8th Std Science Solution in English | Lesson.14 Acids and Bases

Lesson 14. Acids and Bases

Lesson 14. Acids and Bases

I. Choose the best answer.

1. Acids are ________ in taste. 

1. sour
2. sweet
3. bitter
4. salty

Ans : sour

2. Aqueous solutions of _________ conduct electricity.

1. acid
2. base
3. salt
4. salt and base

Ans : salt and base

3. In acidic solutions blue litmus changes into ______ colour.

1. blue
2. green
3. red
4. white

Ans : red

4. Base is a substance that gives _____ on dissolving in water.

1. OH^–
2. H⁺
3. OH
4. H

Ans : OH

5. Sodium hydroxide is a ____________

1. acid
2. base
3. oxide
4. alkali

Ans : alkali

6. Red ant sting contains __________

1. acetic acid
2. sulphuric acid
3. oxalic acid
4. formic acid

Ans : formic acid

7. Magnesium hydroxides are used for treating_______

1. acidity
2. head pain
3. teeth decay
4. None of these

Ans : acidity

8. Acid mixed with base forms ________

1. salt and water 
2. salt
3. water
4. No reaction

Ans : salt and water 

9. We brush our teeth with tooth paste because it is __________ in nature.

1. basic
2. acidic
3. Both a and b
4. None of these

Ans : basic

10. In basic solution turmeric indicator paper changes from yellow to __________

1. blue
2. green
3. yellow
4. red

Ans : red

II. Fill in the blanks.

1. Benzoic acids are used for ___________

Ans : tasteless

2. The word sour refers to __________ in Latin

Ans : 100^oC

3. Bases are _____________ in taste

Ans : boiling

4. Chemical formula of calcium oxide is _______

Ans : 4^oC

5. Wasp sting contains __________

Ans : sedimentation

6. Turmeric is used as a__________

Ans : sedimentation

7. In acidic solution the colour of the hibiscus indicator paper will change to _____

Ans : sedimentation

III. Say True or False. If false, correct the statement.

1. Most of the acids are not soluble in water.

Ans : True

2. Acids are bitter in taste.

Ans : False

Cu. St. : Acids are bitter in taste. Bases are bitter in taste

3. Bases are soapy to touch when they are dry.

Ans : False

Cu. St. : Bases are soapy to touch only in aqueous media

4. Acids are corrosive in nature

Ans : True

5. All bases are alkalis

Ans : False

Cu. St. : All alkalis are base, but all bases are not alkalis

6. Hibiscus flower is an example for natural indicator.

Ans : True

IV. Answer in brief:

1. Acid – Define.

A substance which contains one or more replaceable hydrogen atoms.

2. Write any four physical properties of acids.

• Acids are sour in taste.
• They are corrosive in nature. Strong acids can spoil substances like human skin, clothes and paper.
• Generally acids exist in liquid state but few acids exist in solid state too. E.g. Benzoic acid.
• Acids are colour less.
• Acids change the colour of the indicators. Blue litmus paper turns red and methyl orange turns pink when treated with acids.

3. What are the similarities between acids and bases?

• They are corrosive in nature.
• They undergo ionization in aqueous solution.
• They conduct electricity in aqueous solution.
• They undergo neutralization reaction.

4.State the difference between acids and bases..

5. What is an indicator?

An indicator or acid – base indicator is a chemical substance which indicates the acidic or basic nature of a solution by suitable colour change.

6. What is a neutralization reaction?

Neutralization is a chemical reaction in which an acid and a base react with each other to form water and salt.

7. Write any four physical properties of base.

• Bases generally exist in solid state but some bases exist in liquid state also. E.g. Ammonium hydroxide, calcium hydroxide.
• Bases give soapy touch only in aqueous media not in dry nature.
• Bases are bitter in taste.
• Bases are corrosive in nature. When come in contact with the skin frequently they form painful blisters.
• Bases also change the colour of the indicators. Red litmus paper turns blue when . treated with bases. Similarly, they turn methyl orange yellow and phenolphthalein pink.

VIII. Answer in detail:

1. What are the uses of acids?

• Hydrochloric acid present in our stomach helps in the digestion of foodstuff.
• Vinegar (acetic acid) is used to preserve food materials.
• Benzoic acid is also used to preserve food materials like pickles.
• Sodium or potassium salts of higher fatty acids are used to make washing and bathing soaps.
• Sulphuric acid is called the king of chemicals. It is an effective dehydrating agent. It is used in various industries to make detergents, paints, fertilizers and many more chemicals.
• Hydrochloric acid, Nitric acid and Sulphuric acid are important laboratory reagents.
• Cells of all living organisms contain the fundamental nuclear material called nucleic acids. Animals have deoxyribo nucleic acid (DNA) whereas plants contain ribo nucleic acid (RNA).

2. What are the uses of bases?

• Potassium hydroxide is used to make bathing soaps.
• Sodium hydroxide is used to make washing soaps.
• Sodium hydroxide is also used in paper industries, textile industries and in the preparation of medicines.
• Calcium hydroxide is used for white washing.
• Aluminum hydroxide and magnesium hydroxides are used in antacids to cure acidity problems.
• Ammonium hydroxide is used to manufacture fertilizers, nylon, plastics and rubber.

3. Explain the neutralization reaction in our daily life.

Ant bite:

Whenever bees or red ants bite they inject an acid called formic acid. These acids cause burning sensation and pain.

To suppress the pain, a suitable base in the form of calcium hydroxide (readily available at home) is applied to neutralise the formic acid.

Wasp bite:

When we are bitten by wasp, we feel the burning sensation and pain. It is due to an alkaline substance injected by the insect. To neutralise the alkalinity, we use vinegar which is an acid.

Tooth decay:

The bacteria present in our mouth decompose the food particles stuck in the gaps between our teeth thereby causing acid formation which leads to tooth decay.

When we brush with tooth powder or tooth paste containing weak bases, the acid gets neutralized.

Acidity:

Excessive production of hydrochloric acid in our stomach causes ulcer in stomach and food pipe.

In order to neutralize, antacids which are nothing but weak bases like aluminum and magnesium hydroxides are used.

Agriculture:

Farmers add lime fertilisers such as powdered lime (CaO), limestone (CaCO₃) or ashes of burnt wood to the soil to neutralise the acidity.

Industries:

Effluents from the industries contain acids such as sulphuric acid. It is treated by adding lime to neutralise it before it is discharged into rivers and streams.

4. How will you prepare natural indicator from turmeric powder?

• By adding small amount of water to turmeric powder, a paste is prepared.
• This is applied on a blotting paper or filter paper and dried.
• These strips are used as indicators to find the nature of the solution.
• In acidic solution, turmeric indicator paper has no change in colour.
• That means, it remains yellow. In basic solution, the colour changes from yellow to red.

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