The term 'acid' has its origin in the Latin word acidus, meaning sour. In fact, anything that tastes sour contains an acid. For example, lemon juice, tomato, vinegar, etc., all taste sour. So, each of these substances must contain an acid. Some of the naturally occurring substances that contain acids are given in Table
Aqueous solutions of acids are generally sour in taste. Acids turn blue litmus red, conduct electricity and react with bases to form salts and water. [Bases and salts are discussed a little later.]
An acid may be defined in various ways. Here, we shall study the definition given by Liebig in 1838. According to Liebig, an acid is a compound which contains hydrogen that can be replaced partially or wholly by a metal or a group of elements acting like a metal, to produce a salt.
For example, sulphuric acid (H2
) is an acid because of the following reasons.
(i) It contains hydrogen atoms in its molecule.
(ii) The two hydrogen atoms present in its molecule can be replaced partially or wholly by a metal like sodium (Na) to produce sodium hydrogensulphate or sodium sulphate.
The hydrogen atoms in H2
can also be partially or wholly replaced by a group of elements, like an ammonium ion
to form ammonium hydrogensulphate (NH4
) or ((NH4
The substances NaHSO4
are all salts.
(iii) The acid dissolves in water to make a solution that turns blue litmus red.
(iv) It is sour in taste.
(v) It reacts vigorously with a base to produce a salt.
The hydrogen atoms present in an acid that can be replaced by a metal or a group of elements are called replaceable hydrogen or acidic hydrogen.