Chapter 5 – Acids, Bases, and Salts

Acids and Bases

How many of you love having a chilled glass of lemon juice on a summer day? I knew it, everybody. How does it taste? It is sour in taste because it contains acid. Can you recall some more food items that taste sour? Curd, orange juice, vinegar and a lot more. The chemical nature of these substances is acidic. On the other hand, the substances which are bitter in taste and feel soapy on touching are known as bases. Such substances are basic in nature.

But not every substance is edible. So, how can we know the acidic and basic nature of substances that we cannot taste? Indicators are used to test if any substance is acidic or basic in nature. Some naturally occurring indicators are – litmus, china rose petals, turmeric, etc. In this unit, you will learn about acids, bases, and indicators in detail.

Natural Indicators Around Us

There are various natural indicators that can be used to find the acidic or basic nature of any substance. These indicators change their color accordingly on coming into contact with an acidic or basic substance. Here, we shall learn about some acidic and basic substances in detail.

Litmus: A Natural Dye

Litmus, the most commonly used natural indicator is extracted from Lichens. When litmus is added to an acidic substance, it turns red in colour whereas, when added to a basic substance, it turns blue in colour. Litmus is available in both solution form as well as in the form of paper strips. Some more natural indicators explained in this chapter are:

1. Turmeric

2. China Rose

These concepts are explained in detail through various scientific experiments. Try to perform all the experiments practically, in your school lab, or at home under the supervision of a teacher or parents. Performing the experiments will give you better knowledge and a detailed understanding of the concepts.

Neutralization

Now, you already know about acids, bases, and indicators. Have you ever wondered what happens when an acid is mixed with a base? In this section, we will use a new indicator, called phenolphthalein, and study the effects in detail. The reaction that happens between an acid and a base is known as neutralization. During the process of neutralization, salt and water are produced with the evolution of heat.

Acid + Base → Salt + water (heat is evolved).

NCERT Solutions for Class 7 Science Chapter 3 – Heat

Neutralisation in Everyday Life

In this unit, you will learn about the process of neutralization that occurs in our day-to-day lives. 

1. Indigestion 

2. Ant bite 

3. Soil Treatment 

4. Factory waste 

Question/Answer

1. State differences between acids and bases.

Solution:

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

Solution:

The answer is it is basic in nature.

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

Solution:

Litmus solution is extracted from lichens. Litmus solution is used as an indicator to find the acidic and basic nature of a solution.

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

Solution:

Distilled water is neutral in nature, and this can be tested by using red and blue litmus paper. In either of the cases, the colour remains unchanged.

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

Solution:

Neutralisation is a reaction between an acid and a base. Here, both acids and bases get neutralised. For example, when sodium hydroxide (NaOH) is added to hydrochloric acid (HCl), sodium chloride (NaCl) and water (H₂O) are obtained.

NaOH + HCl → NaCl + H₂O + Heat

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

(i) Nitric acid turns 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) An 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)

Solution:

i) False

ii) False

iii) True

iv) True

v) False

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

Solution:

Dorji can taste a few drops out of soft drink bottles; the acidic solution is sour in taste, the basic solution is bitter in taste, and the neutral solution has no taste. Along with tasting, Dorji can use litmus paper to test the nature of the soft drinks. He should use blue litmus paper to test the acidic solution. Dorji has to put a drop of solution on blue litmus. If it turns red, then the solution will be acidic in nature.

Similarly, he can use red litmus paper to test the basic solution. He has to put a drop of solution on red litmus. If it turns blue, then the solution will be basic in nature.

8. Explain why

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

(b) Calamine solution is applied on the skin when an ant bites.

(c) Factory waste is neutralised before disposing it into the water bodies.

Solution:

1. The antacid tablet contains base-like milk of magnesia, which neutralises the acid produced in the stomach. Hence, it is used while suffering from acidity.
2. When ant bites, it injects formic acid inside the skin. Calamine consists of Zinc carbonate which is basic in nature. Hence, calamine neutralises the effect of formic acid to bring relief to the affected person.
3. Factory wastes are acidic in nature which may cause harm to aquatic life. Hence, they are neutralised by using a base before disposing it into the water bodies.

9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide, and the third is a sugar solution. How will you identify them? You have only turmeric indicator.

Solution:

The following steps are taken to test the given liquids:

• Put a drop of the provided liquid on the turmeric indicator. The solution that changes the colour of the indicator to red is sodium hydroxide, which is basic in nature.
• Now, to make two mixtures, add a drop of sodium hydroxide to the other two liquids individually.
• The drop of each combination added to the turmeric indicator one after another.
• The mixture that changes the indicator to a red colour includes a neutral solution of sugar.
• The mixture contains hydrochloric acid that has been neutralised by the addition of sodium hydroxide, which does not show any colour change in the indicator.

COMBATING GLACIER MELT: A MULTIFACETED APPROACH FROM GLOBAL MITIGATION TO LOCAL ADAPTATION

10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

Solution:

The above solution may be neutral or basic in nature as both will not change the colour of the blue litmus paper.

11. Consider the following statements:

(a) Both acids and bases change colour of all indicators.

(b) If an indicator gives a colour change with an acid, it does not give a change with a base.

(c) If an indicator changes colour with a base, it does not change colour with an acid.

(d) Change of colour in an acid and a base depends on the type of the indicator.

Which of these statements are correct?

(i) All four

(ii) a and d

(iii) b, c and d

(iv) Only d

Solution:

(iv) Only d is correct