Questions are taken verbatim from the NCERT textbook; answers were grounded against the chapter's content during generation. Items needing review are marked.
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1Exercises11 questions
Q.1State differences between acids and bases.v
Answer:Acids are sour in taste and turn blue litmus red. Bases are bitter in taste, feel soapy and turn red litmus blue. Acids and bases neutralise each other to form salt and water.
Q.2Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?v
Answer:Ammonia is basic in nature because it turns red litmus paper blue.
Q.3Name the source from which litmus solution is obtained. What is the use of this solution?v
Answer:Litmus solution is obtained from lichens. It is used as an indicator to test whether a substance is acidic or basic.
Q.4Is the distilled water acidic/basic/neutral? How would you verify it?v
Answer:Distilled water is neutral. It can be verified by testing it with both red and blue litmus paper; neither paper changes colour.
Q.5Describe the process of neutralisation with the help of an example.v
SolutionAcid + Base → Salt + Water. For example: HCl + NaOH → NaCl + H₂O.
Answer:Neutralisation is the reaction between an acid and a base in which salt and water are formed and heat may be evolved. Example: hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water.
Q.6Mark '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) 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)v
SolutionNitric acid is acidic and turns blue litmus red. Sodium hydroxide is basic and turns red litmus blue. Tooth decay is caused by acids produced by bacteria in the mouth.
Answer:(i) F
(ii) F
(iii) T
(iv) T
(v) F
Q.7Dorji 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?v
Answer:Dorji can test each drink with indicators such as litmus. The drink that turns blue litmus red is acidic. The drink that turns red litmus blue is basic. The drink that does not change either red or blue litmus is neutral.
Q.8Explain 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.v
Answer:(a) An antacid is basic, so it neutralises excess acid in the stomach.
(b) An ant sting injects formic acid; basic calamine solution neutralises it and gives relief.
(c) Factory wastes may be acidic or basic and can harm aquatic life, so they are neutralised before disposal.
Q.9Three 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.v
Answer:Put a little turmeric indicator in each liquid. Sodium hydroxide is basic, so it turns turmeric reddish-brown. Hydrochloric acid and sugar solution do not give this colour. Now add the reddish-brown basic liquid drop by drop to the two remaining samples; the one that changes the turmeric colour back because of neutralisation is hydrochloric acid, and the other is sugar solution.
Q.10Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.v
Answer:The solution may be basic or neutral. Blue litmus turns red only in an acidic solution; it remains blue in both basic and neutral solutions. To confirm, red litmus should also be tested: a base turns it blue, while a neutral solution does not change it.
Q.11Consider 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 dv
SolutionDifferent indicators show different colour changes in acids and bases. Therefore the observed change depends on the indicator used.