NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts

1. State differences between acids and bases.

Ans: The substances that taste sour are acids and the substances taste bitter and feel soapy on touching base.

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

Ans: The substances that turn red litmus blue are base. Therefore, ammonium is basic in nature.

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

Ans: Litmus solution is extracted from lichens and is used to test the acidic/basic nature of the solution.

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

Ans: Distilled water is neutral in nature. We can verify it by using litmus paper. When some drops of the distilled water are poured on red/blue litmus paper, its color remains the same.

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

Ans: Neutralization is a reaction between acids and bases. Acids and bases are opposites in nature, when they mix, they neutralize the effects of each other. The substance formed in the neutralization reaction is known as salt.

Example: $\underbrace{HCl}_{acid}+\underbrace{NaOH}_{base}\to \underbrace{NaCl}_{salt}+\underbrace{{{H}_{2}}O}_{water}$.

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

(i) Nitric acid turns blue litmus red.

Ans: True, Nitric acid is acidic in nature and therefore it turns blue litmus red.

(ii) Sodium hydroxide turns blue litmus red. 

Ans: False, Sodium hydroxide is basic in nature and therefore it turns red litmus blue.

(iii) Sodium hydroxide and hydrochloric acid neutralize each other and form salts and water. 

Ans: True

$\underbrace{HCl}_{acid}+\underbrace{NaOH}_{base}\to \underbrace{NaCl}_{salt}+\underbrace{{{H}_{2}}O}_{water}$

Neutralization is a reaction between acids and bases, when they are mixed, they neutralize the effects of each other. 

(iv) An Indicator is a substance that shows different colors in acidic and basic solutions. (T/F)

Ans: True, Litmus is an indicator and it shows different colors in acidic and basic solutions.

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

Ans: False, Tooth decay is caused by the presence of an acid produced by bacteria.

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

Ans: Dorji can use the indicator like litmus which can help him take the informed decision. If a few drops of a drink change blue litmus to red then it is acidic in nature and if it turns red litmus to blue then it is basic in nature. 

Also, if the drops of a drink do not affect the litmus, then it is neutral in nature.

8. Explain why:

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

Ans: An antacid tablet is taken when you suffer from acidity because the antacid tablet is basic in nature; it reacts with the excess acid in our stomach causing a neutralization reaction. Hence, it relieves us from the pain of acidity.

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

Ans: Calamine solution is applied on the skin when ant bites because ant bites inject acid in the skin and calamine solution is basic in nature, it reacts with the acid causing a neutralization reaction. Hence, it relieves us from itching and irritation.

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

Ans: Factory waste is neutralized before disposing it into the water bodies because excess acids or bases can be dangerous and fatal for the organisms living in the water.

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 a turmeric indicator.

Ans: Turmeric is acidic in nature and so it will turn red in contact with a base. From this, we can figure out that sodium hydroxide is a base. 

Now we are left with sugar solution and hydrochloric acid. The red spot created on the litmus due to the base will again turn yellow while in contact with an acid. You will observe that the hydrochloric acid will turn the red spot into yellow again and hence it is an acid. 

Sugar solution will not affect the litmus and hence it is a neutral solution.

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

Ans: We know that acids turn blue litmus into the red. When a blue litmus paper is dipped in a solution and it remains blue then the solution cannot be acidic. 

Hence, the solution can either be basic or neutral.

11. Consider the following statements:

(a) Both acids and bases change color at all indicators.

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

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

(d) The change of color in an acid and a base depends on the type of indicator.

Which of these statements are correct?

(i) All four 

(ii) a and b 

(iii) b and c 

(iv) only d

Ans: (iv) Only d

Litmus is an indicator and it changes red to blue for the basic solution and blue to red for an acidic solution.

The same is with turmeric. It changes yellow to red for the basic solution.

NCERT Solutions for Class 7 Science Chapter 5 – Free PDF Download

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Chapter 5 – Acids, Bases, and Salts

5.1 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.

5.2 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 color whereas, when added to a basic substance, it turns blue in color. 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).

5.4 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 

NOTE: An acid and base neutralize each other to form a salt. Salts may be acidic, basic, or neutral in nature. 

Study in detail and in case you face any problem regarding any experiment or concept mentioned in chapter 5 acid and bases you can reach out to Vedantu experts on the official website to clear your doubts.

Exercise Solutions: 11 Questions (2 short questions and 9 Long questions).

Extended learning: Activities and projects (4 questions).

Key Features of NCERT Solutions for Class 7 Science Chapter 5

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