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Class 7 Science Chapter 2 Exploring Substances Acidic Basic and Neutral Notes & Solutions | Curiosity

Class 7 Science Chapter 2 Exploring Substances: Acidic, Basic and Neutral notes and book solutions from the NCERT Curiosity textbook with easy explanations, important concepts, examples and complete textbook question answers for revision and exam preparation.

Class 7 Science

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

NCERT Curiosity | Reprint 2026–27

Easy Notes, Textbook Questions and Short Solutions

Chapter Overview

In this chapter, we learn how substances can be acidic, basic, or neutral. We also learn how indicators such as litmus, red rose extract, turmeric and smell can help us identify the nature of substances.

We will also learn about neutralisation and how it is useful in everyday life.

2.1 Acids and Bases

Some substances around us have an acidic nature, while others have a basic nature. Some substances are neither acidic nor basic and are called neutral.

We can use special substances called indicators to identify whether a substance is acidic or basic.

Acids

Many acidic substances have a sour taste. Examples include lemon juice, curd, tamarind and vinegar.

Bases

Basic substances include baking soda solution, soap solution and lime water.

Neutral substances

Some substances are neither acidic nor basic. Such substances are called neutral.

2.1.1 Litmus: A Natural Indicator

Litmus is a natural indicator. It is obtained from lichens.

Litmus is commonly available as red and blue litmus paper.

  • Acidic substances turn blue litmus red.
  • Basic substances turn red litmus blue.
  • Neutral substances generally do not change the colour of red or blue litmus.

Activity 2.1

Page 8

Do you observe any change in the colour of the blue litmus pieces?
Solution: Yes. Acidic samples change blue litmus to red. Basic and neutral samples do not turn blue litmus red.

What is Litmus Made From?

Litmus is obtained from lichens. Lichens are living organisms formed by an association between a fungus and an alga or cyanobacterium.

Page 10
I am curious to know — 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?
Solution: Red and blue litmus papers are made using litmus obtained from lichens. They change colour because litmus reacts differently with acidic and basic substances.

Acidic, Basic and Neutral Substances

Different substances can be grouped according to how they behave with indicators.

GroupNatureExamples
Group AAcidicLemon juice, curd, tamarind, vinegar
Group BBasicBaking soda solution, soap solution, lime water
Group CNeutralSubstances that do not show acidic or basic behaviour
Page 11
Are all the substances in Group A of Table 2.2 edible? Have you ever tasted these edible substances? Can you recall their taste?
Solution: Many substances in Group A are edible. They usually have a sour taste.
Find out and write the names of the most common acids present in the following substances — Lemon________, Curd________, Tamarind________, Vinegar________.
Solution:
Lemon — Citric acid
Curd — Lactic acid
Tamarind — Tartaric acid
Vinegar — Acetic acid
What do you observe?
Solution: The indicator shows different colour changes with acidic and basic substances.
If litmus is not available, are there some other natural substances that can serve as acid-base indicators?
Solution: Yes. Red rose petals, turmeric and some other natural substances can act as indicators.

2.1.2 Red Rose Indicator

Extracts from red rose petals can act as natural indicators.

The colour of the red rose extract can change when it is mixed with acidic or basic substances.

Activity 2.3 and Activity 2.4

Page 13
Are the samples that change the colour of the flower extract to a shade of red the same as those that changed the colour of blue litmus paper to red? (Group A, Table 2.2)
Solution: Yes. These are generally acidic substances.
Are the samples that change the colour of the flower extract to a shade of green the same as those that changed the colour of red litmus paper to blue? (Group B, Table 2.2)
Solution: Yes. These are generally basic substances.
Are the samples that do not change the colour of the flower extract the same as those that did not change the colour of red and blue litmus papers? (Group C, Table 2.2)
Solution: Yes. These are generally neutral substances.
Can you now fill in the nature of the substances in Table 2.3?
Solution: The substances can be classified as acidic, basic or neutral according to their indicator colour changes.

Other Natural Indicators

Natural indicators can be obtained from different plants and flowers. Their extracts may show different colours with acids and bases.

Hydrangea flowers can also show different colours depending on the nature of the soil in which the plant grows.

2.1.3 Turmeric: Another Natural Indicator

Turmeric is another natural indicator.

Turmeric usually remains yellow in acidic or neutral conditions but changes to a reddish-brown colour in the presence of a base.

Activity 2.5

Page 15
What did you observe?
Solution: Turmeric changes colour when it comes in contact with a basic substance.
Do all samples change the colour of the turmeric paper?
Solution: No. Turmeric paper mainly changes colour with basic substances.
Group the samples which do not change the colour of the turmeric paper.
Solution: Acidic and neutral samples generally do not change the yellow colour of turmeric paper.
Can turmeric paper be used as an indicator for acidic substances? Discuss your observations with your classmates.
Solution: Turmeric is mainly useful for identifying basic substances. It does not give a clear colour change with acidic substances.

Secret Message Using an Indicator

A special use of indicators is making a hidden message appear.

Secret message revealed using an indicator spray

Secret message revealed using an indicator solution.

Page 16
Ashwin created a greeting card to pay gratitude to his teacher on the day of Guru Purnima. He applied turmeric paste on white paper and dried it. He wrote his wishes in the teacher’s mother tongue (Odia language) on the dried paper using one of the solutions tested in Table 2.4. Which solution can be used to write the message? His teacher applauded his creative use of this concept.
Solution: A basic solution such as soap solution can be used to write the hidden message. When it comes in contact with turmeric, the writing becomes reddish-brown.

The same idea can be used in a science activity where a hidden message appears after a suitable indicator solution is applied.

Olfactory Indicators

Some substances change their smell when mixed with acidic or basic substances. Such substances are called olfactory indicators.

Examples include onion and some other naturally occurring substances.

Are there any substances whose odours change on adding acidic or basic substances?
Solution: Yes. Some substances such as onion show a change in odour when treated with acidic or basic substances.

Activity 2.6

Do you notice any change in the odour of the onion strips before and after putting tamarind water and baking soda solution on them?
Solution: Yes. The odour of onion changes when acidic tamarind water or basic baking soda solution is added.

2.2 Mixing Acids and Bases

When an acid and a base are mixed, they can react with each other. This process is called neutralisation.

The products of a neutralisation reaction are generally a salt and water.

Activity 2.7

Page 17
Do you observe any colour change (Fig. 2.10a)?
Solution: Yes, a colour change can be observed when the indicator responds to the changing nature of the solution.
What do you observe? Is there any change in the colour of the solution?
Solution: The colour of the solution changes as the acidic and basic substances react.
Can you predict why there is a change in colour?
Solution: The colour changes because the nature of the solution changes during neutralisation.

Neutralisation

Acid + Base → Salt + Water

Neutralisation can be useful in many situations in everyday life.

2.3 Neutralisation in Everyday Life

1. Ant Bite

An ant bite can inject an acidic substance into the skin. A mild basic substance may help neutralise the acid.

Page 18
What do you think might be the reason for this?
Solution: The irritation may be caused by the acidic substance injected by the ant.
What remedies do people use to treat ant bites in your region?
Solution: People may use a mild basic substance such as baking soda paste to help neutralise the acid.

2. Treating Soil

Plants grow best in soil that has a suitable pH. If soil is too acidic or too basic, its condition can be improved by suitable treatment.

Basic substances can be added to acidic soil. Acidic substances or suitable organic matter may be used to help correct basic soil, depending on the condition.

Page 19
What do you think might be the causes for this?
Solution: Soil may become acidic or basic because of natural conditions, excessive use of certain fertilisers and other environmental factors.

3. Factory Waste

Industrial waste may contain acidic or basic substances. If such waste enters a lake or river without proper treatment, it can harm aquatic life.

If the factory waste is acidic in nature, what could be done to save the fish in the lake?
Solution: The acidic waste should be treated with a suitable basic substance to neutralise it before it enters the lake.
Now, can you explain why the words ‘Welcome to the Wonderful World of Science’ appeared on Ashwin and Keerthi’s paper sheets when the liquid was sprayed on them?
Solution: The writing was made using a substance that reacted with the sprayed indicator. The reaction caused the hidden writing to become coloured and visible.
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?
Solution: Yes. Soap solution is basic and can make turmeric change from yellow to reddish-brown.

Chapter 2: Let Us Enhance Our Learning

Page 20 — Question 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
Solution: (iii) Vinegar. Vinegar is acidic and can reverse the effect of the basic solution.
Page 20 — 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
Solution: (iv) Basic, basic, and basic.
Page 20 — 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.
Solution: Identify each solution according to the colour shown by the red rose extract paper. Red indicates an acidic solution, green indicates a basic solution, and no significant change indicates a neutral solution.
Page 20 — Question 4
A liquid sample from the laboratory was tested using various indicators:
Indicator Red litmus Blue litmus Turmeric
Change No change Turned red No change in colour
Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
Solution: The liquid is acidic. It turns blue litmus red and does not change red litmus or turmeric in the given tests.
Page 21 — 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?
Solution: She should use an olfactory indicator because she can identify the change through smell without seeing the colour.
Page 21 — 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.
Solution: Different acid-base indicators and suitable acidic or basic writing materials can be combined. For example, turmeric can be used with a basic writing solution such as soap solution, producing a reddish-brown visible message.
Page 21 — 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.
Solution: The colour will change towards the colour shown by the red rose indicator in a basic medium because baking soda is basic.
Page 21 — 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?
Solution: Turmeric or another suitable natural indicator can be used if the hidden message and indicator are chosen so that they react and produce a visible colour.
Page 21 — Question 9
How can natural indicators be prepared? Explain by giving an example.
Solution: Natural indicators can be prepared by crushing or extracting colour from suitable flowers, leaves or other plant materials. For example, red rose petals can be crushed and their coloured extract can be used as an indicator.
Page 21 — 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.
Solution: Yes, partly. Baking soda solution will turn turmeric reddish-brown. Vinegar and sugar solution will generally not change turmeric paper, so turmeric paper alone cannot distinguish those two.
Page 21 — 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?
Solution: Liquid X is basic. Amla juice is acidic, so adding excess amla juice will neutralise the base and make the mixture acidic.
Page 21 — Question 12
Observe and analyse the information given in the following flowchart. Complete the missing information.
Which indicator can be used to test the nature of the soil?
_________________
Imagine a garden with plants showing signs of poor health.
The basic soil can be treated with ________________________.
The acidic soil can be treated with ________________________.
The soil can be ______ in nature. The soil can be ______ in nature.
Solution:
Indicator: Litmus or another suitable soil indicator.
Basic soil: treated with a suitable acidic material.
Acidic soil: treated with a suitable basic material such as lime.
The soil can be acidic in nature. The soil can be basic in nature.

Dive Deeper

Page 22
Aman accidentally spilt vinegar on some pieces of an eggshell or marble and noticed bubbling. He then poured a soap solution on another piece of eggshell or marble, but no bubbles appeared. Why did bubbles occur with vinegar but not with soap solution?
Solution: Vinegar is acidic and reacts with the calcium carbonate present in eggshell and marble. This reaction releases carbon dioxide gas, which produces bubbles. Soap solution is basic and does not produce this bubbling reaction in the same way.

Exploratory Projects

  1. Create rangoli using acidic or basic substances and natural indicators.
  2. Discuss in your class the acidic, basic, or neutral nature of water obtained from various sources. You may like to test the water samples available from sources such as rain, taps, rivers, etc.
  3. Collect a soil sample of your area and find out whether it is acidic, basic, or neutral in nature.

In a Nutshell

  • Acids and bases have different properties.
  • Indicators help us identify acidic and basic substances.
  • Litmus is a natural indicator obtained from lichens.
  • Acids turn blue litmus red.
  • Bases turn red litmus blue.
  • Turmeric is a natural indicator that changes colour with bases.
  • Red rose extract can act as a natural acid-base indicator.
  • Some substances can act as olfactory indicators.
  • Acid and base can react in a neutralisation reaction.
  • Neutralisation has many uses in everyday life.

Quick Revision

TermEasy Meaning
AcidA substance showing acidic properties.
BaseA substance showing basic properties.
NeutralA substance that is neither acidic nor basic.
IndicatorA substance that helps identify acidic or basic nature.
LitmusA natural acid-base indicator obtained from lichens.
Olfactory indicatorA substance whose odour changes in acidic or basic conditions.
NeutralisationThe reaction between an acid and a base.
Important: The textbook questions above are kept as questions and arranged with their textbook page numbers. The solutions are written separately in short, easy language.

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