Class 7 Science

Acid Base Salt

NCERT Solution

Question: 1. State differences between acids and bases.


Sour in taste.Bitter in taste.
Turns blue litmus paper to red.Turns red litmus paper to blue.
No effect on turmeric paper.Turns turmeric paper to red.
Turns China rose solution to magenta.Turns China rose solution to green.

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

Answer: Since window cleaner turns red litmus paper to blue, it is basic in nature.

Question3: Name the source from which litmus solution is obtained. What is the use of this solution?

Answer: Litmus solution is obtained from lichens. Litmus solution is used to detect the acidic and basic characteristic of a substance.

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

Answer: Distilled water is neutral in character, i.e. it is neither acidic nor basic. Neutral nature of distilled water can be verified by the use of blue and red litmus paper. Distilled water does not change the colour of either blue or red litmus paper.

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

Answer: When the solution of acid is mixed with the solution of base in proper ratio, both of them neutralize the effect of each other and a new substance; called salt; is formed; along with water. This is called neutralization or neutralization reaction.

Example: When the solution of sodium hydroxide (a base) is mixed with the solution of hydrochloric acid (an acid) in proper ratio, both neutralize each other and the reaction mixture so obtained is neutral in character. In this reaction, a new substance sodium chloride (common salt) is formed. The reaction involved in this can be written as follows:

HCl + NaOH → NaCl + H2O

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

(i) Nitric acid turns red litmus blue.
(ii) Sodium hydroxide turns blue litmus red.
(iii) Sodium hydroxide and hydrochloric acid neutralize each other and form salt and water.
(iv) Indicator is a substance which shows different colours in acidic and basic solutions.
(v) Tooth decay is caused by the presence of a base.

Answer: (i) F, (ii) F, (iii) T, (iv) T, (v) F

Question 7: Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labeled. 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 would dip blue and red litmus paper in the sample of each of the bottles.

  • Sample of solution of bottle which turns blue litmus paper red is acidic.
  • Sample of solution of bottle which turns red litmus paper blue is basic.
  • Sample of solution of bottle which does not change the colour of either blue or red litmus paper is neutral in nature.
  • After detecting the acidic, basic and neutral nature of soft drink, Dorji would serve the drink to the customers according to their requirement.

Question: 8. Explain why:

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

Answer: Antacid means substance that works against acid. Antacid tablet is taken in the case of acidity to neutralize the excess acid produced in the stomach. Antacid tablets contain basic substances and hence neutralize the acid produced in the stomach and give relief from acidity.

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

Answer: Calamine solution, which is zinc carbonate, is a base. In the case of ant bites, ant injects an acid; called formic acid; in the skin which causes pain and irritation. By applying calamine solution, it neutralizes the effect of acid inject in the course of ant bite and gives relief from pain.

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

Answer: Most of the factory wastes contain acid. It they are flushed into water as they are, then acid present in them would kill the aquatic organisms; along with creating pollution. Thus, it is necessary to neutralize the factory waste before disposing it into the water bodies.

Question 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: Label the test tubes as A, B and C.

  • Dip the turmeric indicator in test tube A. If the turmeric paper changes to red then the liquid in the test tube is sodium hydroxide; which is a base. Otherwise, it is either hydrochloric acid or sugar solution; which is neutral.
  • If the liquid in the test tube A is not sodium hydroxide then keep it aside and repeat above procedure with the test tube B. If it is not sodium hydroxide then the liquid in test tube C must be sodium hydroxide.
  • Take aside the test tube with sodium hydroxide and pour sodium hydroxide in remaining two test tubes. The test tube which becomes warm contains acid. When acid and base neutralize each other, heat is released in the process.

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

Answer: If blue litmus paper is dipped in a solution and it would remain blue then the solution may be basic or neutral in character.

Blue litmus paper does not change its colour with basic solution and neutral solution.

Question 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 and c (iv) only d

Answer: (iv) only d