Introduction to Acids – Classification, Strong Acids and Weak Acids

Understanding chemical bonding is crucial in many Chemistry Topics, as it explores how atoms combine to form molecules.

Explain About Classifications of Acids

If we cut a lemon (neembu) and taste it, the lemon appears to have a sour taste (khatta taste) (see Figure). The sour taste of lemon is due to the presence of an acid in it. The acid present in lemon which gives it a sour taste is called citric acid.

Raw mango, raw grapes, lemon juice, orange juice, curd, sour milk, vinegar and tamarind (imli), etc., are sour in taste due to the presence of acids in them. The acids present in plant materials and animals are natural acids called organic acids. The acids can be defined as follows.

A substance which reacts with a base to form a salt (and water) is called an acid. Acids have sour taste. Acids turn blue litmus to red. Some of the examples of acids are : Acetic acid, Citric acid, Hydrochloric acid, Sulphuric acid and Nitric acid. Acids are of two types : Organic acids and Mineral acids. These are discussed below.

Organic Acids

Organic acids are the naturally occurring acids. They are found in various types of plants and animals.
Some of the important organic acids (which occur in nature) are :

1. Acetic acid.
2. Formic acid
3. Citric acid
4. Lactic acid
5. Tartaric acid
6. Ascorbic acid
7. Oxalic acid

 

1. Acetic acid is found in vinegar (or sirka). Vinegar is used as a preservative in foods.
2. Formic acid is present in ant’s sting. The sharp pain caused by the sting of an ant is due to the formic acid pushed into our skin during the sting.
3. Citric acid is present in citrus fruits such as lemons and oranges.
4. Lactic acid is present in curd and in sour milk.
5. Tartaric acid is present in tamarind (imli), unripe grapes and unripe mangoes.
6. Ascorbic acid is present in amla and citrus fruits (such as lemons and oranges). Ascorbic acid is commonly known as vitamin C.
7. Oxalic acid is present in spinach (palak).

Activity 3

We can test the presence of an acid in a substance by performing the litmus test. For example, we can perform the litmus test to show the presence of acid in lemon juice (or orange juice) as follows :
(i) Take some lemon juice in a test-tube and add a little water to it.

(ii) Put a drop of the lemon juice solution on a strip of red litmus paper with the help of a dropper. We will find that there is no change in the colour of red litmus paper. This means that lemon juice is not basic in nature (because only basic substances or bases turn red litmus to blue).

(iii) Now put a drop of lemon juice solution on a strip of blue litmus paper. The blue litmus paper turns red. This shows that lemon juice is acidic in nature (or lemon juice contains an acid). This is because only acidic substances or acids turn blue litmus to red.

Organic acids (or naturally occurring acids) are weak acids. It is not harmful to eat or drink substances containing naturally occurring acids in them. For example, we can eat oranges or drink orange juice which contain natural acids. Similarly, we can consume lemon juice because it contains organic acid (or natural acid) which does not harm us.

Mineral Acids

The acids prepared from the minerals of the earth are called mineral acids. Mineral acids are the man-made acids. Mineral acids are also known as laboratory acids because they are used in the science laboratory to perform experiments (see Figure). The three most common mineral acids are :

1. Hydrochloric acid,
2. Sulphuric acid, and
3. Nitric acid.

We also use mineral acids in our daily life. For example, hydrochloric acid is used in cleaning kitchen sinks and bathroom sanitaryware (like wash basin and toilet seat). Sulphuric acid is used in making storage batteries for cars, buses, trucks and inverters. Nitric acid is used by goldsmiths for cleaning gold and silver ornaments.

Concentrated mineral acids are very dangerous. These acids can burn our hands and clothes. These acids should be handled with great care. In the laboratory, acids are generally mixed with water to dilute them. Such acids are called dilute acids. Dilute acids are less harmful to us.

Strong Acids and Weak Acids

All the acids can be divided into two groups : strong acids and weak acids, and nitric acid are strong acids. On the other hand, acetic acid, formic carbonic acid are some of the weak acids. This can be written as follows :
Strong acids
Hydrochloric acid
Sulphuric acid
Nitric acid

Weak acids
Acetic acid
Formic acid
Citric acid
Tartaric acid
Carbonic acid

It is obvious that the mineral acids are strong acids. Only one mineral acid, carbonic acid, is a weak acid. Strong acids are very dangerous to drink. Even the dilute solutions of strong acids are extremely harmful to drink. The organic acids are weak acids.

The dilute solutions of weak acids are quite safe to drink. Being weak, the organic acids like acetic acid, citric acid and tartaric acid are used as food ingredients. Many foods like pickle and tomato ketchup contain acetic acid in the form of vinegar.

Vinegar preserves fruits and vegetables. Baking powder used in making cakes and biscuits contains tartaric acid. Though carbonic acid is not an organic’acid, but it is a weak acid. Carbonic acid is used in fizzy soft drinks and soda water. It gives them a pleasant taste.

Acid Rain

The rain which contains a higher level of acid than normal is called acid rain. Acid rain is caused by the acidic gases like sulphur dioxide, nitrogen dioxide and carbon dioxide which are released into the air as pollutants during the burning of various types of fuels. Sulphur dioxide gas dissolves in falling rain drops to form sulphuric acid; nitrogen dioxide gas dissolves in rain drops to form nitric acid whereas carbon dioxide gas dissolves in rain drops to form carbonic acid.

The presence of sulphuric acid, nitric acid and carbonic acid in rain water makes the rain water acidic. And when this acidic rain water falls on the earth, we call it acid rain. Acid rain causes damage to aquatic animals (like fish), trees, crop plants, metal structures and stone buildings and monuments. This happens as follows:

1. Acid rain makes the water of lakes, ponds and rivers too acidic due to which fish and other aquatic animals get killed.
2. Acid rain eats up the leaves of the trees gradually. By losing leaves, the trees die. Acid rain also damages crop plants in the fields.
3. Acid rain damages the metal structures like steel bridges, etc., when it falls on them.
4. Acid rain damages the surfaces of buildings and monuments made of stone.