer: Manya should use an olfactory indicator such as onion, whose odour changes in acidic or basic solutions. Since she is blindfolded, she cannot see colour changes, but she can detect changes in smell.

All 12 questions in Exploring Substances: Acidic, Basic and Neutral are solved step by step in the Solutions PDF above.

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Chapter 2 Solutions — Exploring Substances: Acidic, Basic and Neutral

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Worked examples

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

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?

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