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

1. State differences between acids and bases.

Answer:

Acids	Bases
Acids are sour.	Bases are bitter.
Acids turns blue litmus red.	Bases turns red litmus blue.
Acids are not soapy to touch.	Bases are soapy to touch.
Do not change color of turmeric indicator.	Changes color of turmeric indicator to red.
Dark pink color can be observed using china rose indicator	Green color can be observed with china rose indicator
E.g. Citric acid, lactic acid, etc	E.g. Calcium hydroxide, sodium hydroxide

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

Answer: Bases turns red litmus blue. Hence, ammonia is basic in nature as it turns red litmus blue.

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

Answer: The most commonly used indicator is litmus. It is obtained from lichens. It is used as an indicator to identify the nature of things.

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

Answer: Distilled water is neutral in nature. One can verify it by putting drops of distilled water on red as well as blue litmus paper. It can be observed that both the litmus paper do not change color. No color change in both the litmus paper suggest that distilled water is neither acidic nor basic. Thus, it is neutral.

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 reaction. In neutralisation reaction, acid and base reacts to form salt and water. The general form of this reaction can be given as:

Acid + Base ⟶ Salt + Water

For example: Hydrochloric acid (HCl) reacts with sodium hydroxide(NaOH) to form sodium chloride ( NaCl ) and water (H₂O).

HCl + NaOH ⟶ NaCl + H₂O + heat

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

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

Answer: False. [ Nitric acid turn blue litmus red. ]

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

Answer: False. [ Sodium hydroxide turns red litmus blue. ]

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

Answer: True.

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

Answer: True.

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

Answer: False. [ Tooth decay is caused by the presence of acid. ]

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: Soft drinks are drinkable. So, by tasting them dorji can decide which drink is to be served to whom. If the drink is sour in taste it is acidic, if it taste bitter in taste it is basic and if it is tasteless it is a neutral drink. Also, with the help of litmus paper nature of drink ( acidic, basic or neutral ) can be identified. If drink turns blue litmus red then it is acidic, if drink turns red litmus blue then it is basic, if there is no change in colour of litmus paper then it is neutral.

8. Explain why:

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

Answer: During acidity, excess of acid is produced in stomach. Antacid is basic in nature. After taking antacid, acid produced in stomach reacts with base of antacid, thus neutralising the effect of acid, providing relief.

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

Answer: When ant bites, it injects acidic liquid in our skin. Calamine solution is basic in nature. To neutralize the effect of acid, calamine solution is applied on the skin.

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

Answer: Factory waste contains acids. If thrown directly into water bodies, these acid may harm aquatic life. Thus, these waste are neutralised by basic chemicals, before letting it into water bodies.

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: First check all three liquids by putting a drop of each on separate turmeric paper. Liquid that turns turmeric paper red is a base, thus this liquid will be sodium hydroxide. Now, mix a drop of sodium hydroxide solution with a drop of remaining two solutions respectively. Lets put these drops of mixture on turmeric paper. Solution which turns turmeric paper red is sugar solution. This is because, sugar solution is neutral in nature. When mixed with base it itself acts as a base, turning turmeric paper red. The drop of mixture which does not change the color of turmeric paper is hydrochloric acid. This is because, acid reacts with base to form neutral solution.

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

Answer: Blue litmus paper when dipped in a solution, remains blue suggest that, the nature of given solution is either basic or neutral. This is because, basic and neutral solution do not change colour of blue litmus paper. Now, to identify if solution is neutral or basic, again test solution with red litmus paper. If red litmus paper turns blue then solution is of basic nature, if no colour change is observed, then solution is neutral in nature.

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

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