Acids, Bases and Salts
Science Class Seven
NCERT Exercise Solution
Question (1) State differences between acids and bases.
Answer
Acid taste sour while taste of base is bitter.
Acid turns blue litmus paper red while base turns red litmus paper blue.
Acid does not feel soapy when touch, while base feels soapy in touch.
Question (2) Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer
Base turns red litmus paper blue. Since ammonia turns red litmus blue, so it 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 a plant called Lichens.
The litmus solution is used as an indicator to test the acidic or basic nature of a substance.
Question (4) Is the distilled water acidic/basic/neutral? How would you verify it?
Answer
Distilled water is neutral in nature.
Using a litmus paper the nature of distilled water can be verified.
A neutral solution does not change the colour of any of the litmus paper.
Question (5) Describe the process of neutralisation with the help of example.
Answer
When acidic and basic solution are added together in a proper ratio, both neutralises each other. In this process salt and water are formed and heat is evolved.
This is called the process of neutralisation.
Example:
When hydrochloric acid is mixed with sodium hydroxide solution, sodium chloride (common salt) and water are formed, and heat is evolved in this reaction.
Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) ⇒ Sodium Chloride (NaCl) + Water (H2O) + Heat
Question (6) Mark "T" if the statement is true and "F" if it is false:
(i) Nitric acid turns red litmus blue. (T/F)
Answer F (False)
Explanation :
Acid turns blue litmus paper red. Since nitric acid is an acid so it will turn blue litmus paper red and not the red litmus paper blue. If red litmus paper is dipped into nitric acid no change in colour will take place.
Hence, the given statement is false.
(ii) Sodium hydroxide turns blue litmus red. (T/F)
Answer F (False)
Explanation
Base turns red litmus paper blue. Since sodium hydroxide is a base so it will turn red litmus paper blue and not the blue litmus paper red. If a blue litmus paper is dipped into nitric acid no change in colour will take place.
Hence, given statement is false.
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water (T/F)
Answer T (True)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
Answer T (True)
(v) Tooth decay is caused by the presence of a base. (T/F)
Answer F (False)
Explanation: Tooth decay is caused by the presence of acid. This is the cause that toothpaste contains base which neutralises acid present in mouth when teeth are brushed using toothpaste and prevents teeth decay.
Question (7) Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wands basic and third wants neutral drink. How ill Dorji decide which drink is to be served to whom?
Answer:
Dorji will decide the acidity and basicity or neutrality of the soft drinks using litmus papers.
He will take the samples of all the available drinks from each of the bottle and test them using litmus paper.
Sample of drink which will turn blue litmus red is acidic, which turn red litmus blue is basic, and which will not change the colour of either blue or red litmus is neutral drink.
And after detecting the nature of soft drinks, Dorji will serve them as per demand of cutomers.
Question (8) Explain why:
(a) An antacid tablet is taken when you suffer from acidity.
Answer
Antacid is the combination of two words, i.e. anti + acid. Means antacid is a substance which acts against acid, i.e. an antacid neutralises acid.
Antacid contains base which neutralises the cause of acidity, i.e. extra acid produced in the stomach and provide relief to the sick person suffering from acidity.
Thus, an antacid tablet is taken when anyone suffers from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
Answer
While biting ant injects formic acid in the skin of the victim which causes pain.
Calamine solution contains zinc which act as a base. Thus by applying calamine solution over the place of ant bite, it neutralises the acid injected by the ant while biting and gives relief to the victim.
This is the cause that calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralise before disposing it into the water bodies.
Answer
Factory waste contains acid. If it will be disposed without treatment it will turns the water acidic. The aquatic animals and other organisms living in the water would be killed by the acid.
Thus to save the aquatic organisms and to prevent water pollution, factory waste is neutralise before disposing into the water bodies.
Question (9) Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is the sugar solution. How will you identify them? You have only turmeric indicator.
Answer
A basic solution turns turmeric solution of stain reddish brown. This means a base can be detected using turmeric indicator.
Sample of all the three liquids will tested using turmeric solution to separate the basic solution.
After separating of basic solution, there are two solution left, in which one is acidic solution and other is sugar solution.
Now, some of the basic solution will be added separately in each of the samples undetected solutions. After adding the basic solution, sample of solution which will become slightly warm, is acidic solution. Because after adding the basic solution in the acidic solution, neutralisation takes place in which heat is evolved which will turn the solution warm.
And the solution which has temperature remain same is the sugar solution.
Question (10) Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer
Basic solution does not change the colour of blue litmus paper and a neutral solution also does not change the colour of litmus paper.
Thus, if the litmus paper remains blue after dipped in the given solution, this means the solution is either basic or neutral.
Question (11) Consider the following statements:
(a) Both acids and bases changes 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 types of the indicator.
Which of these statements are correct?
(i) All four
(ii) a and b
(iii) b and c
(iv) only d
Answer (iv) only d
Explanation
Statement (a) Both acids and bases changes colour of all indicators.
Both acids and bases does not change colour of all indicators. For example turmeric does not change colour with acid.
Thus, statement (a) is not correct.
Statement (b) If an indicator gives a colour change with an acid, it does not give a change with a base.
This statement is also incorrect, because China Rose solution changes pink with acid and greenish with a base.
Statement (c) If an indicator changes colour with a base, it does not change colour with an acid.
This statement is also incorrect with taking china rose solution as an indicator.
Thus, only option (d) is only the correct option which states "Change of colour in an acid and a base depends on the types of the indicator. "
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