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VIII SCIENCE ACIDS AND BASES EVALUATION QUESTION AND ANSWERS
ACIDS AND BASES
TEXTBOOK EXERCISES
I. Choose the best answer.
1. Acids are ________ in taste.
a) sour
b) sweet
c) bitter
d) salty
Ans: a) sour
2. Aqueous solutions of _________ conduct electricity.
a) acid
b) base
c) salt
d) All of these
Ans: c) salt
3. In acidic solutions blue litmus changes into ______ colour.
a) blue
b) green
c) red
d) white
Ans: c) red
4. Base is a substance that gives _____ on dissolving in water.
a) OH-
b) H+
c) OH
d) H
Ans: a) OH-
5. Sodium hydroxide is a ____________
a) acid
b) base
c) oxide
d) alkali
Ans: b) base
6. Red ant sting contains __________
a) acetic acid
b) sulphuric acid
c) oxalic acid
d) formic acid
Ans: d) formic acid
7. Magnesium hydroxides are used for treating_______
a) acidity
b) head pain
c) teeth decay
d) None of these
Ans: a) acidity
8. Acid mixed with base forms ________
a) salt and water
b) salt
c) water
d) No reaction
Ans: a) salt and water
9. We brush our teeth with toothpaste because it is __________ in nature.
a) basic
b) acidic
c) Both a and b
d) None of these
Ans: a) basic
10. In basic solution turmeric indicator paper changes from yellow to __________
a) blue
b) green
c) yellow
d) red
Ans: d) red
II. Fill in the blanks.
1. Benzoic acids are used for ___________
Ans: food preservation.
2. The word sour refers to __________ in Latin.
Ans: acidus
3. Bases are _____________ in taste.
Ans: bitter
4. Chemical formula of calcium oxide is _______
Ans: CaO
5. Wasp sting contains __________
Ans: formic acid.
6. Turmeric is used as a__________
Ans: natural indicator.
7. In acidic solution the colour of the hibiscus indicator paper will change to _____
Ans: pink
III. State true or false. If false, correct the statement.
1. Most of the acids are not soluble in water.
Ans: False
Most of the acids are soluble in water.
2. Acids are bitter in taste.
Ans: False
Acids are sour in taste.
3. Bases are soapy to touch when they are dry.
Ans: False
Bases are soapy to touch only in aqueous medium.
4. Acids are corrosive in nature.
Ans: True
5. All bases are alkalis.
Ans: False
All alkalis are bases but all bases are not alkalis.
6. Hibiscus flower is an example for natural indicator.
Ans: True
IV. Answer briefly.
1. Acid - Define.
The term acid is derived from the Latin word ‘acidus’ which means sour. Thus, the
chemical compounds which have sour taste are generally called acids. All acids contain one or more replaceable hydrogen atoms in their molecules and when dissolved in water they release H+ ions. For example, Hydrochloric acid (HCl)
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.
• Acids are colourless.
• Solutions of acids conduct electricity due to ionisation in water.
3. What are the similarities between acids and bases?
There are certain similarities between acids and bases. Some of them are;
• 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?
Natural indicators are chemical substances which are obtained from the natural resources. Litmus, turmeric juice, China rose petals, red cabbage, grape juice and beetroot juice are the indicators obtained from natural resources.
7. Write any four physical properties of base.
• Bases generally exist in solid states but some bases exist in liquid states also.
E.g. Ammonium hydroxide, calcium hydroxide
• Bases give soapy touch only in aqueous medium not in dry nature.
• Bases are bitter in taste.
• Bases are generally colourless.
V. Answer in detail
1. What are the uses of acids?
• Hydrochloric acid present in our stomach helps in the digestion of food materials.
• 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 deoxy ribonucleic acid (DNA) whereas plants contain ribonucleic acid (RNA).
2. What are the uses of bases?
i) Potassium hydroxide is used to make bathing soaps.
ii) Sodium hydroxide is used to make washing soaps.
iii) Sodium hydroxide is also used in paper industries, textile industries and in the
preparation of medicines.
iv) Calcium hydroxide is used for white washing.
v) Aluminum hydroxide and magnesium hydroxides are used in antacids to cure
acidity problems.
vi) Ammonium hydroxide is used to manufacture fertilizers, nylon, plastics and
rubber.
3. Explain the neutralization reactions in our daily life.
Balancing acids and bases is important for our health and for our environment. We come across various neutralization reactions in our daily life. Let us study about the importance of some of those reactions.
Bee bite:
Whenever bees or red ants bite us they inject an acid called formic acid into our body.
This acid cause burning sensation and pain.
To suppress the pain a suitable base in the form of calcium hydroxide (lime paste available at home) is applied so as 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:
We should brush our teeth twice a day, otherwise 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.
To prevent this we have to neutralize the acid. When we brush with tooth powder or toothpaste containing weak bases, the acid gets neutralized.
So our teeth will be strong and healthy.
Acidity:
Hydrochloric acid present in our stomach helps the digestion of food material along with the enzymes secreted by liver, gallbladder and pancreas.
Sometimes due to excessive production of hydrochloric acid in our stomach we feel a burning sensation in the food pipe and in the chest area.
If this happens again and again ulcers will be formed in the stomach and food pipe, which further aggravates the conditions.
In order to neutralize, antacids which are nothing but weak bases like aluminum and magnesium hydroxides are used. As a result the acidity is removed.
Agriculture:
Acidic soil is not suitable for plant growth.
So 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 indicators from turmeric powder?
-Natural indicators are chemical substances which are obtained from the natural resources.
-By adding a 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.
-It remains yellow.
-In basic solution the colour changes from yellow to red.
VI. Higher Order Thinking Questions.
1. Vinu and Priyan take their lunch at school. Vinu eats lemon rice and Priyan eats curd rice. Both lemon rice and curd rice are sour in taste. What is the reason?
-Both lemon rice and curd rice are acidic in nature.
-Both lemon rice and curd rice are sour in nature.
-The citric acid in lemon and the lactic acid in curd are the cause of sourness.
2. Heshna and Keerthi are friends. Keerthi’s teeth are white without caries, but Heshna has teeth with caries. Why? How is it formed?
-Bacteria in our mouths break down food particles trapped between the teeth and thereby produce acid. This leads to tooth decay.
-To prevent tooth decay we need to brush our teeth twice a day.
-The acid is neutralized by brushing the teeth with low-alkaline toothpaste or toothpaste to neutralize the acid produced by the bacteria. Thus, the teeth will be strong and healthy.
-Excessive consumption of soft drinks may also damage the teeth.
-Improper brushing or consuming too much acidic foods may be the reasons for Heshna’s tooth decay.
