The NCERT Solutions for Class 7 Science Chapter 4, "Acids, Bases, and Salts," are essential because of their scientific applications and their influence on students' future studies and CBSE Class 7 exams. These solutions are thorough and aid students in grasping and retaining the concepts of acids, bases, and salts more effectively.
NCERT Solution for Class 7 Science Ch 4 - Acids, Bases & Salts
Exercises
1. State differences between acids and bases.
Answer:
| Acids | Bases |
|---|---|
| Acids are sour in taste | Bases are bitter in taste |
| Acids turn blue litmus paper into red color | The base does not change the colour of blue litmus paper |
| Acids does not change the colour of the red litmus | Bases turn red litmus paper to blue color |
| Acids do not change the colour of turmeric | Bases turn turmeric to red |
2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer:
It's nature is Basic.
3. Name the source from which litmus solution is obtained. What is the use of this solution ?
Answer:
Litmus solution, derived from lichens, serves as an indicator to determine the acidic or basic nature of a solution.
4. Is the distilled water acidic/basic/neutral? How would you verify it?
Answer:
Distilled water is neutral in nature, which can be verified using red and blue litmus paper. In both cases, the color remains unchanged, indicating its neutrality.
5. Describe the process of neutralisation with the help of an example.
Answer:
Neutralization is a reaction that occurs between an acid and a base, resulting in the formation of a salt and water. For instance, when sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl), the products are sodium chloride (NaCl) and water (H2O).
NaOH + HCl → NaCl + H2O + Heat
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)
Answers :
(i) False (ii) False (iii) True (iv) True (v) False
7. Dorji has 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 ?
Answer:
Dorji can test the nature of soft drinks by tasting and using litmus paper. He notes that acidic solutions taste sour, basic solutions taste bitter, and neutral solutions have no taste. To identify the acidity or basicity of a solution using litmus paper, Dorji should follow these steps:
- To test for acidity, he should use blue litmus paper. If a drop of the solution turns the blue litmus red, the solution is acidic.
- To test for basicity, he should use red litmus paper. If a drop of the solution turns the red litmus blue, the solution is basic.
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.
Answers :
(a) The antacid tablet includes milk of magnesia, which acts as a base to neutralize stomach acid. As a result, it is commonly used to relieve symptoms of acidity.
(b) When an ant bites, it injects formic acid into the skin. Calamine, which contains zinc carbonate, is basic in nature. As a result, calamine neutralizes the formic acid, providing relief to the affected area.
(c) Factory wastes are often acidic and can harm aquatic life. Therefore, these wastes are neutralized with a base before being disposed of into water bodies to mitigate their impact.
9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.
Answer :
To test the given liquids, follow these steps:
1. Place a drop of each liquid onto a turmeric indicator. Sodium hydroxide, which is basic, will turn the turmeric indicator red.
2. Next, create two mixtures by adding a drop of sodium hydroxide to each of the other two liquids individually.
3. Test each mixture with the turmeric indicator, one at a time.
- The mixture that turns the turmeric indicator red contains a neutral sugar solution.
- The mixture that does not change the color of the indicator contains hydrochloric acid that has been neutralized by the sodium hydroxide.
10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of solution? Explain.
Answer :
The solution in question could be either neutral or basic, as both types will not alter the color of 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
Answer :
(iv) only d