NCERT Textbook: Acids, Bases & Salts | Science Class 7 PDF Download

 Page 1


Acids, Bases and Salts
4
W
e use in our daily life a large
number of substances such
as lemon, tamarind, common
salt, sugar and vinegar. Do they have
the same taste? Let us recall tastes of
some edible substances listed in
Table 4.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 4.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
4.1 ACIDS AND BASES
Curd, lemon juice, orange juice and
vinegar taste sour. These substances
taste sour because they contain acids.
The chemical nature of such substances
is acidic. The word acid comes from the
Latin word acere which means sour.
The acids in these substances are
natural acids.
What about baking soda? Does it also
taste sour? If not, what is its taste? Since,
it does not taste sour it means, that it
has no acids in it. It is bitter in taste. If
you rub its solution between fingers, it
feels soapy. Generally, substances like
these which are bitter in taste and feel
soapy on touching are known as bases.
The nature of such substances is said to
be basic.
If we cannot taste every substance,
how do we find its nature?
Special type of substances are used
to test whether a substance is acidic or
basic. These substances are known as
indicators. The indicators change their
colour when added to a solution
containing an acidic or a basic
substance. Turmeric, litmus, China rose
petals (Gudhal), etc., are some of the
naturally occurring indicators.
CAUTION
1. Do not taste anything unless asked
to do so.
2. Do not touch anything unless asked
to do so.
Table 4.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
Cucumber
Reprint 2024-25
Page 2


Acids, Bases and Salts
4
W
e use in our daily life a large
number of substances such
as lemon, tamarind, common
salt, sugar and vinegar. Do they have
the same taste? Let us recall tastes of
some edible substances listed in
Table 4.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 4.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
4.1 ACIDS AND BASES
Curd, lemon juice, orange juice and
vinegar taste sour. These substances
taste sour because they contain acids.
The chemical nature of such substances
is acidic. The word acid comes from the
Latin word acere which means sour.
The acids in these substances are
natural acids.
What about baking soda? Does it also
taste sour? If not, what is its taste? Since,
it does not taste sour it means, that it
has no acids in it. It is bitter in taste. If
you rub its solution between fingers, it
feels soapy. Generally, substances like
these which are bitter in taste and feel
soapy on touching are known as bases.
The nature of such substances is said to
be basic.
If we cannot taste every substance,
how do we find its nature?
Special type of substances are used
to test whether a substance is acidic or
basic. These substances are known as
indicators. The indicators change their
colour when added to a solution
containing an acidic or a basic
substance. Turmeric, litmus, China rose
petals (Gudhal), etc., are some of the
naturally occurring indicators.
CAUTION
1. Do not taste anything unless asked
to do so.
2. Do not touch anything unless asked
to do so.
Table 4.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
Cucumber
Reprint 2024-25
ACIDS, BASES AND SALTS 39
(b)
Fig. 4.1  (a) Lichens, and (b) Red and blue
litmus paper
(a)
4.2 NATURAL INDICATORS
AROUND US
Litmus: A natural dye
The most commonly used natural
indicator is litmus. It is extracted
from lichens (Fig. 4.1a). It has a
mauve (purple) colour in distilled
water. When added to an acidic
solution, it turns red and when
added to a basic solution, it turns
blue. It is available in the form of a
solution, or in the form of strips of
paper, known as litmus paper.
Generally, it is available as red and
blue litmus paper (Fig. 4.1b).
Can I taste all substances to
find their taste?
No. Have you not read the
caution? We should not
taste unknown
substances. They could
harm us.
Do you know?
Name of acid Found in
Acetic acid Vinegar
Formic acid Ant’s sting
Citric acid Citrus fruits such
as oranges,
lemons, etc.
Lactic acid Curd
Oxalic acid Spinach
Ascorbic acid Amla, Citrus fruits
(Vitamin C)
Tartaric acid Tamarind, grapes,
unripe mangoes, etc.
All the acids mentioned
above occur in nature
Name of base Found in
Calcium hydroxide Lime water
Ammonium hydroxide Window cleaner
Sodium hydroxide/ Soap
Potassium hydroxide
Magnesium hydroxide Milk of magnesia 
Reprint 2024-25
Page 3


Acids, Bases and Salts
4
W
e use in our daily life a large
number of substances such
as lemon, tamarind, common
salt, sugar and vinegar. Do they have
the same taste? Let us recall tastes of
some edible substances listed in
Table 4.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 4.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
4.1 ACIDS AND BASES
Curd, lemon juice, orange juice and
vinegar taste sour. These substances
taste sour because they contain acids.
The chemical nature of such substances
is acidic. The word acid comes from the
Latin word acere which means sour.
The acids in these substances are
natural acids.
What about baking soda? Does it also
taste sour? If not, what is its taste? Since,
it does not taste sour it means, that it
has no acids in it. It is bitter in taste. If
you rub its solution between fingers, it
feels soapy. Generally, substances like
these which are bitter in taste and feel
soapy on touching are known as bases.
The nature of such substances is said to
be basic.
If we cannot taste every substance,
how do we find its nature?
Special type of substances are used
to test whether a substance is acidic or
basic. These substances are known as
indicators. The indicators change their
colour when added to a solution
containing an acidic or a basic
substance. Turmeric, litmus, China rose
petals (Gudhal), etc., are some of the
naturally occurring indicators.
CAUTION
1. Do not taste anything unless asked
to do so.
2. Do not touch anything unless asked
to do so.
Table 4.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
Cucumber
Reprint 2024-25
ACIDS, BASES AND SALTS 39
(b)
Fig. 4.1  (a) Lichens, and (b) Red and blue
litmus paper
(a)
4.2 NATURAL INDICATORS
AROUND US
Litmus: A natural dye
The most commonly used natural
indicator is litmus. It is extracted
from lichens (Fig. 4.1a). It has a
mauve (purple) colour in distilled
water. When added to an acidic
solution, it turns red and when
added to a basic solution, it turns
blue. It is available in the form of a
solution, or in the form of strips of
paper, known as litmus paper.
Generally, it is available as red and
blue litmus paper (Fig. 4.1b).
Can I taste all substances to
find their taste?
No. Have you not read the
caution? We should not
taste unknown
substances. They could
harm us.
Do you know?
Name of acid Found in
Acetic acid Vinegar
Formic acid Ant’s sting
Citric acid Citrus fruits such
as oranges,
lemons, etc.
Lactic acid Curd
Oxalic acid Spinach
Ascorbic acid Amla, Citrus fruits
(Vitamin C)
Tartaric acid Tamarind, grapes,
unripe mangoes, etc.
All the acids mentioned
above occur in nature
Name of base Found in
Calcium hydroxide Lime water
Ammonium hydroxide Window cleaner
Sodium hydroxide/ Soap
Potassium hydroxide
Magnesium hydroxide Milk of magnesia 
Reprint 2024-25
SCIENCE 40
Turmeric is another natural
indicator
Activity 4.2
? Take a tablespoonful of turmeric
powder. Add a little water and make
a paste.
? Make turmeric paper by depositing
turmeric paste on blotting paper/filter
paper and drying it. Cut thin strips
of the yellow paper obtained.
? Put a drop of soap solution on the
strip of turmeric paper.
What do you observe?
Activity 4.1
? Mix some water with lemon juice in a
plastic cup/tumbler/test tube.
? Put a drop of the above solution on a
strip of the red litmus paper with the
help of a dropper.
Is there any change in colour?
? Repeat the same exercise with the
blue litmus paper.
Note down if there is any change in
colour.
Perform the same activity with the
following substances:
Tap water, detergent solution, aerated
drink, soap solution, shampoo, common
salt solution, sugar solution, vinegar,
baking soda solution, milk of magnesia,
washing soda solution, lime water. If
possible make solutions in distilled water.
Record your observations as in
Table. 4.2.
In your Table, are there any
substances on which litmus had no
effect? Name those substances.
The solutions which do not change
the colour of either red or blue litmus
are known as neutral solutions. These
substances are neither acidic nor basic.
To prepare limewater, take some water in a tumbler and add some lime (chuna)
into it. Stir the solution and keep it for some time. Pour a little from the top.
This is lime water.
Fig. 4.2  Children performing litmus test
Table 4.2
S. No. Test solution Effect on red Effect on blue Inference
litmus paper litmus paper
Reprint 2024-25
Page 4


Acids, Bases and Salts
4
W
e use in our daily life a large
number of substances such
as lemon, tamarind, common
salt, sugar and vinegar. Do they have
the same taste? Let us recall tastes of
some edible substances listed in
Table 4.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 4.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
4.1 ACIDS AND BASES
Curd, lemon juice, orange juice and
vinegar taste sour. These substances
taste sour because they contain acids.
The chemical nature of such substances
is acidic. The word acid comes from the
Latin word acere which means sour.
The acids in these substances are
natural acids.
What about baking soda? Does it also
taste sour? If not, what is its taste? Since,
it does not taste sour it means, that it
has no acids in it. It is bitter in taste. If
you rub its solution between fingers, it
feels soapy. Generally, substances like
these which are bitter in taste and feel
soapy on touching are known as bases.
The nature of such substances is said to
be basic.
If we cannot taste every substance,
how do we find its nature?
Special type of substances are used
to test whether a substance is acidic or
basic. These substances are known as
indicators. The indicators change their
colour when added to a solution
containing an acidic or a basic
substance. Turmeric, litmus, China rose
petals (Gudhal), etc., are some of the
naturally occurring indicators.
CAUTION
1. Do not taste anything unless asked
to do so.
2. Do not touch anything unless asked
to do so.
Table 4.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
Cucumber
Reprint 2024-25
ACIDS, BASES AND SALTS 39
(b)
Fig. 4.1  (a) Lichens, and (b) Red and blue
litmus paper
(a)
4.2 NATURAL INDICATORS
AROUND US
Litmus: A natural dye
The most commonly used natural
indicator is litmus. It is extracted
from lichens (Fig. 4.1a). It has a
mauve (purple) colour in distilled
water. When added to an acidic
solution, it turns red and when
added to a basic solution, it turns
blue. It is available in the form of a
solution, or in the form of strips of
paper, known as litmus paper.
Generally, it is available as red and
blue litmus paper (Fig. 4.1b).
Can I taste all substances to
find their taste?
No. Have you not read the
caution? We should not
taste unknown
substances. They could
harm us.
Do you know?
Name of acid Found in
Acetic acid Vinegar
Formic acid Ant’s sting
Citric acid Citrus fruits such
as oranges,
lemons, etc.
Lactic acid Curd
Oxalic acid Spinach
Ascorbic acid Amla, Citrus fruits
(Vitamin C)
Tartaric acid Tamarind, grapes,
unripe mangoes, etc.
All the acids mentioned
above occur in nature
Name of base Found in
Calcium hydroxide Lime water
Ammonium hydroxide Window cleaner
Sodium hydroxide/ Soap
Potassium hydroxide
Magnesium hydroxide Milk of magnesia 
Reprint 2024-25
SCIENCE 40
Turmeric is another natural
indicator
Activity 4.2
? Take a tablespoonful of turmeric
powder. Add a little water and make
a paste.
? Make turmeric paper by depositing
turmeric paste on blotting paper/filter
paper and drying it. Cut thin strips
of the yellow paper obtained.
? Put a drop of soap solution on the
strip of turmeric paper.
What do you observe?
Activity 4.1
? Mix some water with lemon juice in a
plastic cup/tumbler/test tube.
? Put a drop of the above solution on a
strip of the red litmus paper with the
help of a dropper.
Is there any change in colour?
? Repeat the same exercise with the
blue litmus paper.
Note down if there is any change in
colour.
Perform the same activity with the
following substances:
Tap water, detergent solution, aerated
drink, soap solution, shampoo, common
salt solution, sugar solution, vinegar,
baking soda solution, milk of magnesia,
washing soda solution, lime water. If
possible make solutions in distilled water.
Record your observations as in
Table. 4.2.
In your Table, are there any
substances on which litmus had no
effect? Name those substances.
The solutions which do not change
the colour of either red or blue litmus
are known as neutral solutions. These
substances are neither acidic nor basic.
To prepare limewater, take some water in a tumbler and add some lime (chuna)
into it. Stir the solution and keep it for some time. Pour a little from the top.
This is lime water.
Fig. 4.2  Children performing litmus test
Table 4.2
S. No. Test solution Effect on red Effect on blue Inference
litmus paper litmus paper
Reprint 2024-25
ACIDS, BASES AND SALTS 41
Table 4.3
S. No. Test solution Effect on turmeric solution Remarks
1. Lemon juice
2. Orange juice
3. Vinegar
4. Milk of magnesia
5. Baking soda
6. Lime water
7. Sugar
8. Common salt
You can prepare a greeting card for
your mother on her birthday. Apply
turmeric paste on a sheet of plane
white paper and dry it. Draw a
beautiful flower with soap solution
with the help of a cotton bud. You will
get a beautiful greeting card.
Similarly test the solutions listed in
Table 4.3 and note down your
observations. You may try solutions of
other substances also.
China Rose as Indicator
Activity 4.3
Collect some China rose (Gudhal ) petals
and place them in a beaker. Add some
Fig. 4.3  China rose flower and indicator
prepared from it
Acid Base Indicator
China
rose
Now I understand why a
turmeric stain on my white shirt
is turned to red when it is
washed with soap. It is because
the soap solution is basic.
Turmeric paste
Soap
solution
Reprint 2024-25
Page 5


Acids, Bases and Salts
4
W
e use in our daily life a large
number of substances such
as lemon, tamarind, common
salt, sugar and vinegar. Do they have
the same taste? Let us recall tastes of
some edible substances listed in
Table 4.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 4.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
4.1 ACIDS AND BASES
Curd, lemon juice, orange juice and
vinegar taste sour. These substances
taste sour because they contain acids.
The chemical nature of such substances
is acidic. The word acid comes from the
Latin word acere which means sour.
The acids in these substances are
natural acids.
What about baking soda? Does it also
taste sour? If not, what is its taste? Since,
it does not taste sour it means, that it
has no acids in it. It is bitter in taste. If
you rub its solution between fingers, it
feels soapy. Generally, substances like
these which are bitter in taste and feel
soapy on touching are known as bases.
The nature of such substances is said to
be basic.
If we cannot taste every substance,
how do we find its nature?
Special type of substances are used
to test whether a substance is acidic or
basic. These substances are known as
indicators. The indicators change their
colour when added to a solution
containing an acidic or a basic
substance. Turmeric, litmus, China rose
petals (Gudhal), etc., are some of the
naturally occurring indicators.
CAUTION
1. Do not taste anything unless asked
to do so.
2. Do not touch anything unless asked
to do so.
Table 4.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
Cucumber
Reprint 2024-25
ACIDS, BASES AND SALTS 39
(b)
Fig. 4.1  (a) Lichens, and (b) Red and blue
litmus paper
(a)
4.2 NATURAL INDICATORS
AROUND US
Litmus: A natural dye
The most commonly used natural
indicator is litmus. It is extracted
from lichens (Fig. 4.1a). It has a
mauve (purple) colour in distilled
water. When added to an acidic
solution, it turns red and when
added to a basic solution, it turns
blue. It is available in the form of a
solution, or in the form of strips of
paper, known as litmus paper.
Generally, it is available as red and
blue litmus paper (Fig. 4.1b).
Can I taste all substances to
find their taste?
No. Have you not read the
caution? We should not
taste unknown
substances. They could
harm us.
Do you know?
Name of acid Found in
Acetic acid Vinegar
Formic acid Ant’s sting
Citric acid Citrus fruits such
as oranges,
lemons, etc.
Lactic acid Curd
Oxalic acid Spinach
Ascorbic acid Amla, Citrus fruits
(Vitamin C)
Tartaric acid Tamarind, grapes,
unripe mangoes, etc.
All the acids mentioned
above occur in nature
Name of base Found in
Calcium hydroxide Lime water
Ammonium hydroxide Window cleaner
Sodium hydroxide/ Soap
Potassium hydroxide
Magnesium hydroxide Milk of magnesia 
Reprint 2024-25
SCIENCE 40
Turmeric is another natural
indicator
Activity 4.2
? Take a tablespoonful of turmeric
powder. Add a little water and make
a paste.
? Make turmeric paper by depositing
turmeric paste on blotting paper/filter
paper and drying it. Cut thin strips
of the yellow paper obtained.
? Put a drop of soap solution on the
strip of turmeric paper.
What do you observe?
Activity 4.1
? Mix some water with lemon juice in a
plastic cup/tumbler/test tube.
? Put a drop of the above solution on a
strip of the red litmus paper with the
help of a dropper.
Is there any change in colour?
? Repeat the same exercise with the
blue litmus paper.
Note down if there is any change in
colour.
Perform the same activity with the
following substances:
Tap water, detergent solution, aerated
drink, soap solution, shampoo, common
salt solution, sugar solution, vinegar,
baking soda solution, milk of magnesia,
washing soda solution, lime water. If
possible make solutions in distilled water.
Record your observations as in
Table. 4.2.
In your Table, are there any
substances on which litmus had no
effect? Name those substances.
The solutions which do not change
the colour of either red or blue litmus
are known as neutral solutions. These
substances are neither acidic nor basic.
To prepare limewater, take some water in a tumbler and add some lime (chuna)
into it. Stir the solution and keep it for some time. Pour a little from the top.
This is lime water.
Fig. 4.2  Children performing litmus test
Table 4.2
S. No. Test solution Effect on red Effect on blue Inference
litmus paper litmus paper
Reprint 2024-25
ACIDS, BASES AND SALTS 41
Table 4.3
S. No. Test solution Effect on turmeric solution Remarks
1. Lemon juice
2. Orange juice
3. Vinegar
4. Milk of magnesia
5. Baking soda
6. Lime water
7. Sugar
8. Common salt
You can prepare a greeting card for
your mother on her birthday. Apply
turmeric paste on a sheet of plane
white paper and dry it. Draw a
beautiful flower with soap solution
with the help of a cotton bud. You will
get a beautiful greeting card.
Similarly test the solutions listed in
Table 4.3 and note down your
observations. You may try solutions of
other substances also.
China Rose as Indicator
Activity 4.3
Collect some China rose (Gudhal ) petals
and place them in a beaker. Add some
Fig. 4.3  China rose flower and indicator
prepared from it
Acid Base Indicator
China
rose
Now I understand why a
turmeric stain on my white shirt
is turned to red when it is
washed with soap. It is because
the soap solution is basic.
Turmeric paste
Soap
solution
Reprint 2024-25
SCIENCE 42
Table 4.4
S. No. Test solution       Initial colour   Final colour
1. Shampoo (dilute solution)
2. Lemon juice 
3. Soda water
4. Sodium hydrogencarbonate solution 
5. Vinegar
6. Sugar solution
7. Common salt solution
warm water. Keep the mixture for some
time till water becomes coloured. Use
the coloured water as an indicator. Add
five drops of the indicator to each of the
solutions given in Table 4.4.
What is the effect of the indicator on
acidic, basic and neutral solutions?
China rose indicator (Fig. 4.3) turns
acidic solutions to dark pink (magenta)
and basic solutions to green.
Coffee is brown
And bitter in taste.
Is it an acid?
Or a base?
Don’t give the answer
Without any test,
You are in the dark
With its taste.
Paheli brought the following paheli
(riddle) for you.
I am not getting the same result
when using solid baking soda
on dry litmus paper. Why?
Make a solution of baking
soda and then try.
Activity 4.4
The teacher is requested to get the dilute
solution of the following chemicals from
his/her school laboratory or from a
nearby school: hydrochloric acid,
sulphuric acid, nitric acid, acetic acid,
sodium hydroxide, ammonium hydro-
xide, calcium hydroxide (lime water).
Table 4.5
S. Name of acid Effect on Effect on Effect on
No. litmus paper turmeric paper China rose
solution
1. Dilute hydrochloric acid
2.
3.
Reprint 2024-25