NCERT Book Class 7 Science Chapter 4 Acids, Bases and Salts

4 Acids, Bases and Salts

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