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What is the definition of an acid in chemistry?

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.

Substances containing an acid (from Latin acere meaning sour) are called acidic substances.
Acids are of two types: mineral acids and organic acids.

1. Mineral Acid:
Hydrochloric acid (HCl), sulphuric acid (H₂SO₄), and nitric acid (HNO3) are examples of mineral acids that are used in the laboratory.

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2. Organic Acid:
Organic acids occur naturally in animal and plant materials.
Some of the naturally occurring substances that contain acids are given in Table.

Substance	Acid present
1. Orange, lemon	Citric acid, ascorbic acid (vitamin C)
2. Apple	Malic acid
3. Tamarind (imli), grape	Tartaric acid
4. Vinegar	Acetic acid
5. Curd	Lactic acid
6. Tomato, Spinach	Oxalic acid
7. Gastric juice	Hydrochloric acid
8. Tea	Tannic acid
9. Red ants, Bees	Formic acid

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 (H₂SO₄) 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.
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The hydrogen atoms in H₂SO₄ can also be partially or wholly replaced by a group of elements, like an ammonium ion to form ammonium hydrogensulphate (NH₄HSO₄) or ammonium sulphate ((NH₄)₂SO₄) respectively.
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The substances NaHSO₄, Na₂SO₄, NH₄HSO₄ and (NH₄)₂SO₄ 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.

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