Class 7 Science Chapter 2 NCERT Book - Exploring Substances: Acidic, Basic and Neutral

 Page 1


On 28 February, the school hosted a science fair to celebrate 
National Science Day. At the entry gate, siblings Ashwin and 
Keerthi were greeted with a white sheet of paper. They were 
curious to know why a white sheet of paper was given to them!
A few steps ahead, there was a volunteer spraying a liquid 
on these sheets of paper. The siblings also got their white sheets 
sprayed. To their surprise, the words ‘Welcome to the Wonderful 
World of Science’ appeared on the papers the moment the liquid 
was sprayed (Fig. 2.1). They were excited and eager to know how 
this happened and the reason behind it.
Their curiosity was partly satis? ed at the ‘Colourful World 
of Substances’ stall. They saw many activities showing colour 
changes on mixing di? erent substances. They decided to explore 
these changes further . Let us join them on this learning adventure.
Fig. 2.1: Scene of a science fair
Exploring 
Substances: Acidic, 
Basic, and Neutral
2
Chapter 2.indd   7 4/3/2025   4:50:42 PM
Page 2


On 28 February, the school hosted a science fair to celebrate 
National Science Day. At the entry gate, siblings Ashwin and 
Keerthi were greeted with a white sheet of paper. They were 
curious to know why a white sheet of paper was given to them!
A few steps ahead, there was a volunteer spraying a liquid 
on these sheets of paper. The siblings also got their white sheets 
sprayed. To their surprise, the words ‘Welcome to the Wonderful 
World of Science’ appeared on the papers the moment the liquid 
was sprayed (Fig. 2.1). They were excited and eager to know how 
this happened and the reason behind it.
Their curiosity was partly satis? ed at the ‘Colourful World 
of Substances’ stall. They saw many activities showing colour 
changes on mixing di? erent substances. They decided to explore 
these changes further . Let us join them on this learning adventure.
Fig. 2.1: Scene of a science fair
Exploring 
Substances: Acidic, 
Basic, and Neutral
2
Chapter 2.indd   7 4/3/2025   4:50:42 PM
Curiosity | Textbook of Science | Grade 7
8
2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory
2.1.1 Litmus as an indicator
Activity 2.1: Let us explore
? Collect samples of lemon juice, soap solution, amla juice, 
tamarind water, vinegar, baking soda solution, lime water, 
tap water, washing powder solution, sugar solution, and salt 
solution.
Fig. 2.2(a): Colour change in 
blue litmus paper
Fig. 2.2(b): Colour change in 
red litmus paper
? Take a strip of blue litmus paper 
and cut it into small pieces.
? Spread these pieces on a clean 
and dry white tile.
? Using a dropper, put one drop of 
each of the samples, one-by-one, 
on these litmus paper pieces, as 
shown in Fig. 2.2a.
? Do you observe any change 
in the colour of the blue 
litmus pieces?
? Record your observations 
in Table 2.1.
? Repeat the same activity 
with pieces of red litmus 
paper as shown in 
Fig.  2.2b and record your 
observations in Table 2.1.
How to prepare lime water?
Do not confuse lime water with the word lime, which is a fruit 
similar to lemon.
Lime water (solution of calcium hydroxide in water) can be 
easily prepared by mixing lime (chuna, i.e. calcium oxide) in water 
and leaving it undisturbed for some time, say an hour. Filter the 
liquid into another container and use it as lime water.
Chapter 2.indd   8 4/3/2025   4:50:43 PM
Page 3


On 28 February, the school hosted a science fair to celebrate 
National Science Day. At the entry gate, siblings Ashwin and 
Keerthi were greeted with a white sheet of paper. They were 
curious to know why a white sheet of paper was given to them!
A few steps ahead, there was a volunteer spraying a liquid 
on these sheets of paper. The siblings also got their white sheets 
sprayed. To their surprise, the words ‘Welcome to the Wonderful 
World of Science’ appeared on the papers the moment the liquid 
was sprayed (Fig. 2.1). They were excited and eager to know how 
this happened and the reason behind it.
Their curiosity was partly satis? ed at the ‘Colourful World 
of Substances’ stall. They saw many activities showing colour 
changes on mixing di? erent substances. They decided to explore 
these changes further . Let us join them on this learning adventure.
Fig. 2.1: Scene of a science fair
Exploring 
Substances: Acidic, 
Basic, and Neutral
2
Chapter 2.indd   7 4/3/2025   4:50:42 PM
Curiosity | Textbook of Science | Grade 7
8
2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory
2.1.1 Litmus as an indicator
Activity 2.1: Let us explore
? Collect samples of lemon juice, soap solution, amla juice, 
tamarind water, vinegar, baking soda solution, lime water, 
tap water, washing powder solution, sugar solution, and salt 
solution.
Fig. 2.2(a): Colour change in 
blue litmus paper
Fig. 2.2(b): Colour change in 
red litmus paper
? Take a strip of blue litmus paper 
and cut it into small pieces.
? Spread these pieces on a clean 
and dry white tile.
? Using a dropper, put one drop of 
each of the samples, one-by-one, 
on these litmus paper pieces, as 
shown in Fig. 2.2a.
? Do you observe any change 
in the colour of the blue 
litmus pieces?
? Record your observations 
in Table 2.1.
? Repeat the same activity 
with pieces of red litmus 
paper as shown in 
Fig.  2.2b and record your 
observations in Table 2.1.
How to prepare lime water?
Do not confuse lime water with the word lime, which is a fruit 
similar to lemon.
Lime water (solution of calcium hydroxide in water) can be 
easily prepared by mixing lime (chuna, i.e. calcium oxide) in water 
and leaving it undisturbed for some time, say an hour. Filter the 
liquid into another container and use it as lime water.
Chapter 2.indd   8 4/3/2025   4:50:43 PM
 Exploring Substances: Acidic, Basic, and Neutral
9
Name of the sample S.No.
Colour of blue
litmus paper after 
putting a drop of 
sample
Colour of red
litmus paper after 
putting a drop of 
sample
Table 2.1: Testing the nature of samples with blue and red litmus papers Table 2.1: Testing the nature of samples with blue and red litmus papers
Lemon juice 1.
2. Soap solution
3. Amla juice
4. Tamarind water
6. Baking soda solution
8. Tap water
10. Sugar solution 
5. Vinegar
7. Lime water
9. Washing powder solution
11. Salt solution
12. Any other
Now, let us analyse Table 2.1 and sort the samples into 
three groups as follows?—
? Group A with samples that turn the blue litmus paper to red. 
? Group B with samples that turn the red litmus paper to blue. 
? Group C with samples that do not a? ect either of the two 
litmus papers.
Record the data in Table 2.2.
Group A Group B Group C
Table 2.2: Grouping of samples tested in Table 2.1 Table 2.2: Grouping of samples tested in Table 2.1
Chapter 2.indd   9 4/3/2025   4:50:43 PM
Page 4


On 28 February, the school hosted a science fair to celebrate 
National Science Day. At the entry gate, siblings Ashwin and 
Keerthi were greeted with a white sheet of paper. They were 
curious to know why a white sheet of paper was given to them!
A few steps ahead, there was a volunteer spraying a liquid 
on these sheets of paper. The siblings also got their white sheets 
sprayed. To their surprise, the words ‘Welcome to the Wonderful 
World of Science’ appeared on the papers the moment the liquid 
was sprayed (Fig. 2.1). They were excited and eager to know how 
this happened and the reason behind it.
Their curiosity was partly satis? ed at the ‘Colourful World 
of Substances’ stall. They saw many activities showing colour 
changes on mixing di? erent substances. They decided to explore 
these changes further . Let us join them on this learning adventure.
Fig. 2.1: Scene of a science fair
Exploring 
Substances: Acidic, 
Basic, and Neutral
2
Chapter 2.indd   7 4/3/2025   4:50:42 PM
Curiosity | Textbook of Science | Grade 7
8
2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory
2.1.1 Litmus as an indicator
Activity 2.1: Let us explore
? Collect samples of lemon juice, soap solution, amla juice, 
tamarind water, vinegar, baking soda solution, lime water, 
tap water, washing powder solution, sugar solution, and salt 
solution.
Fig. 2.2(a): Colour change in 
blue litmus paper
Fig. 2.2(b): Colour change in 
red litmus paper
? Take a strip of blue litmus paper 
and cut it into small pieces.
? Spread these pieces on a clean 
and dry white tile.
? Using a dropper, put one drop of 
each of the samples, one-by-one, 
on these litmus paper pieces, as 
shown in Fig. 2.2a.
? Do you observe any change 
in the colour of the blue 
litmus pieces?
? Record your observations 
in Table 2.1.
? Repeat the same activity 
with pieces of red litmus 
paper as shown in 
Fig.  2.2b and record your 
observations in Table 2.1.
How to prepare lime water?
Do not confuse lime water with the word lime, which is a fruit 
similar to lemon.
Lime water (solution of calcium hydroxide in water) can be 
easily prepared by mixing lime (chuna, i.e. calcium oxide) in water 
and leaving it undisturbed for some time, say an hour. Filter the 
liquid into another container and use it as lime water.
Chapter 2.indd   8 4/3/2025   4:50:43 PM
 Exploring Substances: Acidic, Basic, and Neutral
9
Name of the sample S.No.
Colour of blue
litmus paper after 
putting a drop of 
sample
Colour of red
litmus paper after 
putting a drop of 
sample
Table 2.1: Testing the nature of samples with blue and red litmus papers Table 2.1: Testing the nature of samples with blue and red litmus papers
Lemon juice 1.
2. Soap solution
3. Amla juice
4. Tamarind water
6. Baking soda solution
8. Tap water
10. Sugar solution 
5. Vinegar
7. Lime water
9. Washing powder solution
11. Salt solution
12. Any other
Now, let us analyse Table 2.1 and sort the samples into 
three groups as follows?—
? Group A with samples that turn the blue litmus paper to red. 
? Group B with samples that turn the red litmus paper to blue. 
? Group C with samples that do not a? ect either of the two 
litmus papers.
Record the data in Table 2.2.
Group A Group B Group C
Table 2.2: Grouping of samples tested in Table 2.1 Table 2.2: Grouping of samples tested in Table 2.1
Chapter 2.indd   9 4/3/2025   4:50:43 PM
Curiosity | Textbook of Science | Grade 7
10
Let us ? nd out!
Litmus is a natural substance obtained from lichens. It is 
available  both as a solution and in the form of paper strips, 
known as litmus paper. The litmus paper is available in two 
colours—blue and red, as shown in Fig. 2.3. 
Substances that turn blue litmus paper to red are acidic in 
nature, while those that turn red litmus paper to blue are basic
in nature. Since litmus shows di? erent colours in acidic and basic 
solutions, it is called an acid-base indicator.
Some other substances, both natural and synthetic, can 
also be used as indicators. Synthetic indicators are made in 
laboratories, and you will learn more about them in higher 
grades.
HOLISTIC LENS
Lichens are formed by the association of two 
living organisms, a fungus and an alga. They grow 
on rocks and trees in regions that have abundant 
rainfall and clean air. Do you ? nd lichens on trees 
in your neighbourhood?
Lichens
Now, let us classify the substances sorted in Group A, Group B, 
and Group C in Table 2.2.
? The substances in Group A, such as lemon juice, amla juice, 
tamarind water , and vinegar turned the blue litmus paper to 
red, implying that these substances are acidic in nature. 
? The substances in Group B, such as soap solution, baking 
soda solution, lime water, and washing powder solution 
turned the red litmus paper to blue. Hence, these substances 
are basic in nature. 
? The substances in Group C, such as tap water, sugar solution, 
and salt solution, did not change the colour of either litmus 
paper. Can you predict their nature?
These substances are said to be neutral because they are 
neither acidic nor basic.
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?
Fig. 2.3: Blue and red litmus 
paper strips
Chapter 2.indd   10 4/3/2025   4:50:45 PM
Page 5


On 28 February, the school hosted a science fair to celebrate 
National Science Day. At the entry gate, siblings Ashwin and 
Keerthi were greeted with a white sheet of paper. They were 
curious to know why a white sheet of paper was given to them!
A few steps ahead, there was a volunteer spraying a liquid 
on these sheets of paper. The siblings also got their white sheets 
sprayed. To their surprise, the words ‘Welcome to the Wonderful 
World of Science’ appeared on the papers the moment the liquid 
was sprayed (Fig. 2.1). They were excited and eager to know how 
this happened and the reason behind it.
Their curiosity was partly satis? ed at the ‘Colourful World 
of Substances’ stall. They saw many activities showing colour 
changes on mixing di? erent substances. They decided to explore 
these changes further . Let us join them on this learning adventure.
Fig. 2.1: Scene of a science fair
Exploring 
Substances: Acidic, 
Basic, and Neutral
2
Chapter 2.indd   7 4/3/2025   4:50:42 PM
Curiosity | Textbook of Science | Grade 7
8
2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory 2.1 Nature?—?Our Science Laboratory
2.1.1 Litmus as an indicator
Activity 2.1: Let us explore
? Collect samples of lemon juice, soap solution, amla juice, 
tamarind water, vinegar, baking soda solution, lime water, 
tap water, washing powder solution, sugar solution, and salt 
solution.
Fig. 2.2(a): Colour change in 
blue litmus paper
Fig. 2.2(b): Colour change in 
red litmus paper
? Take a strip of blue litmus paper 
and cut it into small pieces.
? Spread these pieces on a clean 
and dry white tile.
? Using a dropper, put one drop of 
each of the samples, one-by-one, 
on these litmus paper pieces, as 
shown in Fig. 2.2a.
? Do you observe any change 
in the colour of the blue 
litmus pieces?
? Record your observations 
in Table 2.1.
? Repeat the same activity 
with pieces of red litmus 
paper as shown in 
Fig.  2.2b and record your 
observations in Table 2.1.
How to prepare lime water?
Do not confuse lime water with the word lime, which is a fruit 
similar to lemon.
Lime water (solution of calcium hydroxide in water) can be 
easily prepared by mixing lime (chuna, i.e. calcium oxide) in water 
and leaving it undisturbed for some time, say an hour. Filter the 
liquid into another container and use it as lime water.
Chapter 2.indd   8 4/3/2025   4:50:43 PM
 Exploring Substances: Acidic, Basic, and Neutral
9
Name of the sample S.No.
Colour of blue
litmus paper after 
putting a drop of 
sample
Colour of red
litmus paper after 
putting a drop of 
sample
Table 2.1: Testing the nature of samples with blue and red litmus papers Table 2.1: Testing the nature of samples with blue and red litmus papers
Lemon juice 1.
2. Soap solution
3. Amla juice
4. Tamarind water
6. Baking soda solution
8. Tap water
10. Sugar solution 
5. Vinegar
7. Lime water
9. Washing powder solution
11. Salt solution
12. Any other
Now, let us analyse Table 2.1 and sort the samples into 
three groups as follows?—
? Group A with samples that turn the blue litmus paper to red. 
? Group B with samples that turn the red litmus paper to blue. 
? Group C with samples that do not a? ect either of the two 
litmus papers.
Record the data in Table 2.2.
Group A Group B Group C
Table 2.2: Grouping of samples tested in Table 2.1 Table 2.2: Grouping of samples tested in Table 2.1
Chapter 2.indd   9 4/3/2025   4:50:43 PM
Curiosity | Textbook of Science | Grade 7
10
Let us ? nd out!
Litmus is a natural substance obtained from lichens. It is 
available  both as a solution and in the form of paper strips, 
known as litmus paper. The litmus paper is available in two 
colours—blue and red, as shown in Fig. 2.3. 
Substances that turn blue litmus paper to red are acidic in 
nature, while those that turn red litmus paper to blue are basic
in nature. Since litmus shows di? erent colours in acidic and basic 
solutions, it is called an acid-base indicator.
Some other substances, both natural and synthetic, can 
also be used as indicators. Synthetic indicators are made in 
laboratories, and you will learn more about them in higher 
grades.
HOLISTIC LENS
Lichens are formed by the association of two 
living organisms, a fungus and an alga. They grow 
on rocks and trees in regions that have abundant 
rainfall and clean air. Do you ? nd lichens on trees 
in your neighbourhood?
Lichens
Now, let us classify the substances sorted in Group A, Group B, 
and Group C in Table 2.2.
? The substances in Group A, such as lemon juice, amla juice, 
tamarind water , and vinegar turned the blue litmus paper to 
red, implying that these substances are acidic in nature. 
? The substances in Group B, such as soap solution, baking 
soda solution, lime water, and washing powder solution 
turned the red litmus paper to blue. Hence, these substances 
are basic in nature. 
? The substances in Group C, such as tap water, sugar solution, 
and salt solution, did not change the colour of either litmus 
paper. Can you predict their nature?
These substances are said to be neutral because they are 
neither acidic nor basic.
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?
Fig. 2.3: Blue and red litmus 
paper strips
Chapter 2.indd   10 4/3/2025   4:50:45 PM
 Exploring Substances: Acidic, Basic, and Neutral
11
Now, I can check 
the nature of ? oor 
cleaning liquid using 
an indicator! 
Activity 2.2: Let us relate and explore
Are all the substances in Group A of Table 2.2 edible? Have you 
ever tasted these edible substances? Can you recall their taste? 
You will ? nd that all these substances taste sour. Thus, we can 
say that substances that taste sour tend to contain acids and are 
acidic in nature.
Caution?—?Do not taste anything until asked to do so. Do not taste any 
unknown substance.
Some common edible substances and the names of the most 
common acids present in them are given in Fig. 2.4.
Tartaric acid
Ascorbic acid (Vitamin C) 
and Citric acid
Citric acid and Oxalic acid
Citric acid
Fig. 2.4: The most common acids present in some edible substances
Find out and write the names of the most common acids 
present in the following substances?—
Lemon________, Curd________, Tamarind________, Vinegar________.
Now, let us take one of the substances?—?baking soda 
solution?—?from Group B. Rub the baking soda solution between 
your ? ngers. What do you observe? 
It feels soapy or slippery. Basic substances are generally 
slippery to touch.
Also, bases generally taste bitter, but 
everything that tastes bitter may not 
contain a base. For example, bitter gourd 
(karela) possesses a bitter taste but is not 
basic in nature.
If litmus is not available, are there 
some other natural substances 
that can serve as acid-base 
indicators?
Chapter 2.indd   11 4/3/2025   4:50:46 PM