NCERT Book Class 7 Science Acids Bases and Salts

5.Acids, Bases and Salts

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. 

5.2 NATURAL INDICATORS AROUND US

Litmus: A natural dye 

The most commonly used natural indicator is litmus. It is extracted from lichens (Fig. 5.1). It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper (Fig. 5.1).

Exercises

1. State differences between acids and bases.

2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?

3. Name the source from which litmus solution is obtained. What is the use of this solution?

4. Is the distilled water acidic/basic/neutral? How would you verify it?

5. Describe the process of neutralisation with the help of an example.

6. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(i) Nitric acid turn red litmus blue. (T/F)

(ii) Sodium hydroxide turns blue litmus red. (T/F)

(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)

(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)

(v) Tooth decay is caused by the presence of a base. (T/F)

7. Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?

8. Explain why:

(a) An antacid tablet is taken when you suffer from acidity

Please refer to attached file for NCERT Class 7 Science Acids, Bases and Salts