Acids and Bases

We 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
50 SCIENCE
(b)
Fig. 5.1 (a) Lichens, and (b) Red and blue
litmus paper