NCERT Solutions for Class 7 Science Chapter 5 Acids Bases and Salts

Exercises

Question:- 1. State differences between acids and bases.

Answer:-

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

Answer:- Ammonia is basic in nature.

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

Answer:- Litmus solution is obtained from lichens. It’s used to determine whether a solution is acidic or basic.

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

Answer:- Distilled water will be neutral. We can prove it by demonstrating that neither blue nor red litmus paper change its colour when dipped in it .

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

Answer:- Neutralisation is the reaction between an acid and a base. With the evolution of heat, salt and water are produced in this process.

For example:-

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

Question:- 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)

Answer:- (i) Nitric acid turn red litmus blue. ( False )

(ii) Sodium hydroxide turns blue litmus red. ( False )

 (iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. ( True )

(iv) Indicator is a substance which shows different colours in acidic and basic solutions. ( True )

(v) Tooth decay is caused by the presence of a base. ( False )

Question:- 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:- Dorji can make the following decision with the use of litmus paper:

• The drink which turn a red litmus to blue would be basic.
• If the drink changes colour from blue to red, it is acidic.
• The drink which affect neither red nor blue litmus would be neutral.

Question:- 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.

Answer:- (a) We take an antacid, such as milk of magnesia, to neutralise the stomach’s excess acid.

(b) When an ant bites, it injects an acidic liquid (Formic acid) into the skin, causing inflammation. By rubbing, the acid’s action can be neutralised. Calamine solution comprises zinc carbonate, a relatively weak base that has no negative effects on the skin.

(c) Acids can be found in factory waste. If acids are dumped in a body of water, they will harm the organisms. As a result, industry waste is neutralised by the addition of basic chemicals.

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:- Name of the substances effect on turmeric indicator

• Hydrochloric acid will turn Yellow to blue.
• Sodium hydroxide will turn Yellow to red.
• Sugar solution will show any change.

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

Answer:-  (i) The following observations can be used to identify it: Litmus paper turns blue when it comes into contact with bases. The solution must be simple because the colour of blue litmus paper is unaffected. Even if the solution is neutral, the litmus colour will not change.

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, c and d (iv) only d

Answer:- (iv) only d