TEXTBOOK EXERCISES
I. Choose the best
answer.
1.
Acids are ________ in taste.
a) sour
b) sweet
c) bitter
d) salty
[Answer: (a) sour]
2.
Aqueous solutions of _________ conduct electricity.
a) acid
b) base
c) salt
d) All of these
[Answer: All the above]
3.
In acidic solutions blue litmus changes into ______ colour.
a) blue
b) green
c) red
d) white
[Answer: (c) red]
4.
Base is a substance that gives _____ on dissolving in water.
a) OH−
b) H+
c) OH
d) H
[Answer: (a) OH−]
5.
Sodium hydroxide is a ____________
a) acid
b) base
c) oxide
d) alkali
[Answer: (d) alkali]
6.
Red ant sting contains __________
a) acetic acid
b) sulphuric acid
c) oxalic acid
d) formic acid
[Answer: (d) formic acid]
7.
Magnesium hydroxides are used for treating _______
a) acidity
b) head pain
c) teeth decay
d) None of these
[Answer: (a) acidity]
8.
Acid mixed with base forms ________
a) salt and water
b) salt
c) water
d) No reaction
[Answer: (a) salt and water]
9.
We brush our teeth with tooth paste because it is __________ in nature.
a) basic
b) acidic
c) Both a and b
d) None of these
[Answer: (a) basic]
10.
In basic solution turmeric indicator paper changes from yellow to __________
a) blue
b) green
c) yellow
d) red
[Answer: (d) red]
II. Fill in the blanks.
1. Benzoic acids are used for
2. The word sour refers to acidus in Latin.
3. Bases are bitter in taste.
4. Chemical formula of calcium oxide
is
5. Wasp sting contains
6. Turmeric is used as a
7. In acidic solution the colour of
the hibiscus indicator paper will change to
III. State true or
false. If false, correct the statement.
1. Most of the acids are not soluble
in water.
2. Acids are bitter in taste.
Correct statement: Acids are sour in taste.
Bases are bitter in taste.
3. Bases are soapy to touch when
they are dry.
Correct statement: Bases are soapy to touch only in aqueous media, not in dry nature.
4. Acids are corrosive in nature.
5. All bases are alkalis.
Correct statement: All alkalis are base, but all bases are not alkalis.
6. Hibiscus flower is an example for
natural indicator.
IV. Answer briefly.
1.
Acid - Define.
Answer: A substance which contains one
or more replaceable hydrogen atoms.
2.
Write any four physical properties of acids.
(i) Acids are sour in taste.
(ii) They are corrosive in nature. Strong acids can spoil
substances like human skin, clothes and paper.
(iii) Generally acids exist in liquid state but few acids exist
in solid state too. E.g. Benzoic acid.
(iv) Acids are colourless.
(v) 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?
Answer:
(i) They are corrosive in nature.
(ii) They undergo ionization in aqueous solution.
(iii) They conduct electricity in aqueous solution.
(iv) They undergo neutralization reaction.
4.
State the difference between acids and bases.
Answer: Difference between acids and bases
Acids
They produce H+ ions in water.
They are sour in taste.
Few acids are in solid state.
Acids turn blue litmus paper red.
Bases
They produce OH− ions in water.
They are bitter in taste.
Most of the bases are in solid state.
Bases turn red litmus paper blue.
5. What is an indicator?
Answer: 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?
Answer: 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.
Answer:
(i) Bases generally exist in solid state but some bases exist in
liquid state also.
E.g. Ammonium hydroxide, calcium hydroxide.
(ii) Bases give soapy touch only in aqueous media not in dry
nature.
(iii) Bases are bitter in taste.
(iv) Bases are corrosive in nature. When come in contact with
the skin frequently they form painful blisters.
(v) 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.
V. Answer in detail
1.
What are the uses of acids?
Answer:
(i) Hydrochloric acid
present in our stomach helps in the digestion of foodstuff.
(ii) Vinegar (acetic acid) is used to preserve food materials.
(iii) Benzoic acid is also used to preserve food materials like
pickles.
(iv) Sodium or potassium salts of higher fatty acids are used to
make washing and bathing soaps.
(v) 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.
(vi) Hydrochloric acid, Nitric acid and Sulphuric acid are
important laboratory reagents,
(vii) 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?
Answer:
(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.
Answer:
Ant bite:
(i) Whenever bees or red ants bite they inject an acid called
formic acid.
(ii) These acids cause burning sensation and pain.
(iii) 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
:
(i) When we are bitten by wasp, we feel the burning sensation
and pain. It is due to an alkaline substance injected by the insect.
(ii) To neutralise the alkalinity, we use vinegar which is an
acid.
Tooth
decay :
(i) 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.
(ii) When we brush with tooth powder or tooth paste containing
weak bases, the acid gets neutralized.
Acidity :
(i) Excessive production of hydrochloric acid in our stomach
causes ulcer in stomach and food pipe.
(ii) 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.
Answer:
(i) Turmeric indicator is one of the natural indicator.
(ii) By adding small amount of water to turmeric powder, a paste
is prepared.
(iii) This is applied on a blotting paper or filter paper and
dried.
(iv) These strips are used as indicators to find the nature of
the solution.
(v) In acidic solution, turmeric indicator paper has no change
in colour.
(vi) That means, 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?
Answer:
(i) Curd contains lactic acid. The lactic acid makes curd rice
sour in taste.
(ii) Lemon juice contains citric acid. The citric acid makes
lemon rice sour in taste.
(iii) Generally acids are sour in taste.
2.
Heshna and Keerthi are friends. Keerthi’s teeth are white without caries, but
Heshna has teeth with caries. Why? How is it formed?
Answer:
(i) Caries is caused by the action of acids on the enamel
surface.
(ii) The acid is produced when sugar in foods or drinks react
with bacteria present on the tooth surface.
(iii) Heshna has not cleaned her teeth well after sipping sugary
drinks and snacking.
(iv) She has to brush after meals and before bed.
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