Science NCERT Grade 7, Chapter 5, Acids, Bases and Salts starts with classifying few substances like lemon, common salt, curd etc as sour, bitter etc in a tabulated form. Thereafter, a variety of examples of acids and bases are specified along with its definitions. Here, the term indicator is also introduced. Examples of substances containing acids and bases are listed in a table. Apart from this, different examples of natural indicators are explained along with their effects on different substances. Few natural indicators mentioned in the chapter, Acids, Bases and Salts are litmus, turmeric and China rose. Effect of each of the indicators on different test solutions is analysed with the help of activities whose conclusions are to be noted down in a tabular format. Moreover, the effect of litmus, turmeric and China rose on acid is also recorded. The chapter further moves on to explain, the concept of neutralization with the help equations and examples. Lastly, daily life applications of neutralization like indigestion, ant bite, soil treatment and treating factory wastes are discussed. A short note on DNA, amino acids and fatty acids are provided in the chapter, Acids, Bases and Salts.
The following topics are covered in the chapter, Acids, Bases and Salts:
- Acids and Bases
- Natural Indicators Around Us
- Neutralisation
- Neutralisation in Everyday Life
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Question 1:
State differences between acids and bases.
Answer:
|
- |
Acids |
Bases |
|
(i) |
These are sour to taste. |
These are bitter to taste. |
|
(ii) |
Acids turn blue litmus red. |
Bases do not change the colour of blue litmus. |
|
(iii) |
Acids do not change the colour of red litmus. |
Bases turn red litmus blue. |
|
(iv) |
With china rose indicator, these give dark pink colour. |
Bases give green colour with china rose indicator. |
|
(v) |
Acids do not change the colour of turmeric indicator. |
Bases turn the colour of turmeric indicator to red. |
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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 as it turns the colour of red litmus paper to blue.
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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. A natural dye extracted from lichens is dissolved in distilled water to obtain litmus solution.
It is used as an indicator to distinguish between acids and bases.
-
Solution
Colour of litmus solution
Acidic
Red
Basic
Blue
Neutral
No change
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Question 4:
Is the distilled water acidic/basic/neutral? How would you verify it?
Answer:
Distilled water is neutral in nature. The same can be verified by using red and blue litmus papers. Neither will show a colour change with distilled water. This proves that distilled water is neutral.
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Question 5:
Describe the process of neutralization with the help of an example.
Answer:
The reaction between an acid and a base is known as neutralization reaction. In this reaction, both acid and base cancel each other’s effect. Neutralisation reaction results in the formation of salt and water. During this reaction, energy in the form of heat is evolved.
Acid + Base → Salt + Water + Heat
For example, when sodium hydroxide (NaOH) is added to hydrochloric acid (HCl), sodium chloride (NaCl) and water (H2O) are obtained.
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Question 6:
Mark ‘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)
Answer:
(i) Nitric acid turns red litmus blue. (F)
(ii) Sodium hydroxide turns blue litmus red. (F)
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T)
(v) Tooth decay is caused by the presence of a base. (F)
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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:
The first way to find the nature of the soft drinks is to taste them one after the other. Upon consumption of a few drops of a particular soft drink out of the three, if Dorji finds out that the drink is sour in taste, then it is acidic in nature. The soft drink which is bitter in taste is basic in nature and the one having no taste is neutral in nature.
The second way to accurately confirm the nature of the drinks is by the litmus paper test. Dorji can dip a blue litmus paper in the soft drink which he found, had a sour taste. If the colour of the blue litmus paper changes to red, then it is confirmed that the soft drink is acidic in nature.
Similarly, Dorji can dip a red litmus paper in the soft drink which he found, had a bitter taste. If the colour of the red litmus paper changes to blue, then it is confirmed that the soft drink is basic in nature.
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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) This is because during acidity, an excess of acids is produced in the stomach. An antacid contains base, such as milk of magnesia. These bases react with excess of acids and neutralize their effect, thus giving us relief.
(b) When an ant bites, it injects formic acid into the skin. Calamine solution contains zinc carbonate which is basic in nature. Therefore, it is applied on the skin to neutralize the effect of formic acid.
(c) Factory wastes contain acids. Therefore, these wastes, when thrown directly to water bodies, harm aquatic lives. Hence, these wastes are neutralised with basic chemicals before disposing to water bodies.
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Question 9:
Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and the third is a sugar solution. How will you identify them? You have only turmeric indicator.
Answer:
We will put a drop each of hydrochloric acid, sodium hydroxide, and sugar solution on the turmeric indicator. The liquid which changes the colour of turmeric indicator to red is basic in nature, that is, sodium hydroxide.
Now, we will put a drop of sodium hydroxide on a drop of each of the other two liquids separately. After that, we will put the drops of these mixtures on turmeric indicator. The drop which will change the colour of the turmeric indicator to red contains sugar solution. This is because the mixture of basic and neutral solutions is basic in nature.
On the other hand, the drop which will not change the colour of turmeric indicator contains hydrochloric acid. This is because hydrochloric acid reacts with sodium hydroxide to form a neutral solution.
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Question 10:
Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer:
We know that basic and neutral solutions do not change the colour of blue litmus paper. Since blue litmus remains blue after dipping in the solution, the solution is either basic or neutral in nature.
Put a drop of this solution on a red litmus paper. If it turns blue, then the above solution is basic in nature and if no colour change occurs, then it is neutral.
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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 is correct.
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