Chapter 5 Acids, Bases and Salts

Class 7 | Science | Chapter 5 | Acids, Bases and Salts | NCERT Solutions

Question 1. State differences between acids and bases.

Answer 1 

Acids Bases
They are sour in taste They are bitter in taste.
They turn blue litmus red. They turn red litmus blue.
They contain hydrogen ions. They contain hydroxyl ions.
They are not soapy to touch. They are soapy to touch.
Example: HCl, H2SO4 , HNO3 Example: NaOH, KOH

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

Answer 2 Ammonia is basic as it turns red litmus to blue.

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

Answer 3 It is obtained from lichens.It is used as an indicator to distinguish between an acid and base.When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. So, it is used to test the acidic or basic nature of solutions.

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

Answer 4 Distilled water is neutral in nature.Litmus paper can be used to verify the nature of distilled water.Water does not change its colour in either red or blue litmus.

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

Answer 5 The reaction between an acid and a base to form salt and water with the evolution of heat is called neutralisation reaction.

HCl + NaOH ———> H2O + NaCl + heat

When the solution of sodium hydroxide and hydrochloric acid react with each other, the acidic nature of acid and basic nature of base are destroyed.The resulting solution is a neutral solution.In this reaction the hydrogen ions from acids reacts with hydroxide ions of base to form water and salt is formed.

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 6 (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 7 Add few drops of drink on litmus paper and take the decision according to the following:

1) If it turns blue, drink is basic.

2) If it turns red, drink is acidic.

3) If it turns green, drink is 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 8 a) Acidity is the excessive amount of acid in stomach which causes pain and discomfort. Antacids contain base like Milk of Magnesia which react with excess of acid and neutralises its effect and brings relief.

b) When an ant bites it releases or inject formic acid into the skin. Calamine lotion contain zinc carbonate which is basic in nature and neutralises the effect of acid injected by ant bite.

c) Factory waste contain acids.If they are allowed to flow into the water bodies, the acids present in them will cause a serious threat to the aquatic organism.Therefore these waste are neutralised before disposing it into 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 9 When a base is added to turmeric solution, the solution turns into pink colour. However turmeric remains yellow when an acid or neutral solution is added to it. Take few drops from each solution and test it with turmeric solution. If the solution turns into pink colour then that solution is base i.e. Sodium Hydroxide. Take a test tube and add few drops of base solution and second solution. Check if test tube becomes warm and then add turmeric solution to it. If the colour does not change, it means the solution added is acidic (HCl). The test tube becomes warm due to neutralisation. If the test tube of the above solution does not warm and it does show pink colour when turmeric is added, the second solution is neutral

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

Answer 10 If a blue litmus paper when dipped in a solution, remains blue, it implies the solution is either basic or neutral.

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 11 (iv) only d

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