NCERT Textbook - Acids, Bases and Salts Class 7 Notes | EduRev

General Science(Prelims) by IRS Divey Sethi

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Class 7 : NCERT Textbook - Acids, Bases and Salts Class 7 Notes | EduRev

 Page 1


Acids, Bases and Salts
5
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 5.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 5.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
5.1 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. 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 5.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
© NCERT
not to be republished
Page 2


Acids, Bases and Salts
5
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 5.1. If you have not tasted any of
these substances taste it now and enter
the result in Table 5.1.
You find that some of these
substances taste sour, some taste bitter,
some taste sweet and some taste salty.
5.1 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. 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 5.1
Substance Taste (sour/bitter/
any other)
Lemon juice
Orange juice
Vinegar
Curd
Tamarind (imli)
Sugar
Common salt
Amla
Baking soda
Grapes
Unripe mango 
© NCERT
not to be republished
SCIENCE 50
(b)
Fig. 5.1  (a) Lichens, and (b) Red and blue
litmus paper
(a)
5.2 NATURAL INDICATORS
AROUND US
Litmus: A natural dye
The most commonly used natural
indicator is litmus. It is extracted
from lichens (Fig. 5.1). 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. 5.1).
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 
© NCERT
not to be republished
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