Science Class 7 Chapter 5 Acids, Bases and Salts – NCERT Exercise Solutions (Question-Answer)

Q1. State differences between acids and bases.

Solution:

Q2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?

Solution:

Its nature is basic.

Q3. Name the source from which the 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 acidic and basic nature of a solution. Litmus paper is either red or blue in colour. Blue litmus paper turns red in the presence of acidic solutions and red litmus turns blue in the presence of basic solution.

Q4. Is the distilled water acidic/basic/neutral? How would you verify it?

Solution: Distilled water is neutral in nature.

This can be tested by using red and blue litmus paper. We observe that in this case colour remains unchanged.

Q5. Describe the process of neutralisation with the help of an example.

Solution: When an acid reacts with a base, salt is formed and this reaction is called the neutralization reaction.

In this process heat is evolved.

 Acid + base →salt + water + heat

Eg: When sodium hydroxide (base) is added to hydrochloric acid

It produces NaCl (salt) and H₂O (water).

  NaOH + HCl → NaCl + H₂O

Q6. Mark ‘T’ if the statement is true and ‘F’ if it is false:

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

Solution: false

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

Solution: false

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

Solution: true

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

Solution: true

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

Solution: false

Q7. 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 an acidic drink, another wants basic and 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 drinks bottles. We know that the acidic solution is sour in taste, the basic solution is bitter in taste, and the neutral solution has no taste.

Dorji can also use litmus paper to test the nature of the soft drink. He should use a 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, if the red litmus change to blue the solution will be basic in nature. If no change will occur of red and blue litmus paper, then the solution will neutral in nature.

Q8. Explain why:

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

Solution: An antacid tablet is taken when we suffer from acidity because the antacid tablet contains base like milk of magnesia which neutralises the acid produced in the stomach.

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

Solution: Calamine solution is applied on the skin when an ant bites because ant injects formic acid inside the skin. Calamine contains bases like Zinc carbonate. Hence calamine neutralises the effect of formic acid to bring relief for the affected person.

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

Solution: Factory waste is neutralised before disposing it into the water bodies because factory wastes are acidic in nature which may cause harm to the aquatic life. Hence, factory waste is neutralised by using a base before disposing it into the water bodies.

Q9. 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.

Solution:

We will identify the solution by this observation:

Add solution to container with turmeric indicator.

We observe that turmeric indicator will red in colour if the solution is NaOH (base).

To the same container, add test solution number 2. If the added solution is sugar solution we observe that turmeric indicator remains red because mixture of basic and neutral solution will remain basic in nature.

Now, add the test solution number 3 to another container with turmeric indicator. If the indicator remains red then the solution will be hydrochloric acid.

Q10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

Solution:

The given solution may be neutral or basic in nature because both of them (base and neutral) will not change the colour of the blue litmus paper.

Q11. 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