Class Seven Science Acid-base salt NCERT Solution

Chapter 5: Acid-base and salt

NCERT Questions-answers

1. State differences between acids and bases.


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

Answer Ammonia has a basic nature.

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

Answer Litmus solution is extracted from lichens. It is used to determine whether the given solution is acidic or basic.

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

Answer Distilled water will be neutral. We can verify it by showing that neither blue nor red litmus paper changes its colour when dipped in it.

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.

Antacids like milk of magnesia (magnesium hydroxide), baking soda, etc. which contain a base are used for reducing acidity in the stomach when excessive acid released by glands.

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) The indicator is a substance which shows different colours in acidic and basicsolutions. (T/F)

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

Answer (1) F (ii) F (iii) T (iv) T (V) F

Acid-base and salt

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 a neutral drink. How will Dorji decide which drink is to be served to whom?

Answer Dorji can decide with the help of litmus paper:

(i) The drink that turns red litmus into the blue is basic.

(ii) The drink that turns blue litmus to red is acidic. (iii) The drink that does not affect both red and blue litmus is neutral.

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 neutralized before disposing it into the water bodies.

Answer (a) We take an antacid such as milk of magnesia to neutralizes the excessive acid released in the stomach.

(b) Ant injects an acidic liquid (Formic acid) into the skin on biting which causes inflammation, to the skin. The effect of the acid can be neutralized by rubbing. Calamine solution contains zinc carbonate which is a very weak base and causes no harm to the skin.

(c) The wastes of factories contain acids. If acids are disposed of in the water body, the acids will harm the organisms. So, basic substances are used to neutralize waste materials.

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 Name of the substances Effect on turmeric indicator

1. Hydrochloric acid Yellow to blue

2. Sodium hydroxide Yellow to red

3. Sugar solution No change

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

Answer (i) It can be identified on the basis of the following observations: Bases change the colour of litmus paper to blue. As the colour of blue litmus paper does not change, the solution must be basic.

(ii) If the solution is neutral, even then the colour of litmus will not change.

11. Consider the following statements:

(a) Both acids and bases change the 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) and (c)

(iv) only (d)

Answer (ii) (a) and (d)