class 8 Chapter 10 Reaching the Age of Adolescence

class 8 Chapter 10 Reaching the Age of Adolescence

Exercise Questions

1. What is the term used for chemical secretions of endocrine glands responsible for changes taking place in the body?

Soln:

The chemical substance secreted by the endocrine glands is called Hormones. Hormones are responsible for the changes taking place in the body.

 2. Define adolescence.

Soln:

The time period which comes between the beginning of adulthood and puberty is called Adolescence. In this time period, the body undergoes different changes alongside reproductive maturity. It starts somewhat at the age of 11 and ends at the age of 18 or19. This period of adolescence may differ from person to person.

3. What is menstruation? Explain

Soln:

The process of shedding of the uterine lining on a regular monthly basis is called menstruation. It starts at puberty and in the reproductive cycle of the female body. The uterus prepares itself to receive a fertilized egg on every month. Due to this the inner lining of the uterus become thick and is supplied with blood to nourish the embryo. If the egg is not fertilized, then the lining of the uterus breaks down and get released in the form of blood through the vagina. The time period for this is about 2 to 8 days every month. This monthly cycle is known as the menstrual cycle.

4. List changes in the body that take place at puberty

Soln:

The changes that take place at the time of puberty are as follows:

a) The height and weight of the body will suddenly increase

b) The hair seems to appear in the area such as legs, hands, face and underarms

c) In female, the enlargement of the ovary takes place and starts producing matured eggs and on the other hand in males, the testis grows and starts producing sperms

d) In males, the shoulder broadens and chest becomes widen, whereas in females the region below the waist becomes wider.

e) The acne seems to appear because of the excessive secretion of the oil from the skin.

f) In boys, the larynx becomes prominent under the influence of hormones, the vocal cord becomes longer and thicker resulting in hoarse voice.

5. Prepare a Table having two columns depicting names of endocrine glands and hormones secreted by them.

Soln:

Endocrine Glands	Hormones
Testis	Testosterones
Ovary	Oestrogen
Pituitary	Growth hormone
Adrenal	Adrenalin
Thyroid	Thyroxin
Pancreas	Insulin

6. What are sex hormones? Why are they named so? State their function

Soln:

The chemical substances produced by the sex organs are called sex hormones. For example, in females, the sex hormones produced by the ovary is called oestrogen and in males, the sex hormones produced by the testis is called testosterone.

Some of the functions performed by the sex hormones:

Oestrogen: This hormone is responsible for the development of secondary sexual characters in females like enlargement of breasts, development of female reproductive organs etc.

Testosterone: this hormone brings the secondary sex character in boys such as the growth of beard, the voice becomes hoarse, development of the reproductive organs etc.

7. Choose the correct option.

(a) Adolescents should be careful about what they eat, because

(i) proper diet develops their brains.

(ii) proper diet is needed for the rapid growth taking place in their body.

(iii) adolescents feel hungry all the time.

(iv) taste buds are well developed in teenagers.

(b) Reproductive age in women starts when their

(i) menstruation starts.

(ii) breasts start developing.

(iii) body weight increases.

(iv) height increases.

(c) The right meal for adolescents consists of

(i) chips, noodles, coke.

(ii) chapati, dal, vegetables.

(iii) rice, noodles and burger.

(iv) vegetable cutlets, chips and lemon drink.

Soln:

a) (iv) taste buds are well developed in teenagers.

b) (iii) body weight increases.

c) (ii) chapati, dal, vegetables.

8. Write notes on—

(a) Adam’s apple.

(b) Secondary sexual characters.

(c) Sex determination in the unborn baby.

Soln:

a) Adam’s apple: In human males, the larynx grows larger at the time of puberty and can be seen as a protruding part of the throat. This protrusion is known as Adam’s apple. In boys under the influence of sex hormones, the larynx becomes prominent. Due to this, the vocal cord becomes thicker and longer which causes the voice hoarse. On the other hand in females, the larynx is of small size and is hardly visible. Therefore, girls have a high pitched voice while the boys has deep voice.

9. Word game :

Use the clues to work out the words.

Across

3. Protruding voice box in boys

4. Glands without ducts

7. Endocrine gland attached to brain

8. Secretion of endocrine glands

9. Pancreatic hormone

10. Female hormone

Down

1. Male hormone

2. Secretes thyroxine

3. Another term for teenage

5. Hormone reaches here through blood stream

6. Voice box

7. Term for changes at adolescence

Soln:

Across

1. Adam’s apple
2. Endocrine
3. Pituitary
4. Hormone
5. Insulin
6. Estrogen

Down

1. Testosterone
2. Thyroid
3. Adolescence
4. Target site
5. Larynx
6. Puberty

8 Science Chapter 10 NCERT Solutions reaching the Age of Adolescence

Section Number	Topics
10.1	Adolescence and Puberty
10.2	Changes at Puberty
10.3	Secondary Sexual Characters
10.4	Role of Hormones in Initiating Reproductive Function
10.5	Reproductive Phase of Life in Humans
10.6	How is the Sex of the Baby Determined
10.7	Hormones other than Sex Hormones
10.8	Role of Hormones in Completing the Life History of Insects and Frogs
10.9	Reproductive Health What is the term used for chemical secretions of endocrine glands responsible for changes taking place in the body? The chemical substance secreted by the endocrine glands is called Hormones. Hormones are responsible for the changes taking place in the body. Define adolescence? The time period which comes between the beginning of adulthood and puberty is called Adolescence. In this time period, the body undergoes different changes alongside reproductive maturity. It starts somewhat at the age of 11 and ends at the age of 18 or19. This period of adolescence may differ from person to person. What is menstruation? The process of shedding of the uterine lining on a regular monthly basis is called menstruation. What are sex hormones? The chemical substances produced by the sex organs are called sex hormones.

cbse class 8 Chapter 9 Reproduction in Animals

class 8 Chapter 9 Reproduction in Animals

Chapter 9 Exercise Questions

1. Explain the importance of reproduction in organisms.

Soln:

Importance of reproduction in organisms are as follows

• Reproduction is termed to be a biological process in which organisms give birth to their off springs.
• It helps in maintaining the continuity of a particular race and helps in increasing the population of the species.
• Reproduction is meant for the survival of all living things.

2. Describe the process of fertilisation in human beings.

Soln:

• Fertilization is defined as the process in which there is a fusion of male gamete and female gamete.
• The male gametes or sperms are released from the male reproductive organ called the penis
• The sperms released to enter the female body through the vagina.
• From the vagina, sperms travel through the fallopian tubes where they meet the eggs.
• From there on, the process of fertilization takes place in the fallopian tube.
• The male gamete cell (sperm) and female gamete cell (egg) fuse together to form a zygote.
• The zygote divides rapidly to form a group of cells called morula, which becomes the embryo after approximately five days. The foetus is present for about eight weeks after the fertilization.

3. Choose the most appropriate answer.

(a) Internal fertilisation occurs

(i) in female body.

(ii) outside female body.

(iii) in male body.

(iv) outside male body.

(b) A tadpole develops into an adult frog by the process of

(i) fertilisation

(ii) metamorphosis

(iii) embedding

(iv) budding

(c) The number of nuclei present in a zygote is

(i) none

(ii) one

(iii) two

(iv) four

Soln:

a) (i) in female body.

b) (ii) metamorphosis

c) (iii) two

4. Indicate whether the following statements are True (T) or False (F).

(a) Oviparous animals give birth to young ones. ( )

(b) Each sperm is a single cell. ( )

(c) External fertilisation takes place in frog. ( )

(d) A new human individual develops from a cell called gamete. ( )

(e) Egg laid after fertilisation is made up of a single cell. ( )

(f) Amoeba reproduces by budding. ( )

(g) Fertilisation is necessary even in asexual reproduction. ( )

(h) Binary fission is a method of asexual reproduction. ( )

(i) A zygote is formed as a result of fertilisation. ( )

(j) An embryo is made up of a single cell. ( )

Soln:

a) False

b) True

c) True

d) False

e) True

f) False

g) False

h) True

i) True

j) False

5. Give two differences between a zygote and a foetus.

Soln:

Zygote:

• It is the earliest stage of development
• It is formed by the fusion of male and female gametes
• It is a single cell
• The zygote divides several times to form an embryo
• The zygote normally lasts a week and then develop into its next stage.

Foetus

• It is the last developmental stage of an organism
• The stage of the embryo that shows all the main recognizable body parts of a mature organism.
• The foetus stage occurs after the embryo stage.
• Foetus mainly undergoes internal development.

6. Define asexual reproduction. Describe two methods of asexual reproduction in animals.

Soln:

In this type of reproduction, the offspring arises from a single organism without the fusion of male and female gametes. It never changes the number of chromosomes.

Binary fission in amoeba

• It is a kind of asexual reproduction in which one cell divides into two halves.
• It is a unicellular organism that has a cell membrane, cell wall and cytoplasm.
• The division of the cell can take place on any plane.
• In this process, the nucleus of the amoeba first divides to form two daughter nuclei by the process called Karyokinesis.
• Finally, the division of the body into two halves having a nucleus respectively takes place.

Budding in hydra

• Organisms such as hydra, use regenerative cells for reproduction in the process of budding.
• The first step is the formation of buds, and it develops as a small outgrowth on the parent’s body.
• As the bud enlarges, it receives the characteristics of the parent organism.
• Once it is developed, it may be detached from the parent body and develop into a new individual.
• In some rare cases, the buds may not be detached forming interconnected buds.
• There are also methods like fragmentation, multiple fission etc.

7. In which female reproductive organ does the embryo get embedded?

Soln:

• In the female reproductive organ, the embryo gets attached to the uterus.
• Once it gets attached, there occurs the formation of body parts like legs, hands, eyes etc.
• The embryo is then called a foetus.

8. What is metamorphosis? Give examples.

Soln:

• It is defined as the process in which an animal’s body structure abruptly changes through cell growth and differentiation.
• It is a biological process.
• Examples of this kind are frog and insects.

The life cycle of a frog:

The tadpole emerging from the egg will have gills, tail etc.

They can swim easily in water.

It undergoes abrupt changes and develops into a mature frog.

9. Differentiate between internal fertilisation and external fertilisation.

Soln:

Fertilization, in general, is defined as the fusion of a male and a female gamete.

Internal fertilization

It occurs inside the female body.

There are high chances of survival of the offspring.

Internal fertilization protects the fertilized egg or embryo from harsh environments.

Examples are cow, humans, dogs, monkeys etc.

External fertilization

It occurs outside the female body.

There are low chances of survival of the offspring.

Most aquatic animals use this type of fertilization and the advantage of external fertilization is that it produces a large number of offspring due to external hazards.

Examples are fish, frog, organisms etc.

10. Complete the crossword puzzle using the hints given below.

Across

1. The process of the fusion of the gametes.

6. The type of fertilisation in hen.

7. The term used for bulges observed on the sides of the body of hydra.

8. Eggs are produced here.

Down

2. Sperms are produced in these male reproductive organs.

3. Another term for in vitro fertilisation.

4. These animals lay eggs.

5. A type of fission in amoeba

Soln:

Across

1) Fertilization

6) Internal

7) Buds

8) Ovary

Down

2) Testis

3) Zygote

4) Oviparous

5) Binary

This chapter comprises of various topics like-

• Reproduction
• Modes of reproduction
• Fertilization
• Development of embryo
• Viviparous and Oviparous animals
• Young ones to adults

1. Reproduction – It is a procedure in which the organisms reproduce their young ones. There are 2 methods of reproduction. Namely – Sexual reproduction and asexual reproduction.
2. Sexual reproduction – Reproduction ensuing from the combination of the female and male gametes is called sexual reproduction. The reproductive organs in the female comprise of – ovaries, oviducts and uterus. And, the reproductive organs in the male comprises of – testes, sperm ducts and penis. The unification of ovum and sperm is called fertilization. The fertilized egg is known as the zygote.
3. Asexual reproduction – When a single parent is involved, then it is termed as asexual reproduction.  New cells develop from buds in hydra. This mode of asexual reproduction is known as budding. Amoeba reproduces itself by dividing into 2 and this type of reproduction is termed as binary fission

4. Explain the importance of reproduction in organisms?

   • Reproduction is termed to be a biological process in which organisms give birth to their off springs.
   • It helps in maintaining the continuity of a particular race and helps in increasing the population of the species.
   • Reproduction is meant for the survival of all living things.

   Describe the process of fertilisation in human beings?

   • Fertilization is defined as the process in which there is a fusion of male gamete and female gamete.
   • The male gametes or sperms are released from the male reproductive organ called the penis
   • The sperms released to enter the female body through the vagina.
   • From the vagina, sperms travel through the fallopian tubes where they meet the eggs.
   • From there on, the process of fertilization takes place in the fallopian tube.
   • The male gamete cell (sperm) and female gamete cell (egg) fuse together to form a zygote.
   • The zygote divides rapidly to form a group of cells called morula, which becomes the embryo after approximately five days. The foetus is present for about eight weeks after the fertilization.

   Differences between a zygote and a foetus?

   Zygote:

   • It is the earliest stage of development
   • It is formed by the fusion of male and female gametes
   • It is a single cell
   • The zygote divides several times to form an embryo
   • The zygote normally lasts a week and then develop into its next stage.

   Foetus

   • It is the last developmental stage of an organism
   • The stage of the embryo that shows all the main recognizable body parts of a mature organism.
   • The foetus stage occurs after the embryo stage.
   • Foetus mainly undergoes internal development.

   Define asexual reproduction?

   In this type of reproduction, the offspring arises from a single organism without the fusion of male and female gametes. It never changes the number of chromosomes.

   What is metamorphosis?

   It is defined as the process in which an animal’s body structure abruptly changes through cell growth and differentiation.

class 8 Chapter 8 Cell – Structure and Functions

class 8 Chapter 8 Cell – Structure and Functions

Exercise Questions

1. Indicate whether the following statements are True (T) or False (F).

(a) Unicellular organisms have one-celled body. (T/F)

(b) Muscle cells are branched. (T/F)

(c) The basic living unit of an organism is an organ. (T/F)

(d) Amoeba has irregular shape. (T/F)

Soln:

a)True

b) False

c) False

d) True

2. Make a sketch of the human nerve cell. What function do nerve cells perform?

Soln:

The main function of the nerve cell is to transmit messages to the brain from receptor organs and vice versa. It has control over the working of different parts of the body.

3. Write short notes on the following.

(a) Cytoplasm

(b) Nucleus of a cell

Soln:

(a) Cytoplasm:

The cytoplasm is a fluid that contains all the cell organelles such as the Mitochondria, Ribosomes, Golgi bodies, etc. It is present between the nucleus and the plasma membrane. It helps in the exchange of materials between cell organelles. It is made up of eighty percent water and is usually clear and colourless.

(b) The nucleus of a cell:

The Nucleus is a double-membrane bound cell organelle present in eukaryotic cells. It contains the DNA, the genetic material. It is the command centre of the cell and is spherical in shape. It has the following components:

• Nuclear membrane
• Nucleolus
• Chromosomes

1. Nuclear membrane:

It has is a double-layered membrane. It separates the contents of the nucleus from the cytoplasm and acts as a wall. It has pores that allow the transfer of certain substances in and out of the cell.

2. Nucleolus:

It is a small dense spherical body. It is not bound by any membrane.

3. Chromosome:

These are thread-like structures that carry genes. All the necessary information required for the transfer of characteristics from the parents to the offspring are stored in the genes. Inheritance of characteristics is possible only because of chromosomes.

4. Which part of the cell contains organelles?

Soln:

Various cell organelles are present in the cytoplasm. It is a clear and colorless fluid that contains organelles like Mitochondria, Ribosomes, Golgi bodies.

5. Make sketches of animal and plant cells. State three differences between them.

Soln:

Plant cell	Animal cell
They are large in size	They are smaller than plant cells
The cell wall is present	The cell wall is absent
Vacuoles are large	Vacuoles are small
Plastids could be seen	Except for Euglena, Plastids could not be seen in animal cells.

6. State the difference between eukaryotes and prokaryotes.

Prokaryotes	Eukaryotes
Most of them are unicellular	Most of them are multicellular
There is no nuclear membrane. So, the nucleus is poorly defined.	There is a nuclear membrane. So, the nucleus is well defined.
Not all cell organelles are present	All the cell organelles are present.
Nucleolus is absent	Nucleolus is present
Eg: Blue-green algae, Bacteria	Eg: Plant, Animal cells and Fungi.

7. Where are chromosomes found in a cell? State their function.

Soln:

Chromosomes are thread-like structures present in the nucleus that carry genes. All the necessary information required for the transfer of characteristics from the parents to the offspring are stored in the genes. Inheritance of characteristics is possible only because of chromosomes.

8. ‘Cells are the basic structural units of living organisms’. Explain.

Soln:

Various components of plants and animals are constituted by cells. It is the smallest unit of life and is capable of all living functions. They are the building blocks of life. That is the reason why cells are referred to as ‘the basic structural and functional blocks of life’.

Cells exist in various shapes and sizes and perform a wide range of activities.

Their shapes and sizes are related to the function it performs.

9. Explain why chloroplasts are found only in plant cells?

Soln:

Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.

10. Complete the crossword with the help of clues given below.

Across

1. This is necessary for photosynthesis.

3. Term for component present in the cytoplasm.

6. The living substance in the cell.

8. Units of inheritance present on the chromosomes.

Down

1. Green plastids.

2. Formed by collection of tissues.

4. It separates the contents of the cell from the surrounding medium.

5. Empty structure in the cytoplasm.

7. A group of cells.

Soln:

Across

1. Chlorophyll
2. Organelle
3. Protoplasm
4. Genes

 Down

1. Chloroplasts
2. Organ
3. Membrane
4. Vacuole
5. Tissue

What is Cytoplasm?

The cytoplasm is a fluid that contains all the cell organelles such as the Mitochondria, Ribosomes, Golgi bodies, etc. It is present between the nucleus and the plasma membrane. It helps in the exchange of materials between cell organelles. It is made up of eighty percent water and is usually clear and colourless.

Define Nucleus of a cell?

The Nucleus is a double-membrane bound cell organelle present in eukaryotic cells. It contains the DNA, the genetic material. It is the command centre of the cell and is spherical in shape.

Which part of the cell contains organelles?

Various cell organelles are present in the cytoplasm. It is a clear and colorless fluid that contains organelles like Mitochondria, Ribosomes, Golgi bodies.

Where are chromosomes found in a cell?

Chromosomes are thread-like structures present in the nucleus that carry genes. All the necessary information required for the transfer of characteristics from the parents to the offspring are stored in the genes. Inheritance of characteristics is possible only because of chromosomes.

Explain why chloroplasts are found only in plant cells?

Chloroplasts are plastids required for the food making process, called photosynthesis, and thus they are only present in plant cells.

class 8 Chapter 7 Conservation of Plants and Animals

 class 8 Chapter 7 Conservation of Plants and Animals

Exercise Questions

1. Fill in the blanks.

(a) A place where animals are protected in their natural habitat is called _________ .

(b) Species found only in a particular area is known as _______ .

(c) Migratory birds fly to faraway places because of __________ changes.

Soln:

(a) A place where animals are protected in their natural habitat is called Sanctuary.

(b) Species found only in a particular area is known as endemic .

(c) Migratory birds fly to faraway places because of climatic changes.

2. Differentiate between the following.

(a) Wildlife sanctuary and biosphere reserve

(b) Zoo and wildlife sanctuary

(c) Endangered and extinct species

(d) Flora and fauna

Soln:

(a)

Wildlife sanctuary	Biosphere Reserve
An area within which wild animals are protected from external dangers like hunting	An area constructed for the conservation of biodiversity
Adequate habitat and protection are provided for the wild animals living in a sanctuary	Several life forms like plants, animals, and micro-organisms conservation are possible in a reserve
Eg: Corbett national park	Eg: Nilgiri Biosphere reserve

(b)

Zoo	Wildlife Sanctuary
Animals are kept in artificially constructed facilities for public exhibition.	An area within which wild animals are protected from external dangers like hunting.
It is an artificial habitat and the animals may or may not adapt to the new conditions.	The animals are conserved within their natural habitat itself. So, they won’t have a problem with their surroundings.

(c)

Endangered Species	Extinct Species
The species which are on the verge of extinction are called as Endangered species.	The species of animals or plants that no longer exist are called Extinct species.
Eg: Bengal Tiger, Blue Whale, etc.	Eg: Caribbean monk seal, Great Auk, Passenger pigeon.

(d)

Flora	Fauna
It is the collective name for the plant life from a particular area.	It is the collective name for the animal life from a particular area
Eg: Spurge and Hogweed from the Nilgiris region	Eg: Langur, Tiger, Tahr from the Nilgiris region.

3. Discuss the effects of deforestation on the following.

(a) Wild animals

(b) Environment

(c) Villages (Rural areas)

(d) Cities (Urban areas)

(e) Earth

(f) The next generation

Soln:

(a) Effects of deforestation on wild animals:

Removal of trees and plant life from a particular area for the construction of industries, agriculture or for other such purposes is termed as Deforestation. Plant life forms an integral part of the ecosystem and these plants form the natural habitat of various animals of that ecosystem. Destroying the plant life will ultimately threaten the existence of animals in that particular ecosystem and may lead to their extinction.

(b) Effects of deforestation on the environment:

In plants, Photosynthesis takes place by the absorption of CO₂ from the earth’s atmosphere. The percentage of CO₂ in the atmosphere increases rapidly if the plant life of an area is destroyed. Increased concentrations of CO₂ result in the trapping of excessive heat radiations within the Earth’s atmosphere contributing to global warming. This results in the rise of the temperature of the Earth and disturbs the water cycle which occurs in nature. Hence, rainfall pattern changes leading to droughts and floods.

(c) Effects of deforestation on villages:

The soil particles are held together in a place by the roots of the plants, trees and other vegetation. In the event of water flow or a high-speed wind over the top layer of the soil, the soil particles get removed easily as there are no plants to hold it in place. Thus, Soil erosion is increased by deforestation activities. Soil loses fertility and loses humus. Hence, a fertile land, fit for agriculture turns into a desert.

(d)Effects of deforestation on cities:

The risk of natural calamities like floods and drought in urban areas is increased by deforestation. It will also lead to global warming because of increased CO₂ levels in the atmosphere. This is a result of continuous pollution from automobiles and industries. The water cycle in the vicinity is also affected.

(e) Effects of deforestation on Earth:

Deforestation leads to increased chances and occurrences of desertification, droughts and floods. Carbon Dioxide level in the earth’s atmosphere also increases which leads to global warming. The water cycle is disrupted and also there is an increased risk of natural calamities.

(f) Effects of deforestation on the next generation:

Our Environment is slowly changed by the activities of deforestation, both directly and indirectly. Soil erosion, global warming, desertification, drought, greenhouse effect, floods and many other problems are caused only because of deforestation. Ultimately, the next generation will be facing the consequences of deforestation.

4. What will happen if

(a) we go on cutting trees

(b) the habitat of an animal is disturbed

(c) the top layer of soil is exposed

Soln:

(a) If we go on cutting the trees

If we go on cutting the trees, the animals will lose their natural habitat as essential part of their ecosystem are destroyed. This reduces the biodiversity of the affected areas. The temperature of the Earth also increases and gives way to global warming. Global warming, in turn, affects the water cycle. Hence, rainfall pattern changes leading to droughts and floods. Risks of soil erosion, global warming, desertification, greenhouse effect increases.

(b) If the habitat of an animal is disturbed

All the basic necessities for an animal, like food, water, shelter and protection is provided by the habitat in which the animal lives-in disturbing the habitat of the animal forces it to move to another place in search of food, water, shelter and protection. The animal, in due course, may get killed by other animals.

(C) If the top layer of the soil is removed

Removing the top layer of the soil gradually exposes the lower layers of the soil. The lower layers of the soil are hard and rocky in nature and are less fertile. This is because of the reduced quantity of humus. Soil erosion on a continued basis will turn it barren and infertile.

5. Answer in brief.

(a) Why should we conserve biodiversity?

(b) Protected forests are also not completely safe for wild animals. Why?

(c) Some tribals depend on the jungle. How?

(d) What are the causes and consequences of deforestation?

(e) What is Red Data Book?

(f) What do you understand by the term migration?

Soln:

(a) The number and variety of various life forms such as plants, animals and micro-organisms in the area are called Biodiversity. Both plants and animals have a mutual dependence on each other for their survival. As they are related to one another, destruction of one will automatically destroy the other. Hence, biodiversity has to be conserved in order to maintain nature’s balance.

(b) People who live near the forests depend on the resources of the forests in order to fulfil their day-to-day needs. Therefore, it is not safe for animals living in protected forests. The animals would be threatened by the presence of the human population. This results in the killing of wild animals and selling their products for huge sums of money.

(c) The forests provide food, fodder and other resources to the tribal people. It is inevitable for them to depend on forests and their resources for everyday needs.

(d) Causes of deforestation:

There are a lot of causes for deforestation. A few of them are listed below.

I. In order to accommodate the ever-expanding urban population, forest areas are often cleared and converted into lands for various uses.

II. For agricultural activities like crop cultivation and cattle grazing, forest lands are cleared.

III. Firewood is a major product from the forests and it is one of the main reasons for cutting the trees on a large scale.

Consequences of deforestation:

There are a lot of fatal consequences caused by deforestation. A few of them are listed below.

1. Soil erosion
2. Loss of Biodiversity
3. Floods
4. Droughts
5. Global warming as a result of climate change
6. Disruption of the water cycle

(e) Red data book is basically a sourcebook which has an international list of all plant and animal species which are endangered, that is, on the verge of extinction. The International Union for Conservation of Nature and Natural resources (IUCN) maintains the books and adds/ removes the names of the species by conducting a comprehensive survey.

(f) The movement of an organism or a group of organisms from its natural habitat to another place on a standard basis at a particular time of each and every year is known as migration. The organisms do so in order to avoid uninhabitable conditions of climate or for the process of breeding.

6. In order to meet the ever-increasing demand in factories and for shelter, trees are being continually cut. Is it justified to cut trees for such projects? Discuss and prepare a brief report.

Soln:

No. Cutting trees in order to meet the ever growing demands of the human population is not at all justified. There are a vast number of organisms like wild animals, insects, and birds living in the forests. The forests provide good quality air for both animals and humans. This is because of the process of plants respiration by which, they consume the harmful carbon dioxide and give out good quality Oxygen. Thus, keeping greenhouse gases and global warming under check. The roots of the flora of the forests help in preventing soil erosion. Natural Calamities such as floods and droughts could also be prevented by their presence. They help in increasing the soil’s fertility and conserving the biodiversity of the ecosystem.

Overutilization of forests and its resources by cutting a large number of trees in order to satisfy the demands of the ever-increasing human population leading to many long-term problems like

• Soil erosion
• Greenhouse effect
• Global warming
• Floods
• Droughts

Besides the above-listed problems, the effects of large-scale deforestation will be leaving an everlasting mark on the face of human civilization. The balance of nature is disturbed by the destruction of trees. Hence, it is of utmost priority and importance to conserve forests.

7. How can you contribute to the maintenance of green wealth of your locality? Make a list of actions to be taken by you.

Soln:

I can take care of the green cover of my locality by growing plants in my locality. I shall motivate the people of my neighbourhood to plant more trees and take care of the existing trees by creating awareness about the importance of plant life in an ecosystem. In order to make this possible, I would initiate the proceedings and organize weekly events on the same. Creating awareness among the children and youth would be of top priority as they will easily understand the issues that we are facing. Planting new trees is as important as looking after the existing ones. So, my volunteers and I would water the plants and trees on a regular basis.

8. Explain how deforestation leads to reduced rainfall.

Soln:

Removal of trees and plant life from a particular area for the construction of industries, agriculture and other purposes is termed as Deforestation. Photosynthesis, in plant life, takes place by the absorption of CO₂ from the earth’s atmosphere. When the plant life is destroyed, the Carbon Dioxide levels in the atmosphere will increase drastically leading trap of heat radiation which causes global warming. This rise in temperature will affect the water cycle. Disturbance in the water cycle will lead to a chain of events and in the end, the rainfall pattern changes and results in droughts.

9. Find out about national parks in your state. Identify and show their location on the outline map of India

Soln:

Periyar National park located in the state of Kerala.

10. Why should paper be saved? Prepare a list of ways by which you can save paper

Soln:

If one ton of clean white paper is to be produced, seventeen full grown trees are cut down. Trees play a major role in maintaining a balance in nature. So, it is essential to save paper in order to protect the trees and avoid the impact of the loss of trees on the living organisms that depend on these trees.

Ways to save paper:

• Collection and recycling of used paper.
• Using both sides of a paper for writing.
• Spreading awareness about the importance of paper among school children and youth.
• Intelligent and proper use of paper.

11. Complete the word puzzle.

Down

1. Species on the verge of extinction.

2. A book carrying information about endangered species.

5. Consequences of deforestation.

Across

1. Species which have vanished.

3. Species found only in a particular habitat.

4. Variety of plants, animals and microorganisms found in an area.

Down

1. Endangered
2. Red Data Book
3. Deserts

Across

1. Extinct
2. Endemic
3. Biodiversity

4. Subtopics of NCERT Solutions Class 8 Science Chapter 7 Conservation of Plants and Animals

   1. Deforestation and Its Causes
   2. Consequences of Deforestation
   3. Conservation of Forest and Wildlife
   4. Biosphere Reserve
   5. Flora and Fauna
   6. Endemic Species
   7. Wildlife Sanctuary
   8. National Park
   9. Red Data Book
   10. Migration
   11. Recycling of Paper
   12. Reforestation.

       Chapter 7 Conservation of Plants and Animals

       Why should we conserve biodiversity?

       The number and variety of various life forms such as plants, animals and micro-organisms in the area are called Biodiversity. Both plants and animals have a mutual dependence on each other for their survival. As they are related to one another, destruction of one will automatically destroy the other. Hence, biodiversity has to be conserved in order to maintain nature’s balance.

       What is Red Data Book?

       Red data book is basically a sourcebook which has an international list of all plant and animal species which are endangered, that is, on the verge of extinction. The International Union for Conservation of Nature and Natural resources (IUCN) maintains the books and adds/ removes the names of the species by conducting a comprehensive survey.

       What are the causes and consequences of deforestation?

       There are a lot of causes for deforestation. A few of them are listed below.

       1. In order to accommodate the ever-expanding urban population, forest areas are often cleared and converted into lands for various uses.
       2. For agricultural activities like crop cultivation and cattle grazing, forest lands are cleared.
       3. Firewood is a major product from the forests and it is one of the main reasons for cutting the trees from on a large scale.

       Consequences of deforestation:

       There are a lot of fatal consequences caused by deforestation. A few of them are listed below.

       1. Soil erosion
       2. Loss of Biodiversity
       3. Floods
       4. Droughts
       5. Global warming as a result of climate change
       6. Disruption of the water cycle

       What will happen if we go on cutting trees?

       If we go on cutting the trees, the animals will lose their natural habitat as essential part of their ecosystem are destroyed. This reduces the biodiversity of the affected areas. The temperature of the Earth also increases and gives way to global warming. Global warming, in turn, affects the water cycle. Hence, rainfall pattern changes leading to droughts and floods. Risks of soil erosion, global warming, desertification, greenhouse effect increases.

       What will happen if the top layer of soil is exposed?

       Removing the top layer of the soil gradually exposes the lower layers of the soil. The lower layers of the soil are hard and rocky in nature and are less fertile. This is because of the reduced quantity of humus. Soil erosion on a continued basis will turn it barren and infertile.

       How does deforestation lead to reduced rainfall?

       Removal of trees and plant life from a particular area for the construction of Industries, agriculture and other purposes is termed as Deforestation. Photosynthesis, in plant life, takes place by the absorption of CO2 from the earth’s atmosphere. When the plant life is destroyed, the Carbon Dioxide levels in the atmosphere will increase drastically leading trap of heat radiations which causes global warming. This rise in temperature will affect the water cycle. Disturbance in the water cycle will lead to a chain of events and in the end, the rainfall pattern changes and results in droughts.

class 8 Chapter 6 Combustion and Flame

Science NCERT class 8 Chapter 6 Combustion and Flame

Exercise Questions

1. List conditions under which combustion can take place.

Soln:

The burning of a substance in the presence of oxygen is defined as combustion.

The conditions under which combustion can take place are

• The presence of air or oxygen.
• The presence of fuel plays a significant role.
• Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)

2. Fill in the blanks.

(a) Burning of wood and coal causes __________of air.

(b) A liquid fuel, used in homes is__________ .

(c) Fuel must be heated to its ____________ before it starts burning.

(d) The fire produced by oil cannot be controlled by___________ .

Soln:

(a) Burning of wood and coal causes Pollution of air.

(b) A liquid fuel, used in homes is Kerosene.

(c) Fuel must be heated to its Ignition Temperature before it starts burning.

(d) The fire produced by oil cannot be controlled by Water.

3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Soln:

CNG played an important role in reducing pollution among automobiles for the following reasons

• CNG is comparatively a cleaner fuel.
• The CNG can be an alternative for diesel, petrol and propane/LPG.
• It usually contains a few undesirable gases than the other fuels mentioned above.
• The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide which leads to respiratory diseases.

4. Compare LPG and wood as fuels

Soln:

Wood

• It is considered as a traditional fuel used for both domestic and industrial purposes.
• Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
• The usage of wood to a large extent causes deforestation.
• The calorific value of wood ranges between 17000 to 22000 kJ/kg
• However, wood may be used as a furnace, stove or fireplace in indoors while it is used for a campfire, furnace at outdoors.

      LPG

• The usage LPG (Liquefied petroleum gas) has replaced wood.
• It doesn’t release smoke and other pollutants
• It is a cleaner fuel
• The fuel efficiency of LPG is more than that of wood.
• The calorific value of LPG is 55000 kJ/kg
• Hence LPG is mostly preferred choice

5. Give reasons.

(a) Water is not used to control fires involving electrical equipment.

(b) LPG is a better domestic fuel than wood.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Soln:

a)

• Water is a good conductor of electricity.
• If added to an electrical fire, the water would just spread the electricity further.
• The person dousing the fire might get an electric shock

b)

• LPG being a cleaner fuel than wood doesn’t release smoke and other pollutants.
• Wood, on the other hand, releases a lot of smoke and fumes polluting the atmosphere causing pollution and leading to respiratory diseases.
• Hence LPG is a better domestic fuel than wood.

c)

• The paper by itself catches fire easily because of its low ignition temperature
• The piece of paper wrapped around an aluminium pipe doesn’t catch fire because aluminium is a good conductor of electricity.
• While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So there is a transfer of heat from paper to the aluminium pipe. Hence it doesn’t catch fire.

6. Make a labelled diagram of a candle flame.

Soln:

7. Name the unit in which the calorific value of a fuel is expressed.

Soln:

Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg

kJ=kilo joules and kg=kilogram

8. Explain how CO₂ is able to control fires.

Soln:

CO₂ is a non-combustible gas and extinguishes fire in two ways:

(i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.

(ii) In cylinders, CO₂ is kept in the liquid form. When released, it expands enormously. This brings down the temperature of the fuel, which helps in controlling the fire.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Soln:

A heap of green leaves contains a lot of moisture in it, hence its ignition temperature is high. Therefore it does not catch fire easily.

But dry leaves have no moisture content in it, hence its ignition temperature is low. Therefore it catches fire easily.

10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Soln:

The goldsmith mainly uses non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the flame.

11. In an experiment, 4.5 kg of a fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.

Soln:

12. Can the process of rusting be called combustion? Discuss.

Soln:

No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.

13. Abida and Ramesh were doing an experiment in which water was to be heated in a beaker. Abida kept the beaker near the wick in the yellow part of the candle flame. Ramesh kept the beaker in the outermost part of the flame. Whose water will get heated in a shorter time?

Soln:

The water placed in the outermost part of the flame will be heated in a short time since it is non-luminous flame and is regarded as the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida who placed the beaker in the luminous flame (yellow flame)is comparatively less hot.

List conditions under which combustion can take place?

The burning of a substance in the presence of oxygen is defined as combustion.

The conditions under which combustion can take place are

• The presence of air or oxygen.
• The presence of fuel plays a significant role.
• Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest temperature.)

Explain how the use of CNG in automobiles has reduced pollution in our cities?

CNG played an important role in reducing pollution among automobiles for the following reasons

• CNG is comparatively a cleaner fuel.
• The CNG can be an alternative for diesel, petrol and propane/LPG.
• It usually contains a few undesirable gases than the other fuels mentioned above.
• The combustion of fuels like petroleum causes many unburnt carbon particles along with carbon monoxide which leads to respiratory diseases.

Comparison of LPG and wood as fuels?

Wood

• It is considered as a traditional fuel used for both domestic and industrial purposes.
• Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
• The usage of wood to a large extent causes deforestation.
• The calorific value of wood ranges between 17000 to 22000 kJ/kg
• However, wood may be used as a furnace, stove or fireplace in indoors while it is used for a campfire, furnace at outdoors.

LPG

• The usage LPG (Liquefied petroleum gas) has replaced wood.
• It doesn’t release smoke and other pollutants
• It is a cleaner fuel
• The fuel efficiency of LPG is more than that of wood.
• The calorific value of LPG is 55000 kJ/kg
• Hence LPG is mostly preferred choice

How the calorific value of a fuel is expressed?

Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg

kJ-kilo joules

kg- kilogram

Which zone of a flame does a goldsmith use for melting gold and silver and why?

The goldsmith mainly uses non-luminous flame which is termed to be the outermost part of the flame. This part of the flame is used because the outermost flame undergoes complete combustion and is considered as the hottest part of the flame.

Can the process of rusting be called combustion? Discuss.

No, because rusting is an exothermic process as heat is liberated during rusting. On the other hand, combustion is a chemical process in which a substance reacts with oxygen to release energy in the form of heat or light.

class 8 Chapter 5 Coal and Petroleum

 class 8 Chapter 5 Coal and Petroleum

Exercise Questions

1. What are the advantages of using CNG and LPG as fuels?

Soln:

The advantages of using CNG and LPG as fuels are as follows

• CNG and LPG burn easily.
• CNG and LPG give a lot of heat energy when burnt.
• CNG and LPG can be transported easily through pipelines.
• CNG and LPG are clean fuels and they do not release smoke when burnt.

2. Name the petroleum product used for surfacing of roads.

Soln:

Bitumen is used for surfacing of roads.

3. Describe how coal is formed from dead vegetation. What is this process called?

Soln:

Dense forests got buried under the soil due to natural processes, millions of years ago. More and more soil got deposited over them and they got compressed more. This led them to get exposed to very high temperature and pressure. They slowly got converted into coal under these extreme conditions. The whole process of formation of coal from dead vegetation is known as carbonization.

 4. Fill in the blanks.

(a) Fossil fuels are  ____________,  ____________, and ____________ .

(b) Process of separation of different constituents from petroleum is called _____________ .

(c) Least polluting fuel for a vehicle is ______________ .

Soln:

(a) Fossil fuels Coal, Petroleum, and natural gas.

(b) Process of separation of different constituents from petroleum is called refining.

(c) Least polluting fuel for a vehicle is Compressed Natural Gas (CNG).

5. Tick True/False against the following statements.

(a) Fossil fuels can be made in the laboratory. (T/F)

(b) CNG is more polluting fuel than petrol. (T/F)

(c) Coke is the almost pure form of carbon. (T/F)

(d) Coal tar is a mixture of various substances. (T/F)

(e) Kerosene is not a fossil fuel. (T/F)

Soln:

a) False

b) False

c) True

d) True

e) False

6. Explain why fossil fuels are exhaustible natural resources.

Soln:

The process of formation of fossil fuels requires millions of years. Dead vegetation and animals that get buried deep inside the earth require high temperature and pressure for the formation of fossil fuels, which cannot be done in the laboratory. Thus, fossils are limited. Therefore, the use of fossil fuels at a rapid rate will lead to their exhaustion.

7. Describe the characteristics and uses of coke.

Soln:

Characteristics:

i) Coke is tough.

ii) Coke is porous.

iii) Coke is black in colour.

Uses:

i) In the manufacture of steel.

ii) In the extraction of metals (as a reducing agent).

8. Explain the process of the formation of petroleum.

Soln:

Dead organisms that got buried in the sea millions of years ago got covered with layers of sand and clay. Due to lack of air, high temperature, and high pressure, these dead organisms got transformed into petroleum and natural gas.

9. The following table shows the total power shortage in India from 1991–1997. Show the data in the form of a graph. Plot shortage percentage for the years on the Y-axis and the year on the X-axis.

S.No.	Year	Shortage (%)
1.	1993	7.7
2.	1994	7.5
3.	1995	8.2
4.	1996	7.1
5.	1997	7.7
6.	1998	9.1
7.	1999	11.2

Soln:

4 sub-units of Class 8 Chapter 5 Coal and Petroleum

• Coal – Story of coal – coke – coal tar – coal gas
• Petroleum – refining of petroleum – – definition of petroleum
• Natural gas
• Some natural resources are limited

List of important keywords to pay attention

• Coal
• Coal gas
• Coal tar
• Coke
• Fossil fuel
• Natural gas
• Petroleum
• Petroleum refinery

• Name the petroleum product used for surfacing of roads?

  Bitumen is used for surfacing of roads.

  What are the advantages of using CNG and LPG as fuels?

  The advantages of using CNG and LPG as fuels are as follows

  • CNG and LPG burn easily.
  • CNG and LPG give a lot of heat energy when burnt.
  • CNG and LPG can be transported easily through pipelines.
  • CNG and LPG are clean fuels and they do not release smoke when burnt.

  Describe how coal is formed from dead vegetation. What is this process called?

  Dense forests got buried under the soil due to natural processes, millions of years ago. More and more soil got deposited over them and they got compressed more. This led them to get exposed to very high temperature and pressure. They slowly got converted into coal under these extreme conditions. The whole process of formation of coal from dead vegetation is known as carbonization.

  Describe the characteristics and uses of coke?

  Characteristics:

  1. Coke is tough.
  2. Coke is porous.
  3. Coke is black in colour.

  Uses:

  1. In the manufacture of steel.
  2. In the extraction of metals (as a reducing agent).

  Explain the process of formation of petroleum?

  Dead organisms that got buried in the sea millions of years ago got covered with layers of sand and clay. Due to lack of air, high temperature, and high pressure, these dead organisms got transformed into petroleum and natural gas.

class 8 Chapter 4 Materials: Metals and Non-Metals

class 8 Chapter 4 Materials: Metals and Non-Metals

Exercise Questions

1. Which of the following can be beaten into thin sheets?

(a) Zinc

(b) Phosphorus

(c) Sulphur

(d) Oxygen

Soln:

Answer is a) Zinc

Explanation:

Here, Zinc is a metal with malleability and ductility whereas Phosphorus, Sulphur and Oxygen are nonmetals which lack malleability and ductility.

2. Which of the following statements is correct?

(a) All metals are ductile.

(b) All non-metals are ductile.

(c) Generally, metals are ductile.

(d) Some non-metals are ductile.

Soln:

Answer is (c) Generally, metals are ductile.

Explanation:

Ductility is a property where a substance can be drawn into thin wires, Generally metals are ductile with mercury as the exception.

3. Fill in the blanks.

(a) Phosphorus is a very _________non-metal.

(b) Metals are conductors of heat and ____________ .

(c) Iron is ____________reactive than copper.

(d) Metals react with acids to produce ____________ gas.

Soln:

(a) Phosphorus is a very reactive non-metal.

(b) Metals are good conductors of heat and electricity.

(c) Iron is more reactive than copper.

(d) Metals react with acids to produce hydrogen gas.

4. Mark ‘T’ if the statement is true and ‘F’ if it is false.

(a) Generally, non-metals react with acids. ( )

(b) Sodium is a very reactive metal. ( )

(c) Copper displaces zinc from zinc sulphate solution. ( )

(d) Coal can be drawn into wires. ( )

Soln:

a) False

b) True

c) False

d) False

5. Some properties are listed in the following table. Distinguish between metals and non-metals on the basis of these properties.

Properties	Metals	Non-metals
1. Appearance
2. Hardness
3. Malleability
4. Ductility
5. Heat Conduction
6. Conduction of Electricity

Soln:

Properties	Metals	Non-metals
1. Appearance	Lustrous	Dull
2. Hardness	Hard	Soft
3. Malleability	Have property of Malleability	Do not have a property of Malleability
4. Ductility	Have property of Ductility	Do not have the property of Ductility
5. Heat Conduction	Good conductor of Heat	Bad Conductor of Heat
6. Conduction of Electricity	Good conductor of Electricity	The bad conductor of Electricity

6. Give reasons for the following.

(a) Aluminium foils are used to wrap food items.

(b) Immersion rods for heating liquids are made up of metallic substances.

(c) Copper cannot displace zinc from its salt solution.

(d) Sodium and potassium are stored in kerosene

Soln:

a) Aluminium is malleable and can be drawn into thin sheets hence Aluminium foils are used to wrap food items

b) Immersion rods for heating liquids are made up of metallic substances because metals are good conductors of heat and electricity.

c) Copper cannot displace zinc from its salt solution because Zinc is more reactive than copper.

d) Sodium and Potassium are highly reactive metals which readily reacts with atmospheric Oxygen to catch fire hence Sodium and Potassium are stored in kerosene.

7. Can you store the lemon pickle in an aluminium utensil? Explain.

Soln:

Pickle consists of acids which react with Aluminium metal to produce salt and Hydrogen. Hence pickle is not stored in aluminium utensil.

8. Match the substances given in Column A with their uses given in Column B.

A	B
(i) Gold	(a) Thermometers
(ii) Iron	(b) Electric wire
(iii) Aluminium	(c) Wrapping food
(iv) Carbon	(d) Jewellery
(v) Copper	(e) Machinery
(vi) Mercury	(f) Fuel

Soln:

A	B
(i) Gold	(d) Jewellery
(ii) Iron	(e) Machinery
(iii) Aluminium	(c) Wrapping food
(iv) Carbon	(f) Fuel
(v) Copper	(b) Electric wire
(vi) Mercury	(a) Thermometers

9. What happens when

(a) Dilute sulphuric acid is poured on a copper plate?

(b) Iron nails are placed in a copper sulphate solution?

Write word equations of the reactions involved.

Soln:

(i) No reaction occurs when dilute sulphuric acid is poured on a copper plate. However, when concentrated sulphuric acid is poured on a copper plate, hydrogen gas evolves along with the formation of blue coloured copper sulphate crystals. The chemical reaction for the reaction between concentrated sulfuric acid and copper is:
Cu + H₂SO₄ (conc.) -> CuSO₄ + H₂

ii) Iron being more reactive displaces copper from copper sulphate. In this reaction, the blue colour of copper sulphate fades and there is a deposition of copper on the iron nail.

Fe + CuSO₄  →      FeSO₄ + Cu

10. Saloni took a piece of burning charcoal and collected the gas evolved in a test tube.

(a) How will she find the nature of the gas?

(b) Write down word equations of all the reactions taking place in this process.

Soln:

a) In a test tube containing gas, add a few drops of water. Now cover the test tube and shake well. After shaking, test the solution with blue litmus. It will change from blue to red. Thus, gas is acidic in nature.

b) Charcoal reacts with oxygen to form carbon dioxide gas.

11. One day Reeta went to a jeweller’s shop with her mother. Her mother gave old gold jewellery to the goldsmith to polish. Next day when they brought the jewellery back, they found that there was a slight loss in its weight. Can you suggest a reason for the loss in weight?

Soln:

In order to polish the gold ornament, it is to be dipped into a liquid called aqua regia (a mixture of hydrochloric acid and nitric acid). On getting dissolved in the environment of aqua regia, the outer layer of gold dissolves and an inner shiny layer appears. The dissolving of the layer causes a reduction in the weight of the jewellery.

 Materials: Metals and Non-Metals

Which of the following can be beaten into thin sheets? (a) Zinc (b) Phosphorus (c) Sulphur (d) Oxygen

(b) Zinc

Can you store the lemon pickle in an aluminium utensil?

Pickle consists of acids which react with Aluminium metal to produce salt and Hydrogen. Hence pickles are not stored in aluminium utensil.

Science NCERT class 8 Chapter 3 Synthetic Fibres and Plastics

Science NCERT class 8 Chapter 3 Synthetic Fibres and Plastics

Exercise Questions

1. Explain why some fibres are called synthetic.

Soln:

Some fibres are called as synthetic fibers because these are manmade fibers prepared by using chemicals. These are made of small units that join together to form long chains. Some of the examples of synthetic fibres are nylon, rayon, acrylic, polyester etc.

2. Mark the correct answer.

Rayon is different from synthetic fibres because

(a) it has a silk-like appearance.

(b) it is obtained from wood pulp.

(c) its fibres can also be woven like those of natural fibres.

Soln:

Answer is (b) it is obtained from wood pulp.

3. Fill in the blanks with appropriate words.

(a) Synthetic fibres are also called __________ or __________ fibres.

(b) Synthetic fibres are synthesised from raw material called __________.

(c) Like synthetic fibres, plastic is also a __________

Soln:

(a) Synthetic fibres are also called artificial or manmade fibres.

(b) Synthetic fibres are synthesised from raw material called petrochemicals.

(c) Like synthetic fibres, plastic is also a polymer.

4. Give examples which indicate that nylon fibres are very strong.

Soln:

Following are the examples that indicate nylon fibres are very strong

• Parachutes and ropes of rock climbing are made of nylon
• Nylon is used in making seat belts, fishing nets and tyre cords.
• Nylon is used in making sports accessories like rackets.

5. Explain why plastic containers are favored for storing food.

Soln:

Because plastic is non-reactive and will not react with the ingredients of the food items.

6. Explain the difference between thermoplastic and thermosetting plastics.

Soln:

The difference between the thermosetting plastics and thermoplastics are

Thermosetting plastic	Thermoplastic
Cannot be bent, it will break if we attempt to bend thermo setting plastic	Thermoplastic can be bent easily
On heating thermosetting plastics, it cannot be softened. This is the reason it cannot be reshaped once it is molded.	On heating the thermoplastics, it becomes softened and can be molded and reshaped easily.

7. Explain why the following are made of thermosetting plastics.

(a) Saucepan handles

(b) Electric plugs/switches/plug boards

Soln:

a) Thermosetting plastics are used to make saucepan handles because they are bad conductors of heat and also on heating, these plastics do not get softened.

b) Bakelite is a kind of thermosetting plastic which is a bad conductor of heat and electricity. Because of this property it is used for making electric plugs, switches, plug boards, etc.

8. Categorise the materials of the following products into ‘can be recycled’ and ‘cannot be recycled’.

Telephone instruments, plastic toys, cooker handles, carry bags, ball point pens, plastic bowls, plastic covering on electrical wires, plastic chairs, electrical switches.

Soln:

Can be Recycled	Cannot be recycled
Plastic chairs	Cooker handles
Plastic bowls	Electrical switches
Plastic toys	telephone
The plastic covering on electrical wires
Carry bags
Ballpoint pens

9. Rana wants to buy shirts for summer. Should he buy cotton shirts or shirts made from synthetic material? Advise Rana, giving your reason.

Soln:

Rana should buy a cotton shirt for the summer and not a synthetic shirt. Cotton is a good absorber of water. So it can soak the sweat coming out of the body and expose it to the environment. Thus, it helps in evaporating the sweat and helps in cooling our body.

10. Give examples to show that plastics are noncorrosive in nature.

Soln:

Plastics are non-corrosive in nature, they do not react even if they come in contact with strong chemicals. For example, the cleaning chemicals that we use at home are stored in plastic bottles, instead of metal containers.

11. Should the handle and bristles of a tooth brush be made of the same material? Explain your answer

Soln:

The handle and bristle of a toothbrush should not be made of the same material as the handle of the toothbrush should be hard and strong while the bristle should be soft and flexible.

12. ‘Avoid plastics as far as possible’. Comment on this advice.

Soln:

We should avoid plastics as far as possible because plastics are non-biodegradable in nature. Once introduced into the environment they cause pollution. If burnt they release poisonous gases and the the plastic bags thrown in the garbage dumb are swallowed by animals choke their respiratory system and causes fatalities in animals.

13. Match the terms of column A correctly with the phrases given in column B

A	B
(i) Polyester	(a) Prepared by using wood pulp
(ii) Teflon	(b) Used for making parachutes and stockings
(iii) Rayon	(c) Used to make non-stick cookwares
(iv) Nylon	(d) Fabrics do not wrinkle easily

Soln:

A	B
(i) Polyester	(d) Fabrics do not wrinkle easily
(ii) Teflon	(c) Used to make non-stick cookwares
(iii) Rayon	(a) Prepared by using wood pulp
(iv) Nylon	(b) Used for making parachutes and stockings

14. ‘Manufacturing synthetic fibres is actually helping conservation of forests’. Comment.

Soln:

The manufacturing of synthetic fibers is helpful in the conservation of forests because if we use natural fibers, the raw materials for them have to be derived from the plants, which require cutting off lots of trees. This proves to be helpful in the conservation of forests.

15. Describe an activity to show that thermoplastic is a poor conductor of electricity

Soln:

In order to show that thermoplastic is a poor conductor of electricity, we will design a circuit. For that, we need a bulb, some wires, a battery, a piece of metal and a plastic pipe (as shown in the figure below). After setting the experiment switch on the current, you will observe that the bulb glows in the former case. In the latter case, the bulb does not glow. Hence a plastic pipe (which is a thermoplastic) is shown to be a poor conductor of electricity.

Synthetic Fibres and Plastics

The different types of clothes which we wear are made of fabrics. These fabrics are made from different types of fibres. There are two types of Fibres – Natural Fibre and Synthetic fibre. Natural fibres are those which are obtained from plants and animal sources. They include cotton, jute, silk, wool, etc. The NCERT Solutions for class 8 science chapter 3 contains all the information about NCERT Solutions for class 8 Science chapter 3 synthetic Fibres and plastics.

Synthetic fibres are those which are man-made and are obtained by the combination of different types of chemical substance and raw materials such as petrochemicals. They include nylon, acrylic, polyester and so on. These fibres are also called as artificial or man-made fibres. There are 4 different types of Synthetic fibre.

1. Rayon — it is synthesized from wood pulp and is dyed using different colours. It is also called as artificial silk as it resembles the silk. Rayon is cheaper than silk.
2. Nylon — it is the first commercially synthesized fibre. It is synthesised by using coal, water and air. Nylon is very strong and its fabric resembles the silk.
3. Polyester — it is one of the most popular man-made fibres, which is widely used for making clothes. Polyesters are generally made of repeating unit of a chemical called esters.
4. Acrylic- It is a man-made fibre which is known as artificial wool or synthetic wool as it resembles wool. Compared to the natural wool, Acrylic is much cheaper and it can be dyed by various colours.

Plastics are the polymers which can be coloured, recycled, reused, mould and can be drawn into wires or various other shapes. They are mainly used in making bags, brush, chairs, tables, toys, and many other countless items. Polythene is one of the most famous types of plastic, which is used in the manufacturing of carrying bags.

Sub-topics of class 8 Science NCERT solutions chapter 3 synthetic Fibres and plastics

1. What are synthetic fibres
2. Types of synthetic fibres
3. Characteristics of synthetic fibres
4. Plastics
5. Plastic as a material of choice
6. Plastics and environment

   why some fibres are called synthetic?

   Some fibres are called as synthetic fibers because these are manmade fibers prepared by using chemicals. These are made of small units that join together to form long chains. Some of the examples of synthetic fibres are nylon, rayon, acrylic, polyester etc.

   which indicate that nylon fibres are very strong?

   Following are the examples that indicate nylon fibres are very strong

   • Parachutes and ropes of rock climbing are made of nylon
   • Nylon is used in making seat belts, fishing nets and tyre cords.
   • Nylon is used in making sports accessories like rackets.

   why plastic containers are favored for storing food?

   Because plastic is non-reactive and will not react with the ingredients of the food items.

   How to show that plastics are noncorrosive in nature?

   Plastics are non-corrosive in nature, they do not react even if they come in contact with strong chemicals. For example, the cleaning chemicals that we use at home are stored in plastic bottles, instead of metal containers

NCERT class 8 Chapter 2 Microorganisms: Friend and Foe

NCERT class 8 Chapter 2 Microorganisms: Friend and Foe

Exercise Questions

1. Fill in the blanks.

(a) Microorganisms can be seen with the help of a ____________.

(b) Blue green algae fix __________ directly from air and enhance fertility of soil.

(c) Alcohol is produced with the help of __________.

(d) Cholera is caused by __________.

Soln:

a) Microscope

b) Nitrogen

c) Yeast

d) Bacteria- Vibrio cholera

2. Tick the correct answer.

(a) Yeast is used in the production of

(i) sugar (ii) alcohol (iii) hydrochloric acid (iv) oxygen

(b) The following is an antibiotic

(i) Sodium bicarbonate (ii) Streptomycin (iii) Alcohol (iv) Yeast

(c) Carrier of malaria-causing protozoan is

(i) female Anopheles mosquito (ii) cockroach (iii) housefly (iv) butterfly

(d) The most common carrier of communicable diseases is

(i) ant (ii) housefly (iii) dragonfly (iv) spider

(e) The bread or idli dough rises because of

(i) heat (ii) grinding (iii) growth of yeast cells (iv) kneading

(f) The process of conversion of sugar into alcohol is called

(i) nitrogen fixation (ii) moulding (iii) fermentation (iv) infection

Soln:

a) (ii) alcohol

b) (ii) Streptomycin

c) (i) female Anopheles mosquito

d) (ii) housefly

e) (iii) growth of yeast cells

f) (iii) fermentation

3. Match the organisms in Column A with their action in Column B.

A	B
(i) Bacteria	(a) Fixing nitrogen
(ii) Rhizobium	(b) Setting of curd
(iii) Lactobacillus	(c) Baking of bread
(iv) Yeast	(d) Causing malaria
(v) A protozoan	(e) Causing cholera
(vi) A virus	(f) Causing AIDS
	(g) Producing antibodies

Sol:-

A	B
(i) Bacteria	(e) Causing cholera
(ii) Rhizobium	(a) Fixing nitrogen
(iii) Lactobacillus	(b) Setting of curd
(iv) Yeast	(c) Baking of bread
(v) A protozoan	(d) Causing malaria
(vi) A virus	(f) Causing AIDS

4. Can microorganisms be seen with the naked eye? If not, how can they be seen?

Soln:

Microorganisms cannot be seen by naked eye because they are very small. They can be seen by using microscope.

5. What are the major groups of microorganisms?

Soln:

Major group of microrganisms are as follows

1. Bacteria
2. Fungi
3. Protozoans
4. Algae
5. Viruses

6. Name the microorganisms which can fix atmospheric nitrogen in the soil.

Soln:

Rhizobium

7. Write 10 lines on the usefulness of microorganisms in our lives.

Soln:

Uses of microorganisms are listed below

• They are helpful in cleaning up of the environment. For example, the organic wastes (vegetable peels, remains of animals, faeces, etc.) are broken down into harmless and usable substances by bacteria.
• Curd contains several microorganisms. Of these, the bacterium, Lactobacillus promotes the formation of curd. It multiplies in milk and converts it into curd.
• Bacteria are also involved in the making of cheese, pickles and many other food items.
• Bacteria and yeast are also helpful for fermentation of rice idlis and dosa batter.
• Yeast reproduces rapidly and produces carbon dioxide during respiration. Bubbles of the gas fill the dough and increase its volume. This is the basis of the use of yeast in the baking industry for making breads, pastries and cakes.
• Microorganisms are used for the large scale production of alcohol, wine and acetic acid (vinegar).
• Antibiotics are being produced from bacteria and fungi.
• Microorganisms are helpful in producing vaccines against disease causing agents.
• Some bacteria (Fig. 2.7) are able to fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.
• Microorganisms decompose dead organic waste of plants and animals to convert them into simple substances. These substances are again used by other plants and animals. Thus, microorganisms can be used to degrade the harmful and smelly substances and thereby clean up the environment.

8. Write a short paragraph on the harmful effects of microorganisms.

Soln:

Microorganisms causes diseases in plants and animals Ex; Cholera in humans is caused by bacteria called Vibrio cholera. In animals virus causes foot and mouth diseases. Microorganisms spoil the foods by decomposing the food material. Some microorganisms spoil leather and clothes. Microorganisms are responsible for food poisoning.

9. What are antibiotics? What precautions must be taken while taking antibiotics?

Soln:

Chemical agents that kill or stop the growth of disease causing microorganisms are called antibiotics. Antibiotics should be taken only on the advice of a qualified doctor. Also one must complete the course prescribed by the doctor.

Sub Topics of NCERT Solutions Class 8 Science Chapter 2 Microorganisms Friend and Foe   

1. Microorganisms
2. Habitats of Microorganisms
3. Microorganisms and us
4. Harmful microorganisms
5. Food preservation
6. Nitrogen fixation
7. Nitrogen cycle

   Can microorganisms be seen with the naked eye? If not, how can they be seen?

   Microorganisms cannot be seen by naked eye because they are very small. They can be seen by using microscope.

   What are the major groups of microorganisms?

   Major group of microrganisms are as follows

   1. Bacteria
   2. Fungi
   3. Protozoans
   4. Algae
   5. Viruses

   Name the microorganisms which can fix atmospheric nitrogen in the soil?

   Rhizobium

   what are the harmful effects of microorganisms?

   Microorganisms causes diseases in plants and animals Ex; Cholera in humans is caused by bacteria called Vibrio cholera. In animals virus causes foot and mouth diseases. Microorganisms spoil the foods by decomposing the food material. Some microorganisms spoil leather and clothes. Microorganisms are responsible for food poisoning.

   What are antibiotics?

   Chemical agents that kill or stop the growth of disease causing microorganisms are called antibiotics.