Raju

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 13 Question Answer – Synthetic Fibres and Plastics are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 13 Question Answer – Synthetic Fibres and Plastics

DAV Class 8 Science Ch 13 Solutions – Synthetic Fibres and Plastics

Something To Know

A. Fill in the bleaks:

Question 1.
________ and ________ are examples of natural fibres.
Answer:
Cotton, silk

Question 2.
The simplest molecule of a polymer is known as a ________ .
Answer:
monomer

Question 3.
Rayon is also known as a ________ fibre.
Answer:
regenerated

Question 4.
Nylon is a ________ fibre.
Answer:
synthetic

Question 5.
________ plastics cannot be remoulded again on heating.
Answer:
Thermosetting

B. State true or false for the following statements:

1. Natural fibres are highly resistant to the effects of chemicals like acids and alkalis.
Answer:
False

2. Synthetic fibres absorb very little water
Answer:
True

3. Thermoplastics can be easily remoulded on heating.
Answer:
True

4. The plastic, used for insulating electrical wires, is teflon.
Answer:
False

5. All types of plastics are biodegradable.
Answer:
False

C. Tick (✓) the correct option:

Question 1.
The correct choice, for the box, in the following sequence is-
thermoplastic: PVC; Thermosetting Plastic : [?]
(a) Nylon
(b) Polystyrene
(c) Bakelite
(d) Polythene
Answer:
(c) Bakelrce

Question 2.
Which of the following is also known as the “wonder polymer’?
(a) Nylon
(b) Polystyrene
(c) Polythene
(d) Acrylic
Answer:
(a) Nylon

Question 3.
A highly durable synthetic fibre used for making parachutes and ropes for rock climbing and fishing nets, is-
(a) Rayon
(b) Nylon
(c) Terylene
(d) PET
Answer:
(b) Nylon

Question 4.
Which of the following is NOT one of the characteristics of synthetic fibres?
(a) Easy to clean and dry quickly.
(b) Long lasting lustre.
(c) Biodegradable.
(d) Do not shrink on washing.
Answer:
(c) Biodegradable.

Question 5.
Melamine is a thermosetting plastic; for it one can say thfat-
(a) It can be easily remoulded on heating.
(b) It cannot resist fire.
(c) It has a cross-linked arrangement of molecules.
(d) Its ‘tolerance’ towards heat is very much lower than those of other plastics.
Answer:
(c) It has a cross-linked arrangement of molecules.

Question 6.
Which of the following is a correct statement about synthetic plastics?
(a) Thermoplastics cannot be deformed easily on heating.
(b) Synthetic plastics are biodegradable.
(c) Synthetic plastics get easily affected by acids and alkalis.
(d) Synthetic plastics have quite high tensile strengths.
Answer:
(d) Synthetic plastics have quite high tensile strengths.

D. Answer the following questions in brief:

Question 1.
Define the term synthetic fibres’.
Answer:
Man-made fibre is called synthetic fibre.

Question 2.
Write two uses of rayon.
Answer:
For making ropes for mountaineering, for making parachute

Question 3.
Name two to three articles that can be made from nylon.
Answer:
Socks, shoes, bags

Question 4.
Why are clothes made from synthetic fibres uncomfortable in summer?
Answer:
Synthetic fibre does not allow air to pass through it. It does not absorb water. A cloth made from synthetic fibre prevents the body heat from escaping the body. Due to this, clothes made from synthetic fibres are uncomfortable in summer.

Question 5.
Give two examples each of the thermoplastics and thermosetting plastics.
Answer:
Thermoplastics: Polythene, PVC
Thermosetting Plastics: Melamine, Bakelite

E. Answer the following questions:

Question 1.
How is a ‘regenerated fibre’ different from a true synthetic fibre? Give one example of each of these two types of fibres.
Answer:
When a fibre is made from processing of natural fibres, it is called regenerated fibres. But a true synthetic fibre is not made from natural fibres. Rayon is a regenerated fibre, while nylon is a true synthetic fibre.

Question 2.
Why do clothes made of nylon last longer?
Answer:
Nylon is highly durable and strong. It does not get affected by acids or alkalis. Due to this, nylon clothes last longer.

Question 3.
Write two points of differences between thermosetting plastics and thermoplastics.
Answer:

Thermosetting plastics	Thermoplastics
Cannot be remoulded by heating.	Can be remoulded by heating.
Has cross-linked arrangement of molecules.	Has linear arrangement of molecules.

Question 4.
State any three advantages of synthetic fibres.
Answer:
Following are the three advantages of synthetic fibres:

• They are durable.
• They are strong.
• They are resistant to action by acids and alkalis.

Question 5.
Give your views on the following: ‘Excessive use of plastics is a threat to environment’.
Answer:
Many types of plastic cannot be recycled. Moreover, plastic is non-biodegradable. So, plastic waste keeps on accumulating in our environment. It turns the soil unfertile. It harms stray animals. It chokes the drains. Due to this, excessive use of plastics is a threat to environment.

Question 6.
Why is Bakelite a good choice for making electrical switches and handles of cooking utensils?
Answer:
Bakelite is resistant to high temperature. It is a bad conductor of heat and electricity. Hence, Bakelite is a good choice for making electrical switches and handles of cooking utensils.

Value Based Question

Ankita is a keen and observant student of Science. She felt concerned when she noticed that the lady, who had joined their household for working in their kitchen, was wearing a nylon saree. She told her mother about the danger of using such a saree in the kitchen. Her mother immediately bought two cotton sarees for that lady and politely advised her to wear them only while working in the kitchen.

Question 1.
State the values displayed by Ankita and her mother.
Answer:
Concern for safety of others and safety at home.

Question 2.
Give reason(s) that make it unsafe to use a nylon saree in the kitchen.
Answer:
Nylon is highly inflammable. It can easily catch fire. Hence, a nylon saree is unsafe in kitchen.

Question 3.
Have a ‘group discussion’ in which different students give their suggestions/ opinions about making the kitchen a ‘safe working place’.
Answer:
Do it yourself.

Something To Do

Question 1.
Take three big plastic containers. In the first, add a few pieces from a polythene bag and fill it with garden soil. In the second, add pieces ot paper and fill it with garden soil. In the third, add vegetable waste from your kitchen and fill it with garden soil. Keep the three containers on the roof and water them for 7-10 days. Check after 10 days and find out which material undergoes decomposition faster. Make a project report on the above activity.
Answer:
Do it yourself.

Question 2.
Visit a few families in your neighbourhood and enquire about the kind of clothes they use, the reason for their choice and advantages of using them. Make a short report on your findings.
Answer:
Do it yourself.

Question 3.
Design posters, with ‘catchy slogans’ and ‘relevant pictures’, to spread awareness about the judicious use of plastics.
Answer:
Do it yourself.

DAV Class 8 Science Chapter 13 Solutions – Synthetic Fibres and Plastics Important Questions and Answers

I. Fill in the blanks:

1. ________ fibres come from either plants or animals.
Answer:
Natural

2. ________ is not a truly synthetic fibre.
Answer:
Rayon

3. ________was the first truly synthetic fibre.
Answer:
Nylon

4. Many monomers are joined to form a ________ .
Answer:
Polymer

5. Thermosetting plastics are ________ monomers.
Answer:
cross-linked

II. Tick (✓) the correct answer:

Question 1.
Which of the following is also known as ‘poor man’s silk’?
(a) Rayon
(b) Acrylic
(c) Nylon
(d) PET
Answer:
(a) Rayon

Question 2.
Which of the following resembles wool?
(a) Rayon
(b) Nylon
(c) Acrylic
(d) Terylene
Answer:
(c) Acrylic

Question 3.
Melamine is used for making which of these?
(a) Toys
(b) Cooker handle
(c) Kitchenware
(d) Insulation for electric wire
Answer:
(c) Kitchenware

Question 4.
Bakelite is used for making which of these?
(a) Toys
(b) Electric switches
(c) Kitchenware
(d) Bottles
Answer:
(b) Electric switches

Question 5.
Which of these is not environment friendly?
(a) Paper bag
(b) Jute bag
(c) Cotton bag
(d) Plastic bag
Answer:
(d) Plastic bag

III. Answer the following questions:

Question 1.
What are monomers?
Answer:
A fibre is composed of simple molecules which are joined to form a long chain. These simple molecules are called monomers.

Question 2.
Why is rayon not considered as truly synthetic fibre?
Answer:
Rayon is made from cellulose which comes from plants. Due to this, rayon is not considered as truly artificial fibre.

Question 3.
What are the various uses of rayon?
Answer:
Uses of Rayon:

• It is mixed with cotton to make bed sheets. It is mixed with wool to make carpets.
• Rayon can be easily dyed in a wide range of colours.
• It is used in making apparels, e.g. dresses, jackets, linings, suits, hats, etc.
• It is used in making many industrial products; iibe surgical products and tyre cords.

Question 4.
What are the various uses of nylon?
Answer:
Uses of Nylon:

• Nylon is highly elastic, tensile and has silk-like appearance. Due to this, it is used in carpets and socks.
• Nylon is used in many military applications; like ropes, parachutes, etc.
• It is highly durable and hence is used in making seat belts, tyre cords, clothes, etc.
• It is used for making many household items; like toothbrushes, combs, hooks, etc.
• It is used for making ropes for rock climbing and fishing nets.

Question 5.
What are the advantages of synthetic fibre?
Answer:
Advantages of Synthetic Fibres:

• They have long-lasting lustre.
• They more durable; compared to natural fibres.
• They are easy to clean and dry very quickly.
• They are wrinkle free and do not need ironing.
• They do not shrink on washing.
• They are less expensive than natural fibres.

Question 6.
What are the disadvantages of synthetic fibre?
Answer:
Disadvantages of Synthetic Fibres:

• They have very low melting point and they burn to form small sticking beads at high temperature. They are a potential fire hazard.
• They don’t absorb sweat and hence it is uncomfortable to wear them; especially in summer and rainy seasons.
  Synthetic fibres are not biodegradable and thus cause lot of pollution.

Question 7.
What are linear and cross-linked arrangements of monomer?
Answer:
There are two types of arrangement of monomers in plastics, i.e. linear arrangement and cross-linked arrangement.
(a) Linear Arrangement: In this arrangement, the monomers are arranged in a straight chain. Plastics formed because of linear arrangement are generally known as thermoplastic.
(b) Cross-linked Arrangement: In this arrangement, the monomers are formed from cross-linked chains of monomers. This type of plastic is generally known as thermosetting plastic.

IV. Projects:

Question 1.
Collect threads of cotton, wool, jute and nylon. Try to break them by your hand.
Find out which of them is having high tensile strength.
Answer:
Do it yourself.

Question 2.
Make a poster on ‘Say No to Plastic Bag’.
Answer:
Do it yourself.

These DAV Class 8 Science Notes and DAV Class 8 Science Chapter 2 Notes – Microorganisms: Friends or Foes act as excellent revision resources, particularly in preparation for board exams.

Microorganisms: Friends or Foes Class 8 DAV Notes

→ Microorganisms: Organisms which are invisible to the naked eye but visible under the microscope are called microorganisms. Examples: bacteria, yeast, protozoans and algae.

→ Types of Microorganisms: Microorganisms can be divided into four groups, i.e. bacteria, fungi, protozoa and some algae.

→ Virus: Viruses are unique because they show characteristics of both living and non-living. When they are outside their host’s body, they are non-living. But when a virus enters a host, it becomes living.

→ Habitat of Microorganisms: Microorganisms are found in almost all kinds of environment like ice-cold water, hot springs and in dry, marshy or saline areas. They are also found in the human body and in the bodies of other animals and plants.

→ Microbial Population in Human Body: A large number of microorganisms are present in the human body. They help in many biological activities.

→ Microorganisms are our friends

• ○Lactobacillus is a bacterium which helps in the formation of curd.
• Yeast: which is a fungus helps in fermentation. Fermentation is a process which is used in bakery for making bread and cakes. It is used in wine industry for making alcohol. Batter of dosa and idli are also fermented with the help of yeast.
• Some bacteria are used in biological treatment of sewage. This process is known as bioaugmentation.

→ Importance of Microorganisms in Human Health

• Many microorganisms are present in the alimentary canal of some animals like cows. They help in digestion and absorption of food. Bacteria are present in our large intestine as well. They help in bowel movement.
  Microorganisms are used in production of antibiotics. Penicillin is an antibiotic which is prepared from a fungus Penicillium notatum.
• When a disease causing microorganism enters our body, the body produces antibodies to fight the disease causing microbe.
• Weak strains of many microorganisms are used for producing vaccines. Vaccines help our bod to fight many infections.

→ Microorganisms in Agriculture:
Some bacteria and blue-green algae (cyanobacteria) help in nitrogen fixation. This is important as this is the only way through which a plant can get nitrogenous nutrients. Rhizobium bacteria live in the root nodules of leguminous plants. They help in nitrogen fixation in soil.

→ Use in Energy/Fuel Production: Many microorganisms produce ethanol by fermentation of sugars and produce methane in the biogas plants. Ethanol and methane are used as fuel.

→ Cleaning the environment: Microorganisms decompose dead remains of plants and animals. While doing so, they return the fundamental raw materials to the environment. Bacteria and fungi are the common decomposers.

→ Microorganisms: The Foe

→ Bacteria and tooth decay: Many bacteria live in our mouth. They eat the food particles which remain stuck between the teeth. During eating the food particles, bacteria produce acids which corrode the tooth enamel and it results in tooth cavities in the long run.

→ Pathogens: Disease causing microorganisms are known as pathogens. The pathogens release harmful chemicals called toxins in the body.

→ Food Poisoning: It happens because of consumption of contaminated food. Vomiting, nausea, severe pain in abdomen, diarrhoea and dehydration are the symptoms of food poisoning. Clostridium and Staphylococci are the bacteria which cause food poisoning and Aspergillus is a fungus which causes food poisoning.

→ Food Preservation: The process of treating and handling food to stop or greatly slow down spoilage of food (which is caused by microorganisms) is called food preservation.

→ Goals of Food Preservation: Prevention of growth of microorganism and retarding the oxidaton of fats are the main goals of food preservation.

→ Some common methods of food preservation:

• Drying: Food is usually dried in the sun. This reduces water content and thus prevents the growth of microorganisms in food.
• Preservation using sugar and salt: Sugar and salt reduce water content of the food. Thus, sugar and salt help in food preservation. Jams and pickles are preserved because of sugar or salt.
• Preservation using acetic acid: Acetic acid is used in the form of vinegar. Sometimes, citric acid and phosphoric acid are also used. These acids prevent the growth of microorganisms in food.
• Preservation using chemical preservatives: Sodium benzoate and potassium meta bisulphate (KMS) are some of the chemicals which are used as food preservatives. These chemicals prevent the growth of microorganisms.
• Heat and cold treatment: Milk is boiled before being stored. Boiling kills many microorganisms. Some food items are stored in refrigerator. Low temperature inhibits the growth of bacteria.
• Pasteurization: Milk is quickly heated and then quickly cooled down in this process. Pasteurization kills many microorganisms and increases the shelf life of milk.
• Vacuum packing: Some food items are stored in airtight packets. Lack of air inhibits the growth of microorganisms.
• Canning: Canning involves cooking the food and sealing it in sterile cans or jars. The container is then boiled to kill or weaken the remaining microorganisms.

→ Microbiology: The branch of science in which we study about microorganisms.

→ Commensals: The friendly microorganisms living in our body.

→ Pathogens: Disease causing microorganisms.

→ AIDS: Acquired Immuno Deficiency Syndrome; is a disease which affects the immune system.

→ HIV: Human Immuno Deficiency Virus which is the causative organism of AIDS.

→ Plasmodium: Malaria parasite.

→ Aedis egypti: Carrier mosquito of dengue virus.

→ Female Anopheles: Carrier mosquito of malaria parasite.

→ Communicable Disease: A disease which can spread from one person to another is called communicable disease.

Infectious Diseases
Microorganisms	Diseases
Bacteria	Tuberculosis, diphtheria, cholera, tetanus, typhoid
Viruses	Common cold, influenza, mumps, polio, AIDS, chickenpox
Fungi	Ringworm, athlete’s foot
Protozoa	Malaria, amoebic dysentery, sleeping sickness

Modes of Transmission
Modes of transmission	Diseases
Air	Tuberculosis, common cold, influenza, swine flu
Water	Typhoid, amoebic dysentery, cholera
Soil	Tetanus
Animals	Rabies, malaria

Prevention of Diseases
Methods of prevention	Diseases which can be prevented
Vaccination	Tuberculosis, polio, mumps, tetanus
Using mosquito nets, mosquito repellants, controlling mosquito population	Malaria, dengue
Proper disposal of waste and using disinfected water	Cholera
Maintaining good personal hygiene	Ringworm and athlete’s foot

Microorganisms and Animal Diseases
Disease	Causative Microorganism	Animats infected
Anthrax	Bacteria	Cattle
Foot and mouth disease	Virus	Cattle
Rabies	Virus	Dogs, monkeys
Tuberculosis	Bacteria	Cattle, poultry
Ringworm	Fungi	Cattle, poultry
Aspergillosis	Fungi	Poultry
Canine distemper	Virus	Dogs

Microorganisms and Plant Diseases
Microorganisms	Plant diseases
Virus	Tobacco mosaic virus (appearance of uneven spots and discolouration on leaves)
Bacteria	Citrus canker (lesions on leaves, stems and fruit with raised, brown, water soaked margins)
Fungus	Rust of wheat (rust coloured orange patches on infected parts)
Fungus	Smut of rice (smut balls in grains which become greenish black, burst and infect other grains)
Fungus	Red rot of sugar cane (dull red colour; interrupted by occasional whitish patches across the stalk; on stem, elongated red lesions on midribs of leaves)

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 14 Question Answer – Reproduction in Animals are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 14 Question Answer – Reproduction in Animals

DAV Class 8 Science Ch 14 Solutions – Reproduction in Animals

Something To Know

A. Fill In The Blanks:

Question 1.
Genetically identical individuals are produced by ________ reproduction.
Answer:
asexual

Question 2.
________ are the specialised cells that take part in sexual reproduction.
Answer:
Gametes

Question 3.
An animal, that produces both male and female gametes, is known as a ________ animal.
Answer:
hermaphrodite

Question 4.
A fertilised egg is also known as the ________ .
Answer:
zygote

Question 5.
The incubation period for a hen’s egg is ________ .
Answer:
21 days

B. Match the following:

Column I	Column II
(1) Hydra	(a) Hermaphrodite
(2) Reptiles	(b) Cal cite
(3) Earthworm	(c) Budding
(4) Gills	(d) Oviparous
(5) Egg shell	(e) Tadpole

Answer:

Column I	Column II
(1) Hydra	(c) Budding
(2) Reptiles	(d) Oviparous
(3) Earthworm	(a) Hermaphrodite
(4) Gills	(e) Tadpole
(5) Egg shell	(b) Calcite

C. Tick (✓) the correct option:

Question 1.
The site of fertilisation, in humans, is the-
(a) Uterus
(b) Oviduct
(c) Vagina
(d) Ovary
Answer:
(b) Oviduct

Question 2.
The reason, why parrot does not belong to the same group as that of monkey, man, cat, is that-
(a) It is not oviparous
(b) It undergoes internal fertilisation it is oviparous
(c) It undergoes external fertilisation.
(d) It is viviparous
Answer:
(a) It is not oviparous

Question 3.
An organism, which reproduces by Budding, is-
(a) Plasmodium
(b) Amoeba
(c) Yeast
(d) Spiro,gya
Answer:
(c) Yeast

Question 4.
A caterpillar develops into a silk moth through the process of-
(a) Fertilise tion
(b) Metamorphosis
(c) Hatching
(d) Parthenogenesis
Answer:
(b) Metamorphosis

Question 5.
A foetus can be best defined as-
(a) A fertilised egg
(b) A developing embryo
(c) A well developed embryo
(d) An unfrrtiliscd egg
Answer:
(c) A well developed embryo

D. Answer the following questions in brief:

Question 1.
Distinguish between asexual and sexual reproduction.
Answer:

Asexual Reproduction	Sexual Reproduction
A single parent is involved.	Two parents are involved.
Gametes are not formed.	Gametes are formed.
Is a fast process.	Is a slow process.
Offsprings are identical to parent.	Olisprings show some variations from parents.

Question 2.
How is reproduction carried out in Hydra?
Answer:
By Budding

Question 3.
How is fertilisation in a hen different from fertilisation in a frog?
Answer:
Internal fertilisation takes place in a hen, while external fertilisation takes place in a frog.

Question 4.
State the difference between oviparous and viviparous animals.
Answer:
Oviparous animals lay eggs, while viviparous animals give birth to young one.

Question 5.
Why is the number of eggs, laid by frogs at a time, much higher than those laid by a hen?
Answer:
A frog lays eggs in water, where eggs are exposed to predators and water current. But a hen lays eggs on land and takes care of them during incubation. Most of the eggs of a frog are destroyed by the time tadpoles come out of some of them. Hence, a frog needs to lay thousands of eggs. This is not the case with a hen.

Question 6.
How will you differentiate between a zygote and an embryo?
Answer:
A zygote is unicellular, while an embryo is multicellular.

Question 7.
It is the mother who gives birth to a child. How do children, then, get features of both the parents?
Answer:
A child gets genes from its father and mother. Hence, a child gets features from both the parents.

E. Answer the following questions:

Question 1.
Define the term ‘Metamorphosis’. List the changes that a tadpole undergoes to develop into a frog.
Answer:
A marked, and more or less abrupt developmental change in the structure and habit of an animal, occurring during its life cycle, is called metamorphosis. Metamorphosis in tadpole begins with the development of hind legs followed by front legs. Then the lungs develop. The tadpole then begins to swim to the surface of water to breathe. The intestine gets shortened to adapt to a carnivorous diet. The tail gets absorbed by the body during the last stages of metamorphosis.

Question 2.
Trace the sequence of events that lead to development of a chick from the fertilised egg of a hen.
Answer:
After fertilisation, the zygote starts dividing and moves down the oviduct. As it moves down, a number of protective layers get formed around it. The egg shell is one of these protective layers. The embryo develops inside the egg for around 21 days. Finally the egg shell breaks down and the chick comes out.

Question 3.
Where does fertilisation occur in human beings? How does the zygote formed eventually develop into an infant?
Answer:
Fertilisation occurs in oviduct in human female reproductive system. The zygote immediately divides into two cells. Then these cells divide again and again over the couple of days. This results in formation of a cluster of cells. This cluster of cells is called embryo. It moves down to the uterus and gets implanted in the lining of the uterus. It undergoes further rounds of cell divisions. Gradually, the embryo develops into the foetus. A foetus resembles a human being. After about 40 weeks of gestation, the fully developed child is ready to take birth.

Question 4.
Draw a neat well labelled diagram of the human sperm. Also, write how its shape and size help it in its functioning.
Answer:
The tail of sperm facilitates its swimming. A structure, at the tip of its head releases an enzyme which helps it penetrate the egg; in order to facilitate fertilisation.

The tail of sperm facilitates its swimming. A structure, at the tip of its head releases an enzyme which helps it penetrate the egg in order to facilitate fertilisation.

Question 5.
Draw a neat and well labelled diagram of
(i) the reproductive system of the human female
(ii) the male reproductive system.
Answer:

Question 6.
Write the functions of each of the following parts of the human female reproductive system:
(a) Ovaries
(b) Oviduct
(c) Uterus
Answer:
(a) Produces egg
(b) It is the site of fertilisation
(c) It is the site of implantation and further development of embryo

Value Based Question

Atul’s mother decides to get their garage cleaned. The maid comes running and informs that there is a pigeon nest in one corner having 3-4 eggs in it. She starts to look for the broom when Atul intervenes. He tells his maid that she should remove the nest only after 3 wrecks. His mother agrees to the suggestion and instructs the maid accordingly.

Question 1.
Why did Atul ask the maid to wait for 3 weeks?
Answer:
Atul knows that the eggs would take 3 weeks to produce chicks. Hence, he asked the maid to wait for 3 weeks.

Question 2.
What is the egg shell made up of?
Answer:
Calcite

Question 3.
State the values shown by Atul.
Answer:
Concern for animals

Something To Do

Question 1.
Visit a hospital and find out if there have been any twins, triplets or quadruplets that have been born there in the past years. Find out how these develop. What are the chances of their survival as compared to the normal single child?
Answer:
Do it yourself.

Question 2.
Although most mammals give birth to young ones, there are some that lay eggs. Find out more about such egg-laying mammals and share the information with your classmates.
Answer:
Platypus, Echidna

Question 3.
Prepare a list of five birds commonly found in your city. Find out about the types of nests they make and the time of the year one is most likely to find eggs in their nests. Also, find out the number of eggs laid at one time, the incubation period and the appearance of the eggs, for the live different birds listed by you.
Answer:
Do it yourself.

Question 4.
‘Dolly’ (Sheep) was the first cloned mammal. Find out where and how it was produced. Also, try to find out about the other animals that have been cloned.
Answer:
Do it yourself.

Question 5.
Complete the table given below:

Animal	What is the young one called?	Average Time taken for development of the young one	Average lifespan (years)
1. Lion
2. Cow
3.Horse
4. Elephant
5. Goat
6. Kangaroo
7. Cat
8. Dog
9. Whale

Answer:

Animal	What is the young one called?	Average Time taken for development of the young one	Average lifespan (years)
1. Lion	Cub	108 days	35
2. Cow	Calf	279 to 292 days	22
3.Horse	Foal	330-342 days	40
4. Elephant	Calf	617-645 days	70
5. Goat	Kid	145-155 days	15
6. Kangaroo	Joey	42 days	9
7. Cat	Kitten	64 days	25
8. Dog	Puppy	61 days	22
9. Whale	Calf	480-590 days	80-110 (Blue whale)

DAV Class 8 Science Chapter 14 Solutions – Reproduction in Animals Important Questions and Answers

I. Fill in the blanks:

1. Amoeba reproduces by ________ .
Answer:
Binary fission

2. Yeast reproduces by ________ .
Answer:
Budding

3. Hydra reproduces by ________ .
Answer:
Budding

4. Single parent is involved in ________ reproduction.
Answer:
Asexual

5. Two parents are involved in ________ reproduction.
Answer:
Sexual

6. Sperm is produced by ________ in the male reproductive system.
Answer:
Testis

7. Egg is produced by ________ in the female reproductive system.
Answer:
Ovary

II. Tick (✓) the correct answer:

Question 1.
How many types of gametes are formed during asexual reproduction?
(a) One
(b) Two
(c) Three
(d) Zero
Answer:
(d) Zero

Question 2.
How many types of gametes are formed during sexual reproduction?
(a) One
(b) Two
(c) Three
(d) Zero
Answer:
(b) Two

Question 3.
Which of these animals shows external fertlisation?
(a) Frog
(b) Crocodile
(c) Bird
(d) Mammal
Answer:
(a) Frog

Question 4.
Which of these is an oviparous animal?
(a) Chimpanzee
(b) Rat
(c) Human
(d) Eagle
Answer:
(d) Eagle

Question 5.
How many ovaries are present in the female reproductive system?
(a) One
(b) Two
(c) Four
(d) Eight
Answer:
(b) Two

Question 6.
Which organs works as a passage for semen?
(a) Testes
(b) Sperm ducts
(c) Penis
(d) Scrotum
Answer:
(c) Penis

III. Answer the following questions:

Question 1.
What do you understand by asexual reproduction?
Answer:
When a single parent is involved and no gamete formation takes place, the reproduction is called asexual reproduction.

Question 2.
What do you understand by sexual reproduction?
Answer:
In sexual reproduction, two parents are involved. Two types of gametes, Le male and female, is formed by the respective parents. The gametes fuse through a process; called fertilisation.

Question 3.
Explain binary fission.
Answer:
Binary Fission: This method is observed in unicellular organisms. During binary fission, the parent cell divides into two daughter cells. Examples: Bacteria and Amoeba.

Question 4.
What are gametes?
Answer:
The specialised cells formed by male and female parents which take part in sexual reproduction are called gametes. The male gamete is called sperm and female gamete is called ova; in multicellular animals.

Question 5.
Which are the different types of fertilisation seen in animals?
Answer:
Types of Fertilisation:
External Fertilisation: When fertilisation takes place outside the animal’s body, it is called external fertlisation. Example: Frogs, fishes.
Internal Fertilisation: When fertilisation takes place inside the animal’s body, it is called internal fertilisation. Examples: humans, crocodiles, birds, etc.

Question 6.
What do you understand by external development?
Answer:
When the embryo develops outside the animal’s body, it is called external development. External development is the norm in case of external fertilisation. In case of internal fertilisation, external development is seen in some animals; which lay eggs.

Question 7.
Explain the structure of a sperm.
Answer:
The sperm is composed of three parts, viz. a head, middle piece and a tail. A structure at the tip of the head produces an enzyme which helps the sperm to penetrate the ovum.

IV. Projects:

Question 1.
Make a list of 10 animals which show external fertlisation.
Answer:
Do it yourself

Question 2.
Make a list of five animals which show metamorphosis.
Answer:
Do it yourself

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 12 Question Answer – Sound are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 12 Question Answer – Sound

DAV Class 8 Science Ch 12 Solutions – Sound

Something To Know

A. Fill in the blanks:

Question 1.
Birds have a ring of cartilage, called ________ in their wind pipes.
Answer:
Syrinx

Question 2.
The SI unit of loudness’ of sound, is ________ .
Answer:
Decibel

Question 3.
The pitch of sound is determined by its ________ .
Answer:
Frequency

Question 4.
Sounds, of frequency more than 20 kHz, are known as ________ .
Answer:
Ultrasound

Question 5.
An irregular, irritating sound is referred to as a ________ .
Answer:
Noise

B. State True or False for the following statements:

1. Human beings, as well as all animals, have a voice box for producing their characteristic sounds.
Answer:
False

2. Of the three mediums, solids, liquids and gases, sound propagates fastest in liquids.
Answer:
False

3. Sound cannot propagate through vacuum.
Answer:
True

4. Light propagates, through vacuum, with a speed that is nearly a million times more than the speed of sound in air.
Answer:
True

5. The frequency of a given sound determines its ‘shrillness’ or ‘flatness’.
Answer:
True

6. Irregular non-periodic vibrations generally produce musical sounds.
Answer:
False

C. Tick (✓) the correct option:

Question 1.
The maximum displacement of a vibrating body, on either side of its mean position, is known as its-
(a) Frequency
(b) Loudness
(c) Amplitude
(d) Pitch
Answer:
(c) Amplitude

Question 2.
The frequency of a given sound is 1.5 kHz. The vibrating body is then-
(a) Completing 1,500 vibrations in one second.
(b) Taking 1,500 seconds to complete one vibration.
(c) Taking 1.5 seconds to complete one vibration.
(d) Completing 1.5 vibrations in one second.
Answer:
(b) Completing 1,500 vibrations in one second.

Question 3.
Sound can propagate through-
(a) Vacuum as well as gases.
(b) Only through gases and liquids.
(c) Only through gases and solids.
(d) All the three states of matter.
Answer:
(d) All three states of matter.

Question 4.
When lightning and thunder take place, they occur-
(a) Together and are also observed together.
(b) One after the other but are observed together.
(c) Together but the thunder is observed a little after the lightning.
(d) Together but thunder is observed a little before the lightning.
Answer:
(d) Together but thunder is observed a little after the lightning.

Question 5.
A given sound is inaudible to the human ear. We can then say that this is so-
(a) Only because its amplitude is too small.
(b) Only because its frequency is below 20 Hz.
(c) Only because its frequency is above 20 Hz.
(d) Because it has any one, or more, of the three characteristics listed above.
Answer:
Only because its frequency is below 20 Hz.

Question 6.
The following are some statements about sound:
(i) Sound propagates faster in steel than in air
(ii) Sounds are produced through vibrations
(iii) Sound propagates faster than light
(iv) Sound does not require a medium ior its propagation Out of these, the correct statements are-
(a) (i) and (ii) only
(b) (i) and (iii) only
(c) (i) and (ii) only
(d) (ii) and (iv) only
Answer:
(c) (i) and (ii) only

D. Answer the following questions in brief:

Question 1.
Name the cause that is regarded as the basic cause of all sounds.
Answer:
Vibrations

Question 2.
Name two musical instruments which produce sound through vibrating strings.
Answer:
Sitar and guitar

Question 3.
Through which state of matter does sound propagate with the
(a) Slowest speed
(b) Fastest speed
Answer:
(a) Gas
(b) Solid

Question 4.
Define the following terms. Also write their SI units.
(a) Amplitude
(b) Time period
(c) Frequency
Answer:
(a) The maximum distance from the mean position to which a vibrating body moves is called amplitude. SI unit of amplitude is meter.
(b) Time taken to complete one oscillation is called time period. The SI unit of time period is second.
(c) Number of oscillations per second is called frequency. The SI unit of frequency is hertz.

Question 5.
Name the characteristic of a vibrating body which determines the- (a) Loudness (b) Pitch of the sound Produced by it.
Answer:
(a) Amplitude
(b) Frequency

Question 6.
The irequencies of the male and female voices in human beings are nearly 1 kHz and 1.5 kHz. Which of the two voices
(a) Is likely to be flatter? (b) Has a higher pitch?
Answer:
(a) Male
(b) Female

Question 7.
When is a given sound regarded as a musical sound or music?
Answer:
When the sound is pleasant it is called music. Musical sounds are produced by regular or periodic vibrations.

E. Answer the following questions:

Question 1.
Explain briefly the mechanism of production of sound in human beings.
Answer:
The lungs force air through the slit between the vocal chords. It causes vibrations in vocal chords and sound is produced.

Question 2.
How does the mechanism of production of sounds in birds differ from that in human beings?
Answer:
Birds have a cartilaginous ring; called syrinx; to produce sound. Humans have voice box (larynx) to produce sound.

Question 3.
Describe a simple experiment to show that sound cannot propagate through vacuum.
Answer:
Take a bell jar, an electric bell, a vacuum pump, battery and wires.

• Fix the electric bell inside the bell jar and connect the bell to the battery.
• Attach the bell jar with vacuum pump.
• Switch on the electric bell to hear its sound.
• With the help of vacuum pump, create vacuum inside the bell jar.
• Now, switch on the electric bell and try to hear its sound.

Observations: When air was present in the bell jar, you could hear the sound of the electric bell. But when vacuum was present in the bell jar, you could not hear the sound of electric bell.
Conclusion: Sound cannot propagate through vacuum.

Question 4.
On a hot summer day, a little mosquito produces a warning sound near your ear by vibrating its wings at the rate of 3,000 wing beats per five seconds. State the frequency, in hertz, of this sound.
Answer:
Given number of oscillations = 3,000, time = 5 s Frequency = Number of oscillations/time = 3,000/5 = 600 Hz

Question 5.
Explain why we always hear the thunder a little after we see the flash of lightning.
Answer:The speed of light is more than the speed of sound. Due to this, we always hear
the thunder a little after we see the flash of lightning.

Question 6.
(a) Define “loudness’ and ‘pitch’.
(b) State the effect on the
(i) Loudness of a sound when the amplitude of the vibrating body increases.
(ii) Pitch of a sound when the frequency of the vibrating body increases.
Answer:
The intensity of sound is called its loudness. Loudness increases with increase in amplitude.
Pitch: The shrillness or flatness of sound is called its pitch. Pitch increase with increase in frequency.

Question 7.
Why is noise pollution regarded as harmful for human beings?
Answer:
Noise pollution results in headache, hypertension and temporary loss of hearing. Hence, noise pollution is regarded as harmful for human beings.

Value Based Question

It was Shikha’s birthday. Her friends and cousins came to her house to wish her and to celebrate the day. They all wanted to dance and enjoy, with loud music in the background. Shikha, however, kept the music only at a moderate level. She was aware of the harms and dangers of noise pollution. She also did not want to cause any discomfort and irritation to the old lady, in the neighbourhood, who was recovering from her illness. She was of the firm view that our happiness and celebrations should not cause any inconvenience and discomfort to others.

Question 1.
State the values displayed by Shikha.
Answer:
Shikha shows concern for others.

Question 2.
Why is ‘noise pollution’ undesirable? Suggest some ways of minimizing it.
Answer:
Noise pollution causes headache, hypertension and temporary loss of hearing. Hence, it is undesirable. Some ways of minimizing noise pollution are as follows:

• Installing sound barriers along the roads and bridges.
• Banning loud music in residential areas.
• Avoiding unnecessary honking of horns.

Question 3.
Have a ‘group discussion’ in which different students think of situations in which they can have their ‘happiness calibrations’ without causing discomfort, or inconvenience to others.
Answer:
Do it yourself.

Something To Do

Question 1.
Take some steel or porcelain bowls of the same size. Put different amounts of water in all the bowls. Beat them slowly with a spoon one by one. You will hear different types of sound from different bowls. This is your Jaltarang.
Answer:
Do it yourself.

Question 2.
Identify the sources of noise pollution in your locality. Suggest ways and means to control the noise pollution caused by them. Discuss your report with your friends, parents and teachers.
Answer:
Do it yourself.

Question 3.
Musical instruments are categorized into three main types- Stringed (Tantu vadya) instruments, wind (Sushir vadya) instruments and membrane (Avnaddhu vadya) instruments. Make a list of some well known musical instruments of each type and try to understand the way they work.
Answer:
Do it yourself

Question 4.
Solve the crossword puzzle.

Across →	Down ↓
1. Sound of frequencies above 20 kHz.	1. Form of energy produced due to vibration.
2. Amplitude of sound.	2. Voice box
3. Ring of cartilage.	3. SI unit of frequency
4. Unpleasant sound.	4. Sound of frequencies below 20 Hz.
5. Reciprocal of time period.	5. Shrillness or flatness of sound.

Answer:

DAV Class 8 Science Chapter 12 Solutions – Sound Important Questions and Answers

I. Fill in the blanks:

1. Sound is a form of ________ .
Answer:
Energy

2. Voice of females is of ________ pitch.
Answer:
High

3. Voice of males is of ________ pitch.
Answer:
Low

4. Sound of guitar is produced because of vibrations in ________ .
Answer:
String

5. Sound of table is produced because of vibrations in ________ .
Answer:
Membrane

II. Tick the correct answer:

Question 1.
Which of these is the SI unit of amplitude?
(a) Metre
(b) Metre per second
(c) Hertz
(d) Decibel
Answer:
(a) Metre

Question 2.
Which of these is the SI unit of frequency?
(a) Metre
(b) Metre per second
(c) Hertz
(d) Decibel
Answer:
(c) Hertz

Question 3.
Decibel is the unit of which of these?
(a) Loudness of sound
(b) Noise
(c) Amplitude
(d) Pitch
Answer:
(a) Loudness of sound

Question 4.
What is the frequency of infrasonic sound?
(a) 20 Hz
(b) 20 Hz to 20,000 Hz
(c) Less than 20 Hz
(d) More than 20,000 Hz
Answer:
(c) Less than 20 Hz

Question 5.
What is the frequency of ultrasonic sound?
(a) 20 Hz
(b) 20 Hz to 20,000 Hz
(c) Less than 20 Hz
(d) More than 20,000 Hz
Answer:
(d) More than 20,000 Hz

III. Answer the following questions:

Question 1.
What is sound?
Answer:
Sound is a form of energy which produces the sensation of hearing in our ears.

Question 2.
How does an insect produce sound?
Answer:
Insects; like mosquitoes and cockroaches produce sound by vibrating their wings.

Question 3.
What is the speed of sound in air?
Answer:
Speed of sound through air is about 330 m/s.

Question 4.
Explain the structure and function of the inner ear in humans.
Answer:
The inner ear is made up of cochlea and semi-circular canals. The sound waves reach the cochlea from where they are sent to the brain via auditory nerve. The brain interprets these signals and we are able to hear sound. The semi-circular canal plays a role in maintaining the balance of the body.

Question 5.
What do you understand by time period?
Answer:
The time taken to complete one oscillation is called time period. One cycle of to and fro motion along a mean position is counted as one oscillation. The SI unit of time period is second.

Question 6.
What do you understand by audible sound?
Answer:
Human beings can hear the sound between 20 Hz to 20,000 Hz. The sound within this range of frequencies is called audible sound.

Question 7.
What do you understand by inaudible sound?
Answer:
Human beings cannot hear sound with frequency below 20 Hz or more than 20,000 Hz. The sound below 20 Hz and more than 20,000 Hz is called inaudible sound.

IV. Projects:

Question 1.
Devise an experiment to show that vibrations produce sound.
Answer:
For this, take a rubber band and stretch it across your fingers. Produce vibrations in the rubber band by striking it. Try to hear the sound.

Question 2.
Device an experiment to show that sound cannot travel through vacuum.
Answer:
For this, conduct the Bell’s experiment in your school laboratory. Take help of your teacher.

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 11 Question Answer – The Human Eye are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 11 Question Answer – The Human Eye

DAV Class 8 Science Ch 11 Solutions – The Human Eye

Something To Know

A. Fill in the blanks:

Question 1.
The ________ of the human eye determines the colour of a person’s eye.
Answer:
iris

Question 2.
The light sensitive screen of the human eye is called ________ .
Answer:
retina

Question 3.
The cone-shaped cells, on the retina, respond to the ________ .
Answer:
bright light

Question 4.
The defect of vision, in which eyeball becomes smaller than its normal size, is called ________ .
Answer:
hypermetropia

Question 5.
A ________ lens is used to correct myopia and a ________ lens is used to correct hypermetropia.
Answer:
convex, concave

B. Tick (✓) the correct option:

Question 1.
The change in focal length of an eye lens, to focus the image of the objects at varying distances, is done by the action of-
(a) Pupil
(b) Iris
(c) Retina
(d) Ciliary muscles
Answer:
(d) Ciliary muscles

Question 2.
The impression of the image formed on the retina of the human eye remains for about-
(a) 1/6th second
(b) 1/24th second
(c) 1/16th second
(d) 1/32nd second
Answer:
(a) 1/16th second

Question 3.
The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This ability of the eye is known as-
(a) Persistence of vision
(b) Far sightedness
(c) Near sightedness
(d) Power of accommodation
Answer:
(d) Power of accommodation

Question 4.
A near sighted person will face difficulty while reading-
(a) A book held in hands
(b) A text message on the mobile phone
(c) A street signboard across the street
(d) An e-mail, on the computer screen
Answer:
(c) A street signboard across the street

Question 5.
The defect of the human eye, in which the eye lens of a person gets progressively cloudy, resulting in blurred vision, is called-
(a) Near sightedness
(b) Far sightedness
(c) Cataract
(d) Night blindness
Answer:
(c) Cataract

Question 6.
The Braille system, for visually challenged persons, is a-
(a) Visual aid
(b) Auditory aid
(c) Tactual aid
(d) Electronic aid
Answer:
(a) Visual aid

C. State true or false for the following statements:

1. Iris is the muscular assembly that controls the size of the pupil.
Answer:
True

2. The eye lens forms a virtual, erect and diminished image of an object, on the retina.
Answer:
False

3. The retina of the human eye has a large number of cone-shaped and rod-shaped cells. The rods respond even in dim light.
Answer:
True

4. Cinematography makes use of the ‘persistence of vision’ of the human eye.
Answer:
True

5. For a normal human eye, the range of vision is from (nearly) 25 cm to infinity.
Answer:
True

6. Myopia is caused due to excessive curvature of the cornea.
Answer:
True

D. Answer the following questions in brief:

Question 1.
Name the part of the human eye which acts as a protective layer for the eye.
Answer:
Cornea.

Question 2.
The pupil appears black. Why?
Answer:
No light is reflected from pupil. Hence, the pupil appears black.

Question 3.
Differentiate between:
(a) Rod-shaped and cone-shaped cells
(b) Near point and far point of human eye
Answer:
(a) Rod-shaped cells get activated in dim light while cone-shaped cells get activated in bright light.
(b) Near point of human eye is 25 cm, while far point is infinity.

Question 4.
State the difference between
(a) Myopia and hypermetropia
(b) Tactual aid and auditory aid
Answer:
(a)

Myopia	Hypermetropia
Difficulty in seeing distant objects.	Difficulty in seeing nearby objects.
Eyeball gets elongated.	Eyeball gets shortened.
Focal length of eye lens decreases.	Focal length of eye lens increases.
Corrected by concave lens.	Corrected by convex lens.

(b) Tactual aid work on the touch receptors, i.e. skin, while auditory aids rely on our ability to hear. Braille system is an example of tactual aids. Audio caste is an example of auditory aid.

Question 5.
Define the terms
(a) Blind spot on the retina
(b) Persistence of vision of human eye
Answer:
(a) This is the spot where the optic nerve meets the retina. No photosensitive cells are present at this spot. So, no image is formed at this spot. Hence, this is called the blind spot.

(b) An image lasts on the retina for 1/16th second. This is called persistence of vision of human eye. Movies and animation work on this concept.

Question 6.
Give the meaning of terms
(a) Power of accommodation of the human eye
(b) Least distance of distinct vision
(c) Range of vision
Answer:
(a) The ability of the human eye to clearly focus on nearby as well as distant objects.
(b) The minimum distance up to which a normal eye can clearly see.
(c) The angle up to which a normal person can see with his/her eyes.

Question 7.
What happens to the image distance in the eye when the distance of the object from the eye increases?
Answer:
Irrespective of the object distance, the image distance in the eye is always same because image is always formed on the retina in a person with normal vision.

Question 8.
When Sahil tried to read a book by keeping it at a distance of 25 cm, the text print appeared to be blurred.
(a) Is the image of the text getting formed before the retina, or beyond the retina of his eye?
(b) What conclusion can be drawn about the ‘least distance of distinct vision’ of Sahil’s eye?
Answer:
(a) Beyond the retina
(b) The point of least distance of distinct vision in Sahil’s eye has increased significantly.

Question 9.
A person, with a myopic eye, cannot see objects beyond 1.2 m distinctly. What can be said about the likely size of his eyeball? State the nature of the corrective lens that needed to restore his proper vision.
Answer:
His eyeball has become elongated. He needs to use a concave lens to correct his vision defect.

E. Answer the following questions:

Question 1.
Draw a labelled sketch of the human eye. Explain the function of the following parts of the human eye.
(a) Cornea
(b) Iris
(c) Pupil
(d) Eye lens
(e) Ciliary muscles
(f) Retina
(g) Optic nerves
Answer:

(a) Protects the eye.
(b) Adjusts the size of pupil.
(c) Allows light to enter the eye.
(d) Focuses the rays of light or retina.
(e) Controls the variations in focal length of eye lens.
(f) Image is formed on retina.
(g) Carries signals from the eye to the brain.

Question 2.
Explain how we perceive objects as they are?
Answer:
Light rays from an object enter our eyes and pass through the lens. The lens focuses these rays on the retina. An inverted, real and smaller image is formed on the retina. This stimulates photo receptors in the eye. The optic nerve picks signals from the eye and sends it to the brain. The brain interprets the signals and we are able to see an object.

Question 3.
State any five precautions that are needed to take care of our eyes.
Answer:
Following are five precautions to take care of our eyes:

• Do not read in dim light or too bright light.
• Do not keep your book too close to your eyes.
• Don’t look directly at the sun.
• Wash your eyes gently with clean water.
• Take balanced diet.

Question 4.
Explain how optical and non-optical aids help visually challenged persons to develop their skills to a higher level.
Answer:
Optical aids can help a partially visually impaired person. Optical aid can enhance the image or text so that the person can see them with some clarity. Non-optical aids rely on other sense organs like skin or ears. Tactual aids; like Braille rely on the sense of touch. Auditoiy aids; like audio cassettes rely on sense of hearing. These aids can help a visually impaired person to learn how to read and how to communicate effectively.

Question 5.
What is the Braille system? How does it help the visually challenged persons?
Answer:
This is a system of raised dots on paper by which a visually impaired person can read or write. There are 63 characters in the Braille system. It proved to be a boon for the visually challenged because books could be made in this system.

Value Based Questions

When Sumit joined his new school, he soon developed a veiy good understanding and friendship with Amit. Some miscreant students did not like this and succeeded in cre¬ating misunderstanding between them. Their Physics teacher observed their resulting ‘indifferent towards each other’ behaviour.

She called both of them to her room and told them about a (generally) age related defect of the eye. She explained to them that, in this defect, an opaque layer comes over the eye lens and blurs the vision. In the same way, she went on, misunderstanding can blur our thinking and weaken our mutual friendship and trust. She advised them to have a clear and open discussion with each other as this would help in clearing their misunderstandings and reviving their friendship.

Question 1.
What values are displayed by the class teacher in the above situation?
Answer:
The class teacher shows a concern for mental well being of her students.

Question 2.
Name the defect of vision caused by the formation of an opaque layer over the eye.
Answer:
Cataract

Question 3.
Have a group discussion on the importance of interpersonal understanding and trust in friendship.
Answer:
Do it yourself

Something To Do

Question 1.
Make a list of the students of Classes VII and VIII who use spectacles. Find out the nature of lens used and type of defect they have.
Answer:
Do it yourself

Question 2.
The human eye can play tricks on our brain. What we see is not always there, or is it so? There are several ‘optical illusions’ that demonstrate this point. Try to create figures/diagrams which give an impression of the optical illusions.

Answer:
Both lines are of equal length

Question 3.
Survey your neighbourhood. Is there any registered eye bank? Find out how this eye bank functions and helps visually impaired persons.
Answer:
Do it yourself

Question 4.
Make friends with some visually challenged students (It is a known fact that a person, with a sensory handicap, is gifted with some talent). Enquire from them how they master their disability and lead a fairly independent life.
Answer:
Do it yourself

Question 5.
Solve the puzzle with the help of clues given below:

Across →	Down 4 ↓
1. Far sightedness	1. Light sensitive cells.
2. An opening through which light enters the eye.	2. Defect corrected by an appropriate concave lens.
3. The “white’ of the eye.	3. The screen of the eye.
4. Eye lens becomes opaque due to development of a membrane over it.	4. System of raised dots on paper to enable the blind to read.
5. Powerhouse of the eye.	5. Sensory nerve cell which gets activated only in bright light.
6. Dark coloured muscular diaphragm.	6. Focuses the light rays on the retina.

Answer:

Across →	Down 4 ↓
1: hypermetropia	1: photoreceptors
2: pupil	2: myopia
3: cornea	3: retina
4: cataract	4: Braille
5: optic nerve	5: cone
6: iris	6. eye lens

DAV Class 8 Science Chapter 11 Solutions – The Human Eye Important Questions and Answers

I. Fill in the blanks:

1. Human eye is nearly ________ in shape.
Answer:
Spherical

2. The colour of the cornea is ________ .
Answer:
White

3. ________ controls the size of the pupil.
Answer:
Iris

4. The amount of light entering the eye is controlled by the ________ .
Answer:
Pupil

5. Eye lens becomes ________ in case of myopia.
Answer:
Enlarged

II. Tick the correct answer:

Question 1.
The image is formed on which part of the human eye?
(a) Cornea
(b) Iris
(c) Retina
(d) Pupil
Answer:
(c) Retina

Question 2.
Which of these decide the c-olour of a person’s eyes?
(a) Cornea
(b) Iris
(c) Pupil
(d) Retina
Answer:
(b) Iris

Question 3.
What is the maximum distance of clear vision for a normal eye?
(a) 10 m
(b) 100 m
(c) 1000 m
(d) Infinity
Answer:
(d) Infinity

Question 4.
Concave lens is prescribed in which of these conditions?
(a) Myopia
(b) Hyperrnetropia
(c) Cataract
(d) Glaucoma
Answer:
(a) Myopia

Question 5.
Convex lens is prescribed in which of these cases?
(a) Myopia
(b) Hyperrnetropia
(c) Cataract
(d) Glaucoma
Answer:
(b) Hyperrnetropia

III. Answer the following questions:

Question 1.
No image is formed at the blind spot in human eye. Why?
Answer:
No image is formed at the blind spot in human eye because there is no photosensitive cell at this spot.

Question 2.
What do you understand by persistence of vision?
Answer:
The image formed on the retina remains on the retina for about 1/ 16th of a second; even after the removal of the object. This phenomenon is called persistence of vision. Animations and movies take advantage of persistence of vision and we get an illusion of motion pictures.

Question 3.
What happens in case of myopia?
Answer:
In this eye defect, a person is unable to see distant objects clearly. In this condition, the light rays from a distant object converge before the retina; after passing through the eye lens. Due to this, a myopic person is unable to clearly see a distant object.

Question 4.
What happens in case of hyperrnetropia?
Answer:
In this eye defect, a person is unable to see nearby objects clearly. In this condition, the light rays from a nearby object converge behind the retina; after passing through the eye lens. Due to this, a hypermetropic person is unable to clearly see a nearby object.

Question 5.
Define a visually challenged person.
Answer:
A person with limited vision or with complete loss of vision is called a visually challenged person.

IV. Projects:

Question 1.
Make a model of human eye by using Styrofoam. Display it in your classroom.
Answer:
Do it yourself

Question 2.
Make a list of persons in your family and from your relatives who use spectacles.
Answer:
Do it yourself

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 10 Question Answer – Refraction and Dispersion of Light are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 10 Question Answer – Refraction and Dispersion of Light

DAV Class 8 Science Ch 10 Solutions – Refraction and Dispersion of Light

Something To Know

A. Fill in the blanks:

Question 1.
A ray of light, passing from one medium to another, does not bend its path if its angle of incidence equals ________ .
Answer:
90 degree

Question 2.
The more is the optical density of a given medium, the ________ is the speed of light through it.
Answer:
less

Question 3.
The phenomenon of splitting of white light into seven colours is known as the phenomenon of ________ .
Answer:
dispersion of light

Question 4.
The point, on the principal axis of a concave lens, from where a beam of incident parallel ray appears to diverge is called the ________ of the concave lens.
Answer:
focus

Question 5.
An object should be placed at the ________ point so that a convex lens forms its real and inverted image of the same size.
Answer:
2F

B. State True or False for the following statements:

1. The basic cause of refraction is the change in the speed of light as it goes from one medium to another.
Answer:
True

2. In an optically denser medium, the speed of light is more than the speed of light in vacuum.
Answer:
False

3. An (obliquely) incident ray always bends away from the normal when it passes from one transparent medium into another.
Answer:
False

4. A coin, kept at the bottom of an empty dry cup, appears to rise up when some water is poured into the cup.
Answer:
True

5. When white light is dispersed by a glass prism, the yellow colour in it bends more than the blue colour.
Answer:
False

C. Tick (v) the correct option:

Question 1.
Which of the following conditions is not necessary for a change in the direction of propagation when a light ray goes from one medium to another?
(a) The light ray must be incident at an angle greater than 0° with respect to the normal at its point of incidence.
(b) The two media, both must be transparent.
(c) The two media must have different refractive indices.
(d) The incident light rays must always have only the blue colour.
Answer:
(d) The incident light rays must always have only the blue colour.

Question 2.
Which of the following diagrams correctly shows the “blending of a light ray’ as it goes from an optically denser medium into an optically rarer medium?

Answer:

Question 3.
The angle, between the incident ray and the emergent ray, for a rectangular glass slab is-
(a) 0°
(b) 90°
(c) An obtuse angle
(d) A non-zero acute angle
Answer:
(a) 0°; because emergent ray is parallel to incident ray

Question 4.
Which of the following figures correctly represents the passage of white light through a glass prism?

Answer:

Question 5.
An incident ray, passing through the optical centre of a concave lens, after refraction through it, will-
(a) move parallel to its principal axis
(b) pass through its principal focus
(c) go undeviated
(d) pass through the 2F point
Answer:
go undeviated

Question 6.
When an object approaches a convex lens, from infinity towards its focus, the image formed by it-
(a) keeps shifting away from the lens
(b) keeps shifting towards the lens
(c) shifts first towards, and then away from the lens
(d) shifts first away, and then towards the lens
Answer:
(a) keeps shifting away from the lens

Question 7.
Tanya’s grandmother needs a lens to read small letters in her book. For this, she should use
(a) a convex lens (of focal length F), kept at a distance between F and 2F from the book.
(b) a concave lens, kept at a distance less than its focal length from the book.
(c) a concave lens, kept at a distance between F and 2F, from the book.
(d) a convex lens, kept at a distance less than its focal length from the book.
Answer:
(d) a convex lens, kept at a distance less than its focal length from the book.

D. Answer the following questions in brief:

Question 1.
Light propagates faster through a Medium A than through another Medium B.
(a) Which of the two media has a higher optical density?
Answer:
Medium B

(b) State the condition under which no change in the direction of propagation of a light ray would take place when it passes from Medium A to Medium B.
Answer:
When the incident ray makes a right angle at the point of incidence.

Question 2.
Why does a pencil appear bent when it is partially dipped in water?
Answer:
When light rays coming from water enter the air, they bend away from normal. Due to this, light rays appear to be coming from a different position than the actual position of the pencil. Due to this, a pencil appears to be bent when it is partially dipped in water.

Question 3.
Trace the path of rays in the following ray diagrams.

Answer:

Question 4.
When a spherical lens is held towards the Sun and a sharp image is formed on a piece of black paper, a hole gets burnt in the black paper, after some time.
(a) Name the lens used in the above activity.
Answer:
Convex lens

(b) What name is given to the distance between the spherical lens and the black paper?
Answer:
Focal length

(c) What is the relative advantage of using a black paper rather than a white paper?
Answer:
Black colour absorbs most of the energy falling on it, while white colour reflects most of the energy falling on it. So, black paper can bum faster than white paper in this experiment.

Question 5.
What kind of lens would form-
(a) An inverted and magnified image of the object?
Answer:
Convex lens

(b) An erect and magnified image of the object?
Answer:
Convex lens

(c) An inverted and diminished image of the object?
Answer:
Convex lens

(d) An erect and diminished image of the object?
Answer:
Concave lens

Question 6.
Draw labeled diagrams to illustrate the differences between the virtual images formed by using
(a) a concave lens
(b) a convex lens.
Answer:

E. Answer the following questions:

Question 1.
A coin is placed at the bottom of a clear glass vessel. An observer moves herself to a position where the coin goes just out of sight of her eyes. Keeping her eyes in that position, she asks her friend to pour some water, gradually into the glass vessel.
(a) What would she observe?
Answer:
Once water is filled in the vessel, the coin becomes visible to the observer.

(b) Name the phenomenon involved in this observation.
Answer:
Refraction of light

(d) Draw a neat labeled diagram to justify your answer.
Answer:

Question 2.
In the following figures, identify the light ray that would be correct emergent/ refracted light ray in each case. Give reason for your answer in each case.

Answer:
In case of Fig (a), P is the emergent ray. This can be shown by following figure.

In case of figure (b) R is the emergent ray because light ray bends towards the normal when it enters a denser medium from a rarer medium.
In case of fig (c), R is the emergent ray because light ray bends away from the normal when it enters a rarer medium from a denser medium.

Question 3.
White light is made to fall on one face of a glass prism. Draw a ray diagram to show the nature of the emergent beam.
Answer:

Question 4.
With the help of a suitable diagram, explain the meaning of terms:
(a) Centres of curvature
(b) Optical centre
(c) Focus
Answer:
Centre of curvature: A lens is generally made up of two curved surfaces. These curved surfaces are parts of spheres. Centres of these spheres are called the centre of curvature. Here, Cl and C2 are the centres of curvature.
Optical Centre: The centre inside the lens is called optical centre. A ray of light passing through the optical centre emerges without deviation.

Focus: When rays of light parallel to the principal axis, they converge at a point in case of convex lens. In case of a concave lens, they rays appear to diverge from a point. This point is called the focus of the lens.

Question 5.
We are given a convex lens of focal length 20 cm. Draw ray diagrams to show the nature, size and position of the image formed when the object is kept at a distance of (a) 55 cm, (b) 35 cm (c) 15 cm
(Note: For drawing ray diagrams, use an appropriate scale.)
Answer:

Question 6.
We are given a concave lens of focal length 15 cm. Draw a ray diagram to show the nature, size and position of the image formed when the object is kept at a distance of (a) 30 cm (b 15 cm (c) 10 cm
(Note: For drawing ray diagrams, use an appropriate scale.)
Answer:

Question 7.
What type of a lens is used as a magnifying glass? How is the object positioned with respect to this lens? Draw the appropriate ray diagram.
Answer:
A convex lens is used as a magnifying glass. Object is kept between F and O to get a magnified image.

Value Based Question

Khushi listened with great attention and interest when her teacher explained the focusing action of a convex lens through the ‘black paper’ experiment. The teacher went on to say that we must have a similar focused and dedicated approach towards the ‘task at hand’ to achieve success in our life.

On reaching home, Khushi shared her teacher’s ideas with her mother. Her mother fully supported her teacher’s suggestion and advised Khushi to do her studies in a focused and concentrated way. She also advised Khushi not to get distracted by messages on social media while she was studying. Khushi promised to do so.

Question 1.
Write the values depicted by (a) Khushi’s teacher (b) Khushi
Answer:
(a) Khushi’s teacher shows the value of focus and dedication to her task.
(b) Khushi shows honesty and a tendency to share ideas with others.

Question 2.
Draw a diagram showing the focusing action of a convex lens in the black paper experiment mentioned above.
Answer:

Question 3.
Have a group discussion with your friends, on the ‘pros and cons’ of social media in our life.
Answer:
Do it yourself

Something To Do

Question 1.
One of the first microscopes invented used a small drop of water as ‘magnifying glass’. The drop was held in a smooth round hole in a sheet of metal or stiff plastic. Design your own magnifying glass using a stiff plastic sheet. Examine a small object (as the text on a page) through the water drop. How does changing the size of the drop affect the magnification of the image? Record your observations.
Answer:
Do it yourself

Question 2.
Use two appropriate convex lenses and two cylindrical/cardboard tubes to design and optical telescope.
Answer:
Do it yourself

Question 3.
Take a tub/trough full of water and place a light source (like a water proof pencil torch) inside it. Look at the light beam from above. Now, tilt the pencil torch to change the angle of incidence. Observe carefully what happens? Do you observe any refracted beam when you increase the angle of incidence beyond a certain value?
Answer:
Do it yourself

DAV Class 8 Science Chapter 10 Solutions – Refraction and Dispersion of Light Important Questions and Answers

I. Fill in the blanks:

1. Glass is optically ________ than water.
Answer:
denser

2. Air is optically ________ than water.
Answer:
rarer

3. The speed of light is highest in ________ .
Answer:
vacuum

4. At least one surface of the lens should be ________ .
Answer:
spherical

5. A concave lens always makes ________ erect and smaller image of an object.
Answer:
virtual

II. Tick the correct answer:

Question 1.
How many colours mix together to make white light?
(a) 5
(b) 7
(c) 9
(d) 11
Answer:
(b) 7

Question 2.
Which phenomena is used in the formation of the rainbow?
(a) Reflection of light
(b) Refraction of light
(c) Dispersion of light
(d) None of these
Answer:
(c) Dispersion of light

Question 3.
Which of the object is used for the dispersion of light?
(a) Glass slab
(b) Prism
(c) Convex lens
(d) Concave lens
Answer:
(b) Prism

Question 4.
Which of these is used as a magnifying glass?
(a) Glass slab
(b) Prism
(c) Convex lens
(d) Concave lens
Answer:
(c) Convex lens

Question 5.
Which type of image do we see when we are reading through a magnifying glass?
(a) Real and inverted
(b) Real and erect
(c) Virtual and inverted
(d) Virtual and erect
Answer:
(d) Virtual and erect

III. Answer the following questions:

Question 1.
Define refraction of light.
Answer:
When a ray of light travels from one transparent medium to another transparent medium, it changes its direction of propagation. This phenomenon is called refraction of light.

Question 2.
What do you understand by refractive index? Explain.
Answer:
Refractive Index: The ratio of speed of light; in a given pair of media gives the value of relative refractive index for the given pair of media.

Relative refractive index of medium 2 with respect to medium 1
_²1μ = \(\frac{\text { Speed of light in medium 1 }}{\text { Speed of light in medium 2 }}=\frac{v_1}{v_2}\)
The refractive index or absolute refractive index of a medium (m), denoted by (μ_m) is given as follows:
μ_m = \(\frac{\text { Speed of light in vacuum C }}{\text { Speed of light in the given medium }}=\frac{C}{v_m}\)

Question 3.
State the various rules of refraction.
Answer:
When a ray of light travels (obliquely) from an optically rarer medium to an optically denser medium, it bends towards the normal.
When a ray of light travels (obliquely) from an optically denser medium to an optically rarer medium, it bends away from the normal.

Question 4.
What is dispersion of white light? Why does it happen?
Answer:
When white light passes through a glass prism, it segregates into its component colours. This phenomenon is called dispersion of light. The speed seven of light of different colours are different. Different colours deviate differently because of the difference in speed. This is the reason of dispersion of white light when it passes through a glass prism.

Question 5.
How is a rainbow formed?
Answer:
Rainbow is formed just after the rain. When the sunlight passes through water droplets, it gets dispersed. The water droplet works as a prism in this case. Rainbow is formed due to dispersion of light.

IV. Projects:

Question 1.
Take a convex lens and try to find its focal length by forming an image of a distant object on the wall. Take the help of your teacher for doing this activity.
Answer:
Do it yourself

Question 2.
List some uses of concave lens.
Answer:
Do it yourself.

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 9 Question Answer – Crop Production and Its Management are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 9 Question Answer – Crop Production and Its Management

DAV Class 8 Science Ch 9 Solutions – Crop Production and Its Management

Something To Know

A. Give one word for the following statements:

Question 1.
________ results in loosening and mixing of soil.
Answer:
Ploughing

Question 2
________ compete with crops for nutrients.
Answer:
weeds

Question 3.
________ bacteria, present in the root nodules of ________ plants, help in fixing atmospheric nitrogen.
Answer:
Rhizobium, leguminous

Question 4.
Harvested grains are ________ to reduce moisture content.
Answer:
dried

Question 5.
Removal of anthers from the flowers is known as ________ .
Answer:
emasculation

B. Match the following:

Column I	Column II
(1) Sickle	(a) Threshing
(2) Tractor	(b) Storage
(3) Seed drill	(c) Ploughing
(4) Combine	(d) Harvesting
(5) Silo	(e) Sowing

Answer:

Column I	Column II
(1) Sickle	(d) Harvesting
(2) Tractor	(c) Ploughing
(3) Seed drill	(e) Sowing
(4) Combine	(a) Threshing
(5) Silo	(b) Storage

C. Tick the correct option:

Question 1.
The practice of growing leguminous plants between successive cereal crops is called-
(a) Agriculture
(b) Crop rotation
(c) Cultivation
(d) Transplantation
Answer:
(b) Crop rotation

Question 2.
Which among the following is a Kharif crop?
(a) Wheat
(b) Gram
(c) Barley
(d) Rice
Answer:
(d) Rice

Question 3.
The organic substance obtained from dead plants, animals and their wastes is known as-
(a) Manure
(b) Crumb
(c) Fertilizer
(d) Chaff
Answer:
(a) Manure

Question 4.
The process of separating grain from chaff is known as-
(a) Threshing
(b) Weeding
(c) Tilling
(d) Winnowing
Answer:
(a) Threshing

Question 5.
Crops, which are grown during the rainy season, are known as-
(a) Rabi crops
(b) Leguminous crops
(c) Kharif crops
(d) Cereal crops
Answer:
(c) Kharif crops

D. Answer the following questions in brief:

Question 1.
Why is the use of a seed drill more advantageous as compared to sowing seeds by broadcasting?
Answer:
Seed drill helps in saving time and labour. Hence, seed drill is more advantageous than sowing seeds by broadcasting.

Question 2.
Babu wants to practice crop rotation. Help him by suggesting what kind of crops can be grown between two consecutive cereal crops? Also, provide him names of one Rabi and one Kharif crop that can be used to replenish the nitrogen content of the soil.
Answer:
Babu should grow pulses between two consecutive cereal crops. Groundnut (Kharif) and Pea (Rabi) crops can be used to replenish the nitrogen content of the soil.

Question 3.
If rice is sown in the month of November, how is the produce likely to be impacted? And why?
Answer:
If rice is sown in the month of November, it will not get adequate sunshine, temperature and water. This will result in low productivity of rice crop.

Question 4.
What are crumbs? Why should they be broken?
Answer:
Crumbs are small lumps of soil in a field after ploughing. They need to be broken down so that the soil becomes more airy and seeds can easily enter and germinate through the soil.

Question 5.
Define the term ‘transplantation? List any two advantages it offers to the farmer.
Answer:
Planting seedlings on different plot of land is called transplantation. This method is used for paddy. It helps farmers to select only the healthy plants for farming. This also helps in maintaining adequate gap between consecutive plants so that the plants get proper amount of sunshine and other resources.

E. Answer the following questions:

Question 1.
State the advantages of ploughing the soil and also state the different ways through which it is done.
Answer:
The advantages of ploughing the soil are:

• It improves air circulation in soil.
• Roots can penetrate deeper into the soil. Thus, plant can have better anchorage in soil.
• It increases water retaining capacity of soil.
• It helps in removing weeds.
• It helps in growth of microbes in soil that is beneficial for farming.

Different Methods of Ploughing are:
By cattle driven plough, and by tractor drawn cultivator.

Question 2.
Both manures and fertilizers replenish soil fertility. Compare and contrast the advantages and disadvantages of using them.
Answer:

Manure	Fertilizer
Slow to act.	Fast to act.
It does not help in providing specific nutrients to plants.	It helps in providing specific nutrients to plants.
Improves soil composition.	It degrades soil composition.
It has a positive impact on ecosystem.	It may be harmful for ecosystem.

Question 3.
Explain how weeds are harmful for plants practiced by farmers. State the different ways of weed removal practiced by farmers.
Answer:
Weeds compete with plants for various resources like sunlight, air, temperature, water, nutrients, etc. Thus, weeds hamper the growth of crops. Some weeds are poisonous and can prove lethal for cattle and humans. Weeds are removed manually by sickle. Weedicides are also sprayed to get rid of weeds.

Question 4.
Give reasons for the following:
(a) Excess water supply can damage crops.
Answer:
Excess water supply results in waterlogging in the field which obstructs air-supply to the roots thus damage the crops.

(b) Grains are not stored without sun-drying.
Answer:
Grains contain moisture which promotes growth of microbes and pests. Sun-drying helps in removing excess moisture from grain. It helps in preventing the growth of microbes and pest during storage. Hence, grains are not stored without sun-drying.

(c) Overuse of pesticides should be avoided.
Answer:
Overuse of pesticides should be avoided because:

• Pesticides are poisonous and can harm cattle and humans.
• Pesticides may destroy some useful insects and animals which help in pollination.
• Too much of pesticide goes to groundwater and contaminates it.
• Harmful chemicals from pesticides end up in food chain which is harmful for animals and humans.

Question 5.
Bhola’s field is ready with a mature wheat crop. List and explain the steps he would have to undertake before selling the grains to the FCI (Food Corporation of India).
Answer:
Bhola will take following steps before selling the grains:
Harvesting: Cutting and gathering of mature crops is called harvesting. Vegetables and fruits are plucked by hand. Cereals and other crops are harvested by sickle.

Threshing: Separating the grains from husk is called threshing. Small amount of harvest is threshed manually. Threshing machine is used for large amounts. Sometimes, combine harvester is used which does harvesting and threshing simultaneously.

Storage: Grains are dried in sun to remove moisture. After that, grains are stored in silos. Perishable items like fruits and vegetables are sent to cold storage.

Question 6.
Define hybridization. List the steps to be followed for obtaining seeds of a hybrid variety.
Answer:
Cross breeding between two varieties of an organism is called hybridization. Steps for obtaining seeds of a hybrid variety are as follows:

• Two varieties of plants of desired characteristics are chosen, say A and B.
• Anthers from plant A are removed. This process is called emasculation.
• Anthers from plant B are taken and its pollen are dusted on stigmas of plant A.
• After that, stigmas of plant A are covered with plastic bags. This process is called bagging.

Value Based Questions

The biology teacher was quick to observe the careless and time-wasting attitude of some of her students. She explained to them the need and importance of proper management of time and all other resources in our life. She then told them about the steps that are taken to properly manage the crops after they have ‘grown up’. The students realized the importance of her advice through this interesting comparison.

Question 1.
State the values conveyed by the Biology teacher to her students.
Answer:
Importance of time management

Question 2.
Name the steps taken to manage the crops after they have ‘grown up’.
Answer:
Irrigation, Weeding, Harvesting, Storage

Question 3.
Let the students organise a ‘play’, highlighting the importance of proper management of time/our resources, in our life.
Answer:
Do it yourself

Something To Do

Question 1.
Visit a field, or a park and collect (using protective gloves) around five types of weeds.
Answer:
Do it yourself

Question 2.
Make a list of Kharif and Rabi crops which are grown in your area. Find out if there are any silos in your city.
Answer:
Do it yourself

Question 3.
Frogs play an important role in. the biological control of insects in the agricultural fields. The decrease in population of frogs results in the increase of the population of insects. Try to find out the role of lizards, owls and bats in the biological control of pests.
Answer:
Lizards, owls and bats also feed on insects. Thus, these animals help in getting rid of many pests.

Question 4.
Find out about ‘Organic Farming’. Learn about the activities involved, and the characteristics of plants grown, in ‘organic farming’.
Answer:
Farming without the use of synthetic fertilizers and synthetic pesticides/weedicides is called organic farming. Organic farming involves using manure, biological pest control and biological methods of weed control. The produce of organic farming is free of toxic materials. Organic farming is a sustainable method of farming.

Question 5.
Visit a nearby farm and observe the various agricultural practices being carried out there. Find out the names and uses of the various machines that are used on the farm.
Answer:
Do it yourself

Question 6.
Try to find out the ‘changes that have come about’ between ‘modem’ and ‘traditional’ farming practices.
Answer:
Do it yourself

DAV Class 8 Science Chapter 9 Solutions – Crop Production and Its Management Important Questions and Answers

I. Fill in the blanks:

1. Paddy is a ________ crop.
Answer:
Kharif

2. Wheat is a ________ crop.
Answer:
Rabi

3. Kharif crops are grown in the ________ season.
Answer:
Rainy

4. Rabi crops are grown in the ________ season.
Answer:
Winter

5. NPK stands for ________,________ and ________.
Answer:
Nitrogen, Phosphorus, Potassium

II. Tick the correct answer:

Question 1.
Which of these is a kharif crop?
(a) Maize
(b) Wheat
(c) Mustard
(d) Linseed
Answer:
(a) Maize

Question 2.
Which of these is a Rabi crop?
(a) Paddy
(b) Soyabean
(c) Potato
(d) Millet
Answer:
(c) Potato

Question 3.
Which of these crops is grown by transplantation?
(a) Wheat
(b) Maize
(c) Paddy
(d) Pea
Answer:
(c) Paddy

Question 4.
Which of these is an example of pest?
(a) Sparrow
(b) Rat
(c) Grasshopper
(d) All of these
Answer:
(d) All of these

Question 5.
Which of these tools is used for harvesting?
(a) Sickle
(b) Hoe
(c) Spade
(d) Axe
Answer:
(a) Sickle

III. Answer the following questions:

Question 1.
What do you understand by agriculture?
Answer:
The practice of cultivating land for growing crops is called agriculture. It also includes growing plants for other requirements like fibre, medicines, flowers and ornamental plants.

Question 2.
Define agricultural practices.
Answer:
In order to maintain a steady supply of food, farmers undertake several systematic activities, spread over a period of time for growing crops. These activities are called agricultural practices.

Question 3.
What is crop rotation?
Answer:
Some crops like wheat and rice use lot of nitrogen from the soil. This leads to loss of nitrogenous nutrients from the soil. The lost nitrogen can be replenished by growing a leguminous crop in between two cereal crops. This practice is called crop rotation.

Question 4.
Explain harvesting.
Answer:
Cutting and gathering of crops after maturation is called harvesting. Crops like wheat and paddy are harvested by sickles. Fruits and vegetables are plucked with hands. After harvesting, grains need to be separated from the harvested crop. This is done by the process of threshing. Threshing can be done manually or by using animals or by threshing machines. For large scale farming, combine harvester is used. This is a machine which does harvesting and threshing at one go. After threshing, grains are separated from chaff. This is known as winnowing.

Question 5.
Write a brief note on irrigation.
Answer:
The supply of water to the plants in the field is called irrigation. Irrigation is done with water from various sources like rains, canals, waterways, wells and pumps. Some crops need more water, while some others need less water.

IV. Projects:

Question 1.
Make a list of food grains which are available in your home. Group them as Kharif and Rabi crops.
Answer:
Do it yourself

Question 2.
Make a list of different tools which are used by a farmer during various agricultural practices.
Answer:
Some tools are: plough, sickle, axe, spade, seed drill, etc.

The DAV Class 8 Science Book Solutions and DAV Class 8 Science Chapter 8 Question Answer – Conservation of Plants and Animals are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 8 Question Answer – Conservation of Plants and Animals

DAV Class 8 Science Ch 8 Solutions – Conservation of Plants and Animals

Something To Know

A. Fill in the blanks:

Question 1.
International Day for biological diversity is observed on ________ .
Answer:
May 22

Question 2.
A group of organisms, capable of interbreeding under natural conditions, is known as a ________ .
Answer:
species

Question 3.
________ species are the species that are found only in a particular area.
Answer:
endemic

Question 4.
________ is an important reserve for Project Tiger.
Answer:
Sunderbans Biosphere Reserve

Question 5.
________ sanctuary in Odisha is the breeding ground for the ‘giant olive ridley turtle’ which migrate here from the Pacific Ocean.
Answer:
Gahirmatha Turtle Sanctuary

B. Match the following:

Column I	Column II
(1) Rhinoceros	(a) Vulnerable
(2) Giant Panda	(b) Kaziranga National Park
(3) Flamingos	(c) Extinct
(4) Cheetah	(d) Endangered
(5) Passenger pigeon	(e) Migration

Answer:

Column I	Column II
(1) Rhinoceros	(b) Kaziranga National Park
(2) Giant Panda	(d) Endangered
(3) Flamingos	(e) Migration
(4) Cheetah	(a) Vulnerable
(5) Passenger pigeon	(c) Extinct

C. Tick (✓) the correct option:

Question 1.
Protection, and preservation of wildlife and natural resources is known as-
(a) Afforestation
(b) Preservation
(c) Deforestation
(d) Conservation
Answer:
(d) Conservation

Question 2.
The cheetah is a vulnerable species because-
(a) It is found in selected areas.
(b) It will be ‘extinct’ soon.
(c) It will be ‘endangered’ soon.
(d) Its hunting is banned.
Answer:
(c) It will be ‘endangered’ soon.

Question 3.
Plants found in a particular area are referred to as-
(a) Species
(b) Forest
(c) Flora
(d) Fauna
Answer:
(c) Flora

Question 4.
Which one of the following is not an outcome of deforestation in an area?
(a) Increased rainfall
(b) Increased temperature
(c) Decreased rainfall
(d) Decreased soil erosion
Answer:
(d) Decreased soil erosion

Question 5.
A region in which ‘conservation of biodiversity’ in its natural habitat is practiced is known as a—
(a) Botanical garden
(b) National park
(c) Aquarium
(d) Zoo
Answer:
(b) National Park

D. Answer the following questions in brief:

Question 1.
How have ‘urbanisation and industrialisation’ led to a large scale deforestation?
Answer:
Large areas of forest have been cleared to make way for urbanization and industrialization. Thus, urbanization and industrialization have led to a large scale deforestation.

Question 2.
A new species X is introduced in a forest. How is it likely to affect the local species of the area?
Answer:
The new species may prey on, or compete for resources with the existing species. Thus, introduction of a new species is generally harmful for the ecosystem of a forest.

Question 3.
What is a Biosphere Reserve? Name two biosphere reserves in India.
Answer:
Biosphere reserve is an international conservation designation given by the UNESCO under its programme on ‘Man and the Biosphere (MAB)’.
Two examples of biosphere reserves in India are: Nilgiri Biosphere Reserve and Nanda Devi Biosphere Reserve.

Question 4.
How does the ‘Red Data Book’ help in protecting biodiversity?
Answer:
Red Data Book provides the list of animals and plants whose continued existence is under threat. Thus, it helps in identifying the species at risk. Tirriely corrective measures can be taken to protect such species.

Question 5.
Why do some birds and animals migrate?
Answer:
Some birds and animals need to migrate in search of food and breeding places when conditions at a particular place become too harsh to continue living there.

Question 6.
How do birds ‘get helped’ in navigation while undertaking long distance flights?
Answer:
The precise methods by which animals and birds navigate are still obscure. But it appears that the birds and animals take the help of the Sun and stars, and possibly the earth’s magnetic field. This is achieved through an inbuilt ‘magnetic compass’, which is a tiny mass of tissue, between the eye and the brain in birds.

Question 7.
Distinguish between a National Park and a Wildlife Sanctuary.
Answer:
Most of the human activities are prohibited in a National Park. It is profected from human development and pollution. Whereas some human activities are permitted in a Wildlife Sanctuary. Hunting, killing or capturing of animals is prohibited in Wildlife Sanctuary.

E. Answer the following questions:

Question 1.
Explain how deforestation can result in droughts and floods.
Answer:
Trees help in reducing the flow of water. Thus, trees help in preventing soil erosion. Trees also help in rains by making immense contribution to atmospheric moisture by way of transpiration. Deforestation results in reduced and erratic rainfall which often results in droughts. Absence of obstruction from trees also results in excess water flow causing flood.

Question 2.
‘Extinction of tiger can destabilize the entire forest ecosystem and result in a large scale loss of biodiversity. ’ Justify this statement.
Answer:
Tigers are important components of an ecosystem. Being at the top of the food pyramid, tigers check the population of herbivores. This helps in preventing the green plants from being wiped out. Absence of tigers from the ecosystem would result in population explosion of herbivores. This will have disastrous consequences on the ecosystem.

Question 3.
Why are endemic species at a greater risk of becoming extinct? List the “human action related causes’ that are responsible for their extinction.
Answer:
Endemic species are adapted to live in limited areas. Any disturbance to their natural habitat can prove to be dangerous for their existence. Hence, they are at a greater risk of becoming extinct. Human related actions that are responsible for the extinction of endemic species are:

• Farming
• Deforestation
• Overgrazing
• Construction activities

Question 4.
The growing human population poses a grave threat to biodiversity.’ Justify this statement.
Answer:
Growing human population means that more land is required for farming, for making new houses and for making roads and industries. This necessitates deforestation. This also increases industrial activities which result in increased levels of environmental pollution. Thus, it destroys the biodiversity in the long run. Hence, it can be said that growing human population poses a grave threat to biodiversity.

Extinct species	Endangered species	Vulnerable species
These species do not longer exist on the earth.	These species face high risk of extinction in near future.	These species face high risk of extinction in medium term.
Examples: Dodo, passenger pigeon	Examples: Giant panda, Snow leopard	Examples: Cheetah, sloth bear

Question 6.
List five ways in which you, as an individual, can contribute to conserving forests and wildlife.
Answer:
The five ways by which one can contribute to conserving forests and wildlife are:

• Reducing the consumption of various items.
• Reusing old items.
• Recycling discarded items.
• By using public transport.
• By using bicycle wherever possible.

Value Based Question

Anil went to the house of his friend Rohan. They decided to draw a picture of their choice on a sheet of white paper. Not happy with their efforts, they crossed many sheets and threw them around. When Rohan’s elder sister saw them wasting paper, she, in a sweet voice, told them that they should not waste paper, just like that. She not only helped them to draw good pictures but also guided them to make interesting toys from the sheets they had thrown around. She explained to them how, by recycling and reusing paper, they can help in conserving/improving our ecosystem.

Question 1.
State the values displayed by Rohan’s elder sister.
Answer:
Rohan’s sister is polite, helpful and possess knowledge about the judicious use of resources.

Question 2.
Why do we say that wasting paper can damage our ecosystem?
Answer:
Paper is made from wood pulp. Trees need to be cut to make paper. So, wasting paper results in deforestation which harms the ecosystem. Hence, it is said that wasting paper can damage our ecosystem.

Question 3.
Have a group discussion in which each student gives his/her suggestions on recycling/reusing waste paper.
Answer:
Do it yourself

Something To Do

Question 1.
Find out about the different zones of a Biosphere Reserve and their significance. Also, prepare a list of all the Biosphere Reserves in India.
Answer:
Do it yourself

Question 2.
Collect information about ‘Hotspots of Biodiversity’. Find out about the flora and fauna of the ones in India and locate them on the map.
Answer:
Do it yourself

Question 3.
Prepare and present a photo essay on any one of the following topics:
(a) Medicinal plants: Preserving traditional Indian Wisdom.
(b) No good air to inhale: Deteriorating air quality of my city.
(c) Living in harmony; Reducing carbon footprints.
Answer:
Do it yourself

Question 4.
Project Tiger has been taken up on a massive scale by the Indian Government. Visit the website http://www.projecttiger.nic.in to find out what you can do to protect the tiger.
Answer:
Do it yourself

Question 5.
Visit the website http://www.wwfindia.org/help/greenliving_tips/ to gather more information about conservation of plants and animals.
Answer:
Do it yourself

DAV Class 8 Science Chapter 8 Solutions – Conservation of Plants and Animals Important Questions and Answers

I. Fill in the blanks:

1. Trees help in recharge of ________ .
Answer:
Groundwater

2. Greenhouse gases result in ________ .
Answer:
Global warming

3. Olive Ridley turtle is found in the state of ________ .
Answer:
Odisha

4. One-horned rhinoceros is found in the state of ________ .
Answer:
Assam

5. Gir National Park is the home of Asiatic ________ .
Answer:
Lions

II. Tick the correct answer:

Question 1.
Which of these is an extinct species?
(a) Dodo
(b) Blue whale
(c) Lion tailed macaque
(d) Asiatic lion
Answer:
(a) Dodo

Question 2.
Which of these is an endangered species?
(a) Blue whale
(b) Giant panda
(c) Snow leopard
(d) All of these
Answer:
(d) All of these

Question 3.
Which of these is an example of endemic species?
(a) Tiger
(b) Lion
(c) Elephant
(d) Kangaroo
Answer:
(d) Kangaroo

Question 4.
Red Data Book is mainly published by which organization?
(a) WWF
(b) UNESCO
(c) IUCN
(d) Department of Forest and Wildlife
Answer:
(c) IUCN

III. Answer the following questions:

Question 1.
What is an ecosystem?
Answer:
A natural unit consisting of all biotic and abiotic factors in an area is called ecosystem.

Question 2.
What do you understand by conservation?
Answer:
The protection, preservation, management or restoration of wildlife resources is known as conservation.

Question 3.
What is a national park?
Answer:
A national park is a reserve of land which is usually declared and owned by the national government. It is protected from most human development and pollution.

Question 4.
What is a wildlife sanctuary?
Answer:
A wildlife sanctuary is an officially designated territory, which provides protection and sustainable living conditions for wild animals. Hunting, killing or capturing of animals is strictly prohibited in such areas.

Question 5.
What is the difference between endangered species and vulnerable species?
Answer:
An endangered species is at the risk of extinction in near future, while a vulnerable species is at the risk of extinction in the middle term.

IV. Projects:

Question 1.
Find the name of national park or wildlife sanctuary in your state. Make a list of protected animal in this.
Answer:
Do it yourself

Question 2.
Make a project report about roles of WWF (World Wide Fund for Nature).
Answer:
Do it yourself

The DAV Class 8 Science Book Solutions Pdf and DAV Class 8 Science Chapter 7 Question Answer – Combustion are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 7 Question Answer – Combustion

DAV Class 8 Science Ch 7 Solutions – Combustion

Something To Know

A. Fill in the blanks:

Question 1.
A good fuel should have its ignition temperature ________ the room temperature.
Answer:
more than

Question 2.
Burning of cowdung cakes is an example of ________ combustion.
Answer:
slow

Question 3.
The ________ zone of a candle flame, is the coldest.
Answer:
innermost

Question 4.
The non-luminous zone of a candle flame is the ________ part of the flame.
Answer:
hottest

Question 5.
During incomplete combustion of a fuel, ________ gas is formed.
Answer:
carbon monoxide

B. State True or False for the following statements:

1. Coal is an inflammable fuel.
Answer:
False

2. The outermost part of a candle flame contains unburnt carbon particles.
Answer:
False

3. The heat produced, by burning completely one kilogram of a given fuel, is known as the efficiency of that fuel.
Answer:
True

4. All solid fuels have a higher calorific value than liquid and gaseous fluids.
Answer:
False

5. Sulphur dioxide gas, produced during combustion of fossil fuels, causes acid rain.
Answer:
True

C. Tick the correct option:

Question 1.
An example of a non-combustible substance is-
(a) Petrol
(b) Coal
(c) Wood
(d) Iron
Answer:
(d) Iron

Question 2.
A fuel starts burning only when it is heated to-
(a) Its boiling point
(b) Its ignition temperature
(c) Its melting point
(d) A temperature double of the room temperature
Answer:
(b) Its ignition temperature

Question 3.
The highest temperature zone of a candle flame, is its-
(a) Yellow zone
(b) Red zone
(c) Blue zone
(d) Orange zone
Answer:
(c) Blue zone

Question 4.
The amount of heat produced, on complete burning of 10 kg of a given fuel, in pure oxygen, equals H kilojoule. The calorific value, of that fuel, would equal-
(a) (0.1 H) kJ/kg
(b) (10 H) kJ/kg
(c) (H + 10) kJ/kg
(d) (H – 10) kJ/kg
Answer:
(a) (0.1 H) kJ/kg

Question 5.
The gas produced, due to incomplete combustion of carbon fuels, is-
(a) Carbon dioxide
(b) Carbon monoxide
(c) Sulphur dioxide
(d) Nitrogen dioxide
Answer:
(b) Carbon monoxide

D. Answer the following questions in brief:

Question 1.
Define the term ‘combustible material’? Give two examples of combustible materials.
Answer:
A material which can undergo combustion is called a combustible material.
Examples: Wood, coal

Question 2.
State the three conditions that are needed for combustion to take place.
Answer:
Following are the three conditions needed for combustion:

• A combustible substance
• Temperature should be more than ignition temperature
• Oxygen

Question 3.
State the condition under which ‘slow combustion’ of a material takes place.
Answer:
Slow combustion takes place in insufficient supply of air.

Question 4.
Define ‘calorific value of a fuel’.
Answer:
The amount of heat energy produced on burning 1 kg of a fuel is called the calorific value of a fuel.

Question 5.
State any two characteristics of an ideal fuel.
Answer:
The two characteristics of an ideal fuel are:

• It should have high calorific value.
• It should be cheap and easily available.

Question 6.
Why is water not used in controlling electrical fires?
Answer:
Normal water is a good conductor of electricity because it contains some dissolved salt. Using water against electrical fire has the risk of getting electric shock. Hence, water is not used in controlling electrical fires.

E. Answer the following questions:

Question 1.
Draw a diagram showing different zones of candle flame. Label the zone which contains:
(a) Unburnt vapours of wax
(b) Unburnt carbon particles
Answer:

Question 2.
Calorific value of wood is 18,000 kJ/kg. How much of wood is required to produce 360,000 kJ of heat energy?
Answer:
Calorific value of wood = 18,000 kJ/kg
∴ Amount of wood needed to produce 360,000 kJ of heat energy = \(\frac{3,60,000}{18,000}\) = 20 kg

Question 3.
Why are gaseous fuels considered better than liquid or solid fuels?
Answer:
Gaseous fuels are better than liquid or solid fuels because:

1. Gaseous fuels have higher calorific value.
2. These produce less amount of pollutants.
3. It is easier to transport and store gaseous fuels.

Question 4.
Why do we say that ‘burning of fuels’ can cause ‘health hazards’?
Answer:
Burning of fuels causes air pollution. Air pollution is detrimental to our health. Hence, it can be said that burning of fuels can cause health hazards.

Value Based Question

Ameeta was upset because she could not find hear black pencil and her coloured pencils set for completing her painting. Her elder sister, Suneeta helped her to locate them and guided her in colouring her painting. She then told her that we need to be careful as we often need to fulfill a number of conditions for completing our tasks. She gave the example of the phenomenon of ‘combustion’ and told her that combustion can be started and sustained, only if three conditions are simultaneously satisfied.

Question 1.
State the values displayed by Suneeta.
Answer:
Suneeta is helpful and intellectual.

Question 2.
State the three conditions that need to be simultaneously satisfied for starting and sustaining combustion.
Answer:
The three conditions which must be satisfied for starting and sustaining combustion are:
(i) A combustible substance
(ii) Temperature should be more than ignition temperature
(iii) Supply of oxygen or air

Question 3.
Ask the students to think of situations/tasks which need two or more conditions for their completion.
Answer:
Do it yourself

Something To Do

Question 1.
Conduct a survey of houses in your locality and find their average monthly consumption of LPG/PNG.
Answer:
One cylinder is generally sufficient for a household for a month. Students should <A» tkss.

Question 2.
Prepare a report on the precautions that should be taken while using CNG fuel in vehicles.
Answer:
Precautions that should be taken while using CNG fuel in vehicles are:
(i) Do not allow anybody to smoke near the CNG storage unit.
(ii) Do not burn firecrackers near CNG pumps.

Question 3.
Wastes’ have been traditionally disposed off by burning them. Waste combustors’ are used to dispose off hospital and industrial waste. Find out how these combustors work and their effects on the environment.
Answer:
Do it yourself

Question 4.
Find out about the different types of fire extinguishers. Write the type and locations of the fire extinguishers kept in your school.
Answer:
The different types of fire extinguishers are:

• Class A – fires involving solid materials such as wood, paper or textiles.
• Class B – fires involving flammable liquids such as petrol, diesel or oils.
• Class C – fires involving gases.
• Class D – fires involving metals.
• Class E – fires involving live electrical apparatus

DAV Class 8 Science Chapter 7 Solutions – Combustion Important Questions and Answers

I. Fill in the blanks:

1. Petrol is a highly ________ material.
Answer:
inflammable

2. Coal is capable of ________ combustion.
Answer:
slow

3. Spirit is capable of ________ combustion.
Answer:
rapid

4. Phosphorus is capable of ________ combustion.
Answer:
spontaneous

5. Calorific value of LPG is ________ than the calorific value of firewood.
Answer:
higher

II. Tick the correct answer:

Question 1.
Which of these is not required for combustion?
(a) Oxygen
(b) Combustible substance
(c) Carbon dioxide
(d) Ignition temperature
Answer:
(c) Carbon dioxide

Question 2.
Which of these shows spontaneous combustion?
(a) CNG
(b) LPG
(c) Diesel
(d) Sodium
Answer:
(d) Sodium

Question 3.
Which of these shows slow combustion?
(a) Firewood
(b) Petrol
(c) Kerosene
(d) LPG
Answer:
(a) Firewood

Question 4.
Which of these gases is evolved on incomplete combustion of a fuel?
(a) Sulphur dioxide
(b) Nitrogen oxide
(c) Carbon monoxide
(d) Hydrogen
Answer:
(c) Carbon monoxide

Question 5.
What is the colour of the innermost zone of a flame?
(a) Dark black
(b) Blue
(c) Yellow
(d) Red
Answer:
(a) Dark black

III. Answer the following questions:

Question 1.
What is ignition temperature?
Answer:
The lowest temperature at which a substance starts burning is called its ignition temperature.

Question 2.
What is slow combustion?
Answer:
When a combustive material burns at slow or moderate rate, it is called slow combustion. Examples: burning of cowdung cakes, wood, etc.

Question 3.
What is rapid combustion?
Answer:
When a combustible material burns at fast rate, it is called rapid combustion. Examples: burning of petrol and LPG.

Question 4.
What is spontaneous combustion?
Answer:
When a combustible material starts burning at room temperature, it is called spontaneous combustion. Example: Sodium and phosphorus catch fire at room temperature instantaneously.

Question 5.
What is explosive combustion?
Answer:
When a combustible substance completely burns in a very short period of time and releases lot of heat, light and gases, it is called explosive combustion. Examples: burning of firecrackers, gunpowder, etc.

IV. Projects:

Question 1.
Make a list of five combustible substances and five non-combustible substances in your household.
Answer:
Do it yourself

Question 2.
Look for items in your home which have warning sign of inflammable material on their packs. Make their list.
Answer:
Do it yourself

The DAV Class 8 Science Book Solutions Pdf and DAV Class 8 Science Chapter 6 Question Answer – Sources of Energy are essential study tools for DAV public school students in Class 8.

DAV Class 8 Science Ch 6 Question Answer – Sources of Energy

DAV Class 8 Science Ch 6 Solutions – Sources of Energy

Something To Know

A. Fill in the blanks:

Question 1.
________ and ________ are non-renewable sources of energy.
Answer:
coal, petroleum

Question 2.
________ is a major renewable, natural resource.
Answer:
water

Question 3.
The process in which coal is strongly heated in the absence of air is known as ________ .
Answer:
destructive distillation

Question 4.
In oil wells ________ is always found above the layer of petroleum.
Answer:
natural gas

Question 5.
________ is a black viscous liquid, obtained by destructive distillation of coal.
Answer:
coal tar

B. State True or False for the following statements:

1. Fossil fuels are renewable natural resources.
Answer:
False

2. Anthracite has higher carbon content than lignite.
Answer:
True

3. Coke can be used to make fuel gases.
Answer:
True

4. Natural gas is found below the petroleum layers.
Answer:
False

5. Burning of fossil fuels is a major source of pollution.
Answer:
True

C. Tick (✓) the correct option:

Question 1.
The highest grade of coal is known as-
(a) Lignite
(b) Bitumen
(c) Anthracite
(d) Peat
Answer:
(c) Anthracite

Question 2.
Coke and coal tar are obtained from-
(a) Petroleum
(b) Coal
(c) Natural gas
(d) Diesel
Answer:
(b) Coal

Question 3.
Different types of coal generally have the-
(a) Same content of volatile material and same percentage of carbon
(b) Same content of volatile material but different percentage of carbon
(c) Different content of volatile material but same percentage of carbon
(d) Different content of volatile material as well as different percentage of carbon
Answer:
(d) Different content of volatile material as well as different percentage of carbon

Question 4.
A product, that is not obtained through the refining of petroleum, is
(a) Diesel
(b) Kerosene
(c) Coal gas
(d) Petrol
Answer:
(c) Coal gas

Question 5.
Which of the following is a renewable source of energy?
(a) Tidal energy
(b) Coal
(c) Diesel
(d) Petrol
Answer:
Tidal energy

D. Answer the following questions in brief:

Question 1.
Define the term ‘fuel’.
Answer:
A substance which produces heat on burning in air is called fuel.

Question 2.
Name any two renewable natural sources of energy.
Answer:
Wind energy, Solar energy

Question 3.
Define the term ‘carbonisation’.
Answer:
The process of conversion of dead plant materials into coal is called carbonization.

Question 4.
Name the three main products obtained from destructive distillation of coal.
Answer:
Petrol, diesel and kerosene

Question 5.
State two uses of each of the following:
(a) Asphalt
Answer:
Asphalt is used for making road and painting the underside of electric pole.

(b) Paraffin wax
Answer:
For making ointments and candles

Question 6.
Write the full forms of CNG, LPG and PNG.
Answer:
CNG: Compressed Natural Gas
LPG: Liquefied Petroleum Gas
PNG: Piped Natural Gas

E. Answer the following questions:

Question 1.
Describe, in brief, the process of ‘coal formation.
Answer:
Millions of years ago, plants got buried under the earth’s crust. In due course of time, those dead remains of plants got converted into coal due intense pressure and temperature inside the earth. The process of conversion of remains of plants into coal is called carbonization.

Question 2.
Differentiate between lignite and bituminous coal.
Answer:

Lignite	Bituminous coal
It contains about 38% carbon.	It contains about 65% carbon.
It is mainly used for power generation.	It is used for power generation and for making coke.

Question 3.
How do petroleum and natural gas occur in nature?
Answer:
Petroleum occurs in rock formations as crude oil. Crude oil is a dark, viscous and foul smelling liquid. Natural gas is found in oil wells above the crude oil.

Question 4.
State the main advantages of using natural gas.
Answer:
Advantages of using natural gas:

• It is a clean fuel.
• It is easy to transport natural gas through pipelines.
• Natural gas is economical than petrol and diesel.

Question 5.
Keeping in mind their availability in nature, explain the need of using coal and petroleum judiciously.
Answer:
Coal and petroleum are non-renewable resources. Their stock is going to be finished in near future. Hence, it is necessary to use them judiciously.

Value Based Question

The chemistry teacher, explained to her students that when petroleum, a naturally occurring fossil fuel, is separated into its different components, each of these components is useful in one way or the other. She then told her students that our daily life too can be thought of as ‘made up’ of a number of activities, actions and tasks. She suggested that ‘all of us’ should try to ensure and aim that all these ‘components’ of our daily life, are useful/helpful to the society at large.

Question 1.
State the values that you think are contained in the teacher’s talk and suggestions.
Answer:
Positive contribution of our activities to the society.

Question 2.
Name any three components, obtained from petroleum, and state any one use of each of them.
Answer:
Three components obtained from petroleum and their uses are as follows:

• Petrol: As fuel in cars.
• Diesel: As fuel in buses.
• Asphalt: For making road surface.

Question 3.
Have a ‘group discussion’ in which different students suggest “ways and means’ of making their different daily life actions and activities, useful to the society at large.
Answer:
Do it yourself

Something To Do

Question 1.
Make a report on ‘electricity consumption’ in your house. Suggest some ways that can help to reduce the consumption.
Answer:
Make the report yourself. Some ways to reduce electricity consumption are as follows:

• Switch off lights and fans when not in use.
• Do not use airconditioner until it is absolutely necessary.
• Use LED or CFL instead of incandescent bulbs.
• Use appliances with good energy rating.

Question 2.
Unlike thermal power plants, which burn fossil fuels and emit large amounts of carbon dioxide (a greenhouse gas) into the atmosphere, nuclear power plants use uranium (a metal) to produce huge amounts of energy from very small amount o fuel. However, they release harmful radiations that can pose a high risk to the population.
Do you think nuclear energy can be an answer to the problems caused by the use of fossil fuels? Try to prepare a report, listing points that suggest that nuclear power could be/could not be a better way of generating electricity as compared to fossil fuels.
Answer:
Replacing fossil fuels with nuclear energy can indeed help in reducing air pollution. But long term harms by potential leak of radiation is too costly for human life. Hence, nuclear energy cannot be a better way of generating electricity as compared to fossil fuels. We need to adapt other approaches for this; such as solar energy and hydel energy.

Question 3.
Try to find out information about the places/activities for which solar energy or wind energy is being put to use in place of conventional fossil fuel based energy sources.
Answer:
India has progressed significantly in the fields of solar energy and wind energy. Many solar power plants have come up in Rajasthan. Similarly, Tamil Nadu is the leading state in terms of wind energy.

DAV Class 8 Science Chapter 6 Solutions – Sources of Energy Important Questions and Answers

I. Fill in the blanks:

1. Battery is a ________ source of energy.
Answer:
Man-made

2. Firewood is used as ________ fuel in rural areas.
Answer:
Kitchen

3. LPG is used as ________ fuel.
Answer:
Domestic

4. CNG is used as fuel for ________ .
Answer:
Automobiles

5. Coal is a type ________ of rock.
Answer:
Sedimentary

II. Tick (✓) the correct answer:

Question 1.
Which of these is a man-made source of energy?
(a) Petrol
(b) Diesel
(c) Battery
(d) Wind energy
Answer:
(c) Battery

Question 2.
Which of these is non-renewable source of energy?
(a) Coal
(b) Hydel energy
(c) Solar energy
(d) Wind energy
Answer:
(a) Coal

Question 3.
Which of these produces lot of polluting gases on burning?
(a) LPG
(b) CNG
(c) Diesel
(d) All of these
Answer:
(c) Diesel

Question 4.
Coal mines are located at which of these places?
(a) Krishna Godavari Basin
(b) Bombay High
(c) Digboi
(d) Jharia
Answer:
(d) Jharia

Question 5.
A petroleum refinery is located at which of these places?
(a) Mathura
(b) Bokaro
(c) Bhilai
(d) Durgapur
Answer:
(a) Mathura

III. Answer the following questions:

Question 1.
What do you understand by non-renewable sources of energy?
Answer:
A source of energy which gets used up and cannot be recreated in a short period of time is called non-renewable source of energy. These are also called exhaustible source of energy, e.g. fossil fuels.

Question 2.
What is a fossil fuel?
Answer:
The fuels which were formed from dead remains of plants and animals are called fossil fuels.

Question 3.
What are the different types of coal?
Answer:
Peat, lignite, bituminous and anthracite are different types of coal.

Question 4.
What are the uses of coal gas?
Answer:
Uses of coal gas:

• It is used as domestic fuel.
• It is used as industrial fuel in those industries which are located near coal processing units.

Question 5.
Write a brief note on processing of coal.
Answer:
Coal is processed through destructive distillation. This is done by strong heating of coal in the absence of air. This helps in retaining useful contents of coal and removing unwanted components so that coal can produce optimum amount of energy. Coke, coal tar, and coal gas are also obtained during destructive distillation of coal.

IV. Projects:

Question 1.
Using a basket and aluminium foil, make a solar cooker. Demonstrate its working in your school.
Answer:
Do it yourself

Question 2.
What changes would you bring in your daily life to reduce environmental pollution? Make a project report.
Answer:
Do it yourself