NCERT Class 7 Science Chapter 4 lets us understand the definition of acids and bases. In addition, we also explore their properties, types, and sources. Furthermore, we get to know about natural indicators used to identify acidic and basic solutions. With this knowledge, we will dive deeper into neutralisation reaction and salt formation. To help you understand all these scientific terms, we have tried to explain essential concepts in a simplified manner. In addition, we have also answered all questions following the Class 7 Science Chapter on Acids, Bases, and Salts.
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Acids and Bases
Let us start understanding NCERT Class 7 Science Chapter 4 by learning about acids and bases, their types, and their natural sources.
The term “acid” is derived from the Latin word ‘acere’ which refers to sour. Some of the naturally occurring acidic substances are sour-tasting items like lemon, curd, oranges, and vinegar.
Types and Sources of Acids
Different types of acids are present in the environment. Let us learn more about the kinds of naturally occurring acids mentioned in the NCERT Class 7 Science Chapter 5: Acids, Bases, and Salts, as mentioned in the following table:
|Types of Acids||Sources|
|Ascorbic Acid||Citrus fruits and Amla|
|Tartaric Acid||Raw mangoes, Tamarind, Grapes|
|Citric Acid||Citrus fruits|
Substances which taste bitter and have a soapy texture are termed “bases”. These items are “basic” in nature.
Types and Sources of Bases
Here are some examples of bases and their sources:
|Types of Bases||Sources|
|Magnesium Hydroxide||Milk of magnesia|
|Calcium hydroxide||Lime water|
|Potassium hydroxide or Sodium hydroxide||Soap|
|Ammonium hydroxide||Window cleaner|
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Natural Indicators for Acids and Bases
Now that we have learnt about acids and bases, let us explore different types of natural indicators explained in the NCERT Class 7 Science Chapter 4.
Litmus is derived from Lichens. In distilled water, litmus acquires a mauve or purple colour. It is one of the most commonly utilised natural indicators of acids and bases.
- When added to an acidic solution, litmus turns red.
- Whereas, when it comes into contact with a basic solution or a base, it turns blue.
- On the other hand, when it is added to a neutral solution, it does not change its colour.
As litmus is a commonly used indicator, it is easily available as a solution or a litmus paper. Litmus papers are available in blue and red colours.
Turmeric when comes in contact with a basic solution, turns red due to neutralisation. Whereas, when added to an acidic solution, turmeric does not change colour because this indicator itself is acidic in nature.
China Rose or hibiscus is also used as an indicator for acids and bases.
- China Rose when added to acidic solutions turns these solutions to magenta or dark pink.
- However, when hibiscus is added to a basic solution, the solution turns green.
When an acidic solution and a basic solution are combined, the effects of both solutions are neutralised. When an acid solution and a base solution are combined in the right proportions, both the acidic and basic properties of the solutions are destroyed. The outcome is a neutral solution, neither acidic nor basic. This neutral solution is known as “Salt” and the reaction is known as “Neutralisation Reaction”. In the reaction, heat and water are also released.
Neutralisation Reaction Equation:
|Acid+Base——>Salt+Water (Heat is released in the reaction)|
Example of Neutralisation Reaction:
|Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) ——> Sodium chloride (NaCl) + Water (H2O)|
Neutralisation in Daily Life
We witness neutralisation in our everyday life. Thus, to help you understand the process in our day-to-day life, we have tried to explain all neutralisation reactions in the NCERT Class 7 Science Chapter 4.
Hydrochloric (HCl) acid can be found in our stomachs. It is essential for food digestion. However, excess HCl acid results in indigestion. Occasionally, indigestion hurts. To treat indigestion we can take antacids as these counteract the effects of too much acid.
An ant injects the acidic liquid (formic acid) into the skin when it bites. To neutralise the acidic effect we can rub calamine solution, which contains zinc carbonate, or moist baking soda (sodium hydrogen carbonate) on the affected part.
The soil becomes acidic when chemical fertilisers are used excessively. When the soil is either too acidic or too basic, plants do not grow well. Bases like slaked lime (calcium hydroxide) or quick lime (calcium oxide) are used to cure too-acidic soil. Organic matter (compost) is added to the soil if it is a basic one. Acids are released by organic matter, which balances the soil’s basic character.
Acids are present in the waste of many companies. The acids will destroy fish and other species if they are permitted to enter the water bodies. As a result, adding basic chemicals neutralises the production byproducts.
NCERT Solutions Class 7 Science Chapter 4: Acids, Bases, and Salt
Ques 1. State differences between acids and bases.
Ans: Here are the key differences between acids and bases:
|Sour in taste||Bitter in taste|
|Turn litmus to red||Turn litmus to blue|
|Do not change the colour of turmeric||Turn turmeric to red|
Ques 2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ans: Window cleaners are basic in nature as basic solutions turn red litmus to blue.
Ques 3. Name the source from which litmus solution is obtained. What is the use of this solution?
Ans: Litmus is obtained from Lichens.
Litmus solution tests the acidic and basic nature of any substance. The acidic solution turns blue litmus red, whereas, a basic solution turns red litmus blue.
Ques 4. Is the distilled water acidic/basic/neutral? How would you verify it?
Ans: Distilled water is neutral. To verify it we can use litmus paper. As the distilled water will not change the colour of red or blue litmus paper, it can be proved that it is neutral.
Ques 5. Describe the process of neutralisation with the help of an example.
Ans: When an acid solution and a base solution are combined in the right proportions, both the acidic and basic properties of the solutions are destroyed. The outcome is a neutral solution, neither acidic nor basic. This neutral solution is known as “Salt” and the reaction is known as “Neutralisation Reaction”. In the reaction, heat and water are also released.
Ques 6. Which of the following statements are True or False:
a. Nitric acid turns red litmus blue.
b. Sodium hydroxide turns blue litmus red.
c. Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water.
d. The indicator is a substance which shows different colours in acidic and basic solutions.
e. Tooth decay is caused by the presence of a base.
Ques 7. Dorji has a few bottles of soft drinks in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants an acidic drink, another wants a basic and the third one wants a neutral drink. How will Dorji decide which drink is to be served to whom?
Ans: Dorji will have to taste a few drops of soft drinks. He will observe that the acidic solution tastes sour, the basic solution tastes bitter, and the neutral solution tastes like nothing at all. Thereafter, Dorji can examine the nature of the soft drinks using litmus paper in addition to taste. He should check the acidic solution with blue litmus paper. To test the blue litmus, Dorji must apply a drop of solution. The solution is going to be acidic if it becomes red.
Similar to that, he can test the fundamental solution using red litmus paper. On red litmus, he must dab a drop of the solution. If it becomes blue, the answer will be straightforward.
Ques 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 being disposed it into the water bodies.
a. An antacid tablet has base milk of magnesia which neutralises HCl present in our stomach. Owing to this, it is used when we suffer from acidity.
b. Ants inject formic acid into the skin when they bite. Calamine is made up of zinc carbonate, which is naturally basic. As a result, calamine balances the effects of formic acid to provide relief for the afflicted.
c. As factory wastes are acidic these can have an adverse impact on the aquatic flora and fauna. Thus, to reduce the impact of the acids, the wastes are neutralised by adding a base before disposing of wastes into seas and oceans.
Ques 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 a turmeric indicator.
- When a turmeric indicator does not change its colour, then the solution is hydrochloric (HCl) acid.
- Whereas, if the indicator turns to red the solution is either sodium hydroxide, a base, or a sugar solution, a neutral solution.
Ques 10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Ans: The solution may be basic or neutral as both these will not transform the colour of the blue litmus paper.
Ques 11. Consider the following statements:
a. Both acids and bases change the 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 the above-mentioned statements is correct?
(i) All four
(ii) a and b
(iii) b,c, and d
(iv) Only d
Ans: Option (iv) Only d
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Ans: The term “acid” is derived from the Latin word ‘acere’ which refers to sour. Some of the naturally occurring acidic substances are sour-tasting items like lemon, curd, oranges, and vinegar.
Ans: Acid+Base——>Salt+Water (Heat is released in the reaction)
Ans: Turmeric is acidic in nature. This can be determined by the fact that a turmeric indicator does not change its colour when put in an acidic solution.
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