CLASS 7 SCIENCE CHAPTER 5 ACIDS, BASES AND SALTS NCERT EXERCISE QUESTIONS SOLVED

Exercise:

1. State differences between acids and bases.
Answer:

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

Answer: As the ammonia turns red litmus blue, the nature of the ammonia is basic.

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

Answer:

• i) The litmus is an indicator solution which is obtained from lichens.
• ii) The indicator solution is used to identify whether the substance is acidic or basic.
• iii) If the substance is acidic, then blue litmus turns red and if the substance is basic, then red litmus turns blue. If the substance neither acidic nor basic, then litmus becomes neutral.

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

Answer: The distilled water is neutral in nature. It is verified by using litmus indicator that means, red and blue litmus papers. If both the litmus papers dipped in a distilled solution, then they do not change their colours. This indicates that distilled water is neutral.

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

Answer: The reaction between an acid and a base is known as neutralisation. Salt and water are formed in this process with the evolution of heat.

The reaction of the Neutralisation process is as follows:

Acid + Base → Salt + Water
(Heat is evolved)

Example:
Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) →Sodium chloride (NaCl) + Water (H2O)

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

a. Nitric acid turn red litmus blue. (T/F)
Answer: F

b. Sodium hydroxide turns blue litmus red. (T/F)
Answer: F

c. Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
Answer: T
d. Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
Answer: T

e. Tooth decay is caused by the presence of a base. (T/F)
Answer: 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?

Answer:

• i) As the Dorji has a few bottles of soft drinks, he can tastes each of them to identify the nature of the bottles (acidic, basic, neutral).
• Acidic drinks will be taste sour, basic drinks will be bitter in taste and neutral drinks will have no tastes.
• ii) Also the Dorji will decide the nature of the bottles by using litmus solution or paper.
• Dorji should put one drop of each drinks on blue litmus paper.
• If the colour of the litmus paper changes to red, then it will be an acidic drink.
• Means acidic drink will be separate out from total drinks. Left will be basic and neutral.
• Again, he should put one drop of remaining drinks on red litmus paper.
• If the colour of the litmus paper changes to blue, then that drink will be basic drink and others will be neutral.
• Accordingly, the Dorji will be decide drinks to serve all his customers on the demand.

8. Explain why:

i. An antacid tablet is taken when you suffer from acidity.

ii. Calamine solution is applied on the skin when an ant bites.

iii. Factory waste is neutralised before disposing it into the water bodies.

Answer:

• i) An antacid tablet is taken when we suffer from acidity because it neutralises the effect of excessive acid and relieves from indigestion.

• ii) When an ant bites, it injects the acidic liquid (formic acid) into the skin. Hence, to neutralise the effect of the acid, Calamine solution contains zinc carbonate which is basic in nature is applied on the skin.
• iii) Factory waste contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances before disposing it into the water bodies.

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:

10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer: If blue litmus is dipped in a solution and it remains blue, then the nature of the solution is basic because they do not change the colour of blue litmus. The blue litmus paper do not change the colour when dipped in basic as well as neutral solutions.

• 11. Consider the following statements:
• i.) Both acids and bases change colour of all indicators.
• ii.) If an indicator gives a colour change with an acid, it does not give a change with a base.
• iii.) If an indicator changes colour with a base, it does not change colour with an acid.
• iv.) 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