CBSE Class 8 Science Chapter 12 Some Natural Phenomena Very Short Answer Type Questions

Question 1. If a charged Plastic straw is brought near an uncharged plastic straw, what will happen?
Answer: Any charged plastic straw will attract another uncharged plastic straw when brought near it.

Question 2. Mention the name of the nature of the charges on a glass rod and a silk cloth when both of them are rubbed with each other.
Answer: When a glass rod and a silk cloth are rubbed with each other, then the positive charge gets induced on the glass rod and the negative charge on the silk cloth.

Question 3. Plastic straws A and 8 are rubbed with dry cotton cloth. What will happen if they are brought near each other?
Answer: The rods will acquire a similar charge, and hence they will repel each other.

Question 4. If a charged plastic straw is brought near another uncharged plastic straw, then what happens?
Answer: If a charged plastic straw is brought near the uncharged
plastic straw, then they will attract each other.

Question 5. The aluminium strips in an electroscope, as shown in the figure, are replaced by plastic strips, and a charged body is brought in contact with the metal clip. What will happen?

Answer: If aluminium strips are replaced by plastic strips, we cannot predict whether the object is charged or not.

Class 8 Science Chapter 7: Reaching The Age Of Adolescence Fill In the Blanks

Question 1. After attaining puberty, boys develop a voice.
Answer: Deep

Question 2. The sex hormones and oestrogen are responsible for the development of sexual characteristics.
Answer: Testosterone

Question 3. Release of sex hormones is under the control of a hormone secreted from the
Answer: Pituitary gland

Read And Learn More CBSE Class 8 Science Question And Answers

Question 4. Endocrine glands release hormones directly into……for transportation to the
Answer: Bloodstream, target site

Question 5. In females, the uterine wall thickens to receive the……
Answer: Fertilised egg

Question 6. AIDS is caused by…
Answer: Virus

Question 7. The legal age for marriage is … years for girls and … years for boys.
Answer: 18,21

Class 8 Science Chapter 7: Reaching The Age Of Adolescence True/False

Question 1. Puberty starts later in girls than in boys.
Answer: False, it starts earlier in girls than in boys

Question 2. Girls have a high-pitched voice, whereas boys have a deep voice.
Answer: True

Question 3. Mood swings take place during adolescence.
Answer: True

Question 4. Oestrogen is a female hormone.
Answer: False, oestrogen is a female hormone

Question 5. Males have both X and Y chromosomes
Answer: True

Question 6. Sex hormones are under the control of hormones produced by the pituitary gland.
Answer: True

Question 7. Chromosomes are present inside the cytoplasm of every cell
Answer: False

Class 8 Science Chapter 7: Reaching The Age Of Adolescence Match The Columns

Question 1. Match the hormones given in Column I with their deficiency diseases given in Column II.

Answer: A-3, B-4, C-2, D-1

Question 2. Match Column I with Column II.

Answer: A-4,B-3,C-2,D-1

Class 8 Science Chapter 7: Reaching The Age Of Adolescence

1. The following questions consist of two statements: Assertion and Reason (R). Answer these questions by selecting the appropriate option given below.

1. Both A and R are true, and R is the correct explanation of A
2. Both A and R are true, but R is not the correct explanation of A
3. A is true, but R is false
4. A is false, but R is true

Answer: 10. A-4, B-3.C-2.D-1

Question 11. Assertion Menstrual cycle is controlled by hormones. Reason (R) The first menstrual flow began at puberty and is termed menarche.
Answer: Assertion Menstrual cycle is controlled by hormones. Reason (R) The first menstrual flow began at puberty and is termed menarche

Question 12. Assertion: Testosterone is a female hormone. Reason (R) It is involved in the development of the reproductive organs and controls secondary sexual characteristics

Answer: Assertion: Testosterone is a female hormone. Reason (R) It is involved in the development of the reproductive organs and controls secondary sexual characteristics.

2. The following questions consist of two assertions, find Roaton (R). Answer the following questions by selecting the appropriate option given below

1. Both A and it are true, and it is the correct explanation of A.
2. Both A and it are true, but K is not the correct explanation of A.
3. A is true, but it is false.
4. A is false, but it is true

Question 1.

Assertion: The Height of an individual is roughly comparable to that of a family member.
Reason (R): Height depends on the genes inherited from parents

Answer: Both A and R are true, and R is the correct explanation of A.

Question 2.

Assertion: Young people get acne and pimples on the face at the time of puberty.
Reason (R): It is caused due to increased activity of the adrenal gland.

Answer: A is true, but R is false. R can be corrected as Young people get acne and pimples on the face due to increased activity of sweat and sebaceous glands

Question 3.

Assertion: Exocrine glands are also called ductless glands.
Reason (R): They release hormones into blood bloodstream to reach a target site.

Answer: A is false, but R is true. A can be corrected as Endocrine glands are called ductless glands. They release hormones directly into the bloodstream to reach a target site.

Question 4.

Assertion: Diet for an adolescent has to be carefully planned.
Reason (R): Chips and packed or tinned snacks should be included in a regular meal.

Answer: A is true, but R is false. R can be corrected as the diet for an adolescent has to be carefully planned because it is a stage of rapid growth and development. Meals should include proteins, carbohydrates, fats and vitamins in requisite proportions.

Question 5.

Assertion: Teenagers should take care of their hygiene.
Reason (R): The increased activity of sweat glands makes their body smell.

Answer: Both A and R are true, and R is the correct explanation of A

Easy-To-Understand Notes On Adolescence Chapter Class 8 Reaching the Age Of Adolescence, Adolescence, And Puberty

Growth is a process that begins from the day a baby is born. It is a slow and natural process. The period oflife, when the body changes, leading to reproductive maturity, is called adolescence.

It begins around the age of 11 to IS or 19 years of age. Since the adolescence period includes the teenage period (i.e.13-19 years of age), adolescents are also called teenagers.

The period of adolescence varies from person to person. In girls, it may begin a year or two earlier than in boys.

Puberty

It is a period during which the human body undergoes certain changes by which adolescents become capable of reproduction. The puberty ends when an adolescent reaches reproductive maturity.

Creative Way To Teach Adolescence Chapter Class 8 CBSE Ganges At Puberty

Growth of the body is an important characteristic during puberty. The rapid growth is due to two main types of hormones, i.e., sex hormones and growth hormones. Some of the conspicuous changes that take place during puberty are given below.

Increase in Height

A sudden increase in height is the most prominent change during puberty. The long bones (i.e., the bones of the arms and the legs) elongate at this time, making a person tall.

The rate of growth in height varies from person to person. Girls grow faster than boys initially, but by about 18 years of age, both reach their maximum height.

Read And Learn More CBSE Class 8 Science Notes

Sometimes the arms and the legs or the hands and the feet of adolescent boys and girls look oversized and out of proportion with the body. But soon the other parts catch up with them in growth, resulting in a more proportionate body.

The average rate of growth in height of boys and girls with age is given in the table.

The table gives the approximate percentage of maximum or lull height a person has achieved particular age.

The maximum height ot a person (t.e, both girls and boys) can he calculated by the following formula

Maximum height (cm)= Present height (cm) % of full height at this age x 100

Example: A boy is years old and 120 cm tall. At the end of the growth period, he is likely to be

120/75 x 100 cm lot) 160 tall

It is to be noted that the height of an individual also depends upon the genes inherited from their parents.

However, one must take a nutritious diet for the proper growth of bones, muscles, and other parts of the body during the growing period.d

Change in Body Shape

During adolescence, changes in body shape take place. These changes are different in boys and girls as follows.ws

In boys, the shoulders become broader and the chests become wider. The muscles of the body grow more prominently.

In girls, the pelvic region (region below the waist) widens, hips broaden, breasts develop, lop, and increase in size.

Change In Voice

The voice box or larynx produces sound when we are talking. At puberty, it begins to grow. It is bigger in boys and can be seen as a protruding part of the throat called Adam’s apple.

In girls, the larynx is smaller and hardly visible from the outside. This is the reason why boys develop a deep voice, whereas girls have a high-pitched voice. In adolescent boys, sometimes, the muscles of the growing voice box go out of control and the voice becomes hoarse. This state may remain for a few days or weeks, after which the voice becomes normal.

Increased Activity of Sweat and Sebaceous Glands

There is an increase in the activity of sweat and sebaceous (ml) glands. This leads to increased sweating, often resulting in body odour. Many young people get acne and pimples on the face at this time because of the increased activity of these glands in the skin.

Development of Sex Organs

The reproductive system starts honing at puberty due to the complete growth and development of organs. In boys and girls,

1. In boys the male sex organs like tester and penis develop completely. The testes start producing sperm.
2. In girls, the ovaries enlarge and eggs begin to mature. Also, ovaries start releasing mature eggs (only one egg is released per month by either ovary)

Reaching Mental, Intellectual, and Emotional Maturity

Adolescence is also a period that brings about development in a person’s way of thinking. The pc becomes more independent and self-conscious.

Intellectual development also occurs, and they tend to spend considerable time on thinking, planning, analysing, exchanging views and ideas with others. It is the time in one’s life when the brain has the highest capacity for learning.

There are a lot of emotional and physical changes during puberty. These changes in body and mind can lead to confusion and insecurity in adolescents. There is, however, no reason to feel insecure, as puberty is a natural process of growing up that everyone passes through.

How To Remember Adolescence Changes For Class 8 Exam: Primary And Secondary Sexual Characteristics

Sexual characteristics are the physical traits of an organism that indicate its biological sex. These mainly include the following characters

1. Primary sexual characteristics are present since birth and determine a child’s gender. Example: testes in males and ovaries in females.
2. Secondary sexual characteristics appear later at the tiiue ot puberty and are not directly Involved In the process of reproduction.
3. Some of the secondary sexual characteristics that develop in boys and girls are as follows.

In Boys

1. Development of facial hair, such as beards and moustaches.
2. Muscles develop, and shoulders become broad.
3. Increase in weight.
4. The voice becomes deeper.
5. Hair develops under the armpit, on the chest, and in the pubic region

In Girls

1. Development and enlargement of the breast.
2. The hip region becomes broader, and the pelvic region widens.
3. Initiation of the menstrual cycle.
4. The voice becomes high-pitched.
5. Hair develops under the armpit and in the pubic region

Creative Way To Teach Adolescence Chapter Class 8 CBSE Hormones

These are chemical substances. They are secreted by the endocrine glands or endocrine system and are released into the bloodstream to reach the target site (i.e., a particular body part), which then responds to the hormone.

The changes that occur at adolescence are controlled by two hormones, i.e., testosterone in males and oestrogen/ estrogen in females.s

Role of Hormones in Initiating Reproductive Function

Puberty begins with a sudden increase in the production of hormones. Apart from being reproductive organs, the testes (in males) and ovaries (in females) are also endocrine glands, which secrete sex hormones that bring about changes at puberty and make their reproductive systems functional.

The testes release male sex hormone, called testosterone, while the ovaries produce female sex hormone called oestrogen. The production of these sex hormones is under the control of another hormone secreted from an endocrine gland called the pituitary gland.

The pituitary gland secretes many hormones. One of these hormones stimulates the formation of sperm in the testes and the maturation of egg cells (or ova) in the ovaries. Another hormone develops the mammary glands (milk-producing glands) inside the breasts in females.

Reproductive Phase Of Life In Humans

Adolescents become capable of reproduction at puberty when their testes and ovaries begin to produce gametes. In males, the capacity to produce gametes (sperm) usually lasts longer than in females.

In females, the reproductive phase of life begins at puberty (10-12 years of age) and generally continues till they reach the age of approximately 45-50 years.

During the reproductive phase, the following changes occur in the ovary and the uterus of a female.e

1. With the onset of puberty, the ova begin to mature in the ovaries of a woman. One mature ovum is released by one of the ovaries once in about 28-30 days.
2. During this period, the wall of the uterus becomes thick to receive the fertilised egg. The fertilised egg gets implanted in the thick wall of the uterus for further development, resulting in pregnancy.
3. If fertilisation does not occur, then the released egg and the thickened lining of the uterus, along with its blood vessels, are shed off

This causes bleeding in women, which is termed menstruation. Menstruation occurs once in about 28 to 30 days.

The cyclic events starting from one menstruation till the next one is called the menstrual cycle. This cycle is controlled by hormones. Initially, the menstrual cycle may be irregular, it becomes regular after some time. The first menstrual flow begins at puberty and is termed as menarche.

At 45-50 years of age, the menstrual cycle stops. This is called menopause. Thus, the reproductive life ofa woman starts at menarche and ends at menopause.

Determination Of The Sex Of The Baby

The sex ofthe baby is determined by the nature of the sex chromosomes present in the fertilised egg. You have already studied that thread-like structures called chromosomes are present inside the nucleus of the cell.

The nucleus of each human cell contains 23 pairs of chromosomes. One pair, out of these, are the sex chromosomes, which are of two types named as X and Y. A female has two X chromosomes, while a male has one X and one Y chromosome. The gametes (egg and sperm) have only one set of chromosomes.

The unfertilised egg always has one X chromosome, but sperm are of two kinds, i.e., X-chromosome and Y chromosome.

The sex of a child depends on what happens at fertilisation. If a sperm carrying an X chromosome fertilises an egg cell (or ovum), then the zygote formed will have an XX combination of sex chromosomes, due to which the child born will be a girl (or female)

If a sperm carrying a Y chromosome fertilises an egg cell (or ovum), then the zygote formed will have an XY combination of sex chromosomes, due to which the child born will be a boy (or male).

From the above discussion, we can see that the sex chromosome of the male parent determines the sex of an unborn baby. Traditionally, it is believed that a mother is responsible for the sex of the baby. This belief is completely wrong and unjustified.

Hormones Other Than Sex Hormones

Certain hormones secreted by endocrine glands are required in extremely small quantities. They control growth, development, behaviour, and in human beings.

Some endocrine glands and their functions are given below in the table.e

Hormones are required in a proper amount. A less (hypo) secretion or more (hyper) secretion of a particular hormone may lead to disorders in the body. Some of these are given below in the table.

The pancreas plays a crucial role in regulating blood sugar levels by secreting insulin and glucagon. While insulin lowers blood sugar, glucagon raises it by prompting the release of glucose.

Role of Hormones In Completing The Life History Of Insects And Frogs

We have studied the life cycle of a frog. The tadpole pa through certain stages to become an Irog. IDs change from larva to adult is called metamorphosis. The process of metamorphosis in insects is controlled by Insect hormones.

In a frog, the process is regulated by the thyroxine hormone produced by the thyroid gland. Thyronine production requires the presence of iodine in water. A tadpole cannot become an adult if iodine is deficient in the water where it is growing.

Reproductive Health

Health is regarded as proper health.

Nutritional Needs of Adolescents

Each individual should take a balanced diet to stay healthy, but it is more important for adolescent to plan their diet carefully.

A typical balanced diet includes carbohydrates, fats, proteins, vitamins, minerals, roughage, and water in adequate proportions.

The various types of food items which should be included in the diet of adolescent to meet their nutritional needs are as follows.s

1. Cereals (like wheat and rice,) which provide carbohydrates for energy, example chapati, dosa, cooked rice, etc.
2. Milk, meat, nuts, and pulses, which provide protein for growth, Example dal, paneer, fish, etc.

Fats and Sugars, which give energy, such as butter, ghee, groundnut, etc.

Fruits and Vegetables, which are protective loud and provide many vitamins and minerals. Examples: Citrus fruits like oranges (vitamin-C rich), carrots (vitamin-A rich).

The iron-rich foods such as leafy vegetables, Indian gooseberry (amla), meat, and jaggery (gud) are good for adolescents. Iron contributes to the formation of blood.

Milk contains a lot of calcium mineral which is necessary for making healthy bones in growing adolescents.

Our Indian meal of roti (or rice), pulses (dal), and vegetables is balanced. Milk is a balanced food in itself. For intants (small babies), mother’s milk provides all the necessary’ nourishment which they need.

Avoid consuming junk foods such as burgers, noodles, etc. Chips and packed (or tinned) snacks, though very tasty, should never be taken in place of regular meals because they do not have adequate nutritional value.

Personal Hygiene

Good personal hygiene is one of the most effective ways to protect ourselves and others from illness. The adolescents are advised to adopt the practices given below to maintain personal hygiene.

1. Keep all the parts of the body clean by taking a bath daily. It is more necessary for teenagers because the increased activity of sweat glands sometimes makes the body smell. Bathing removes the sweat, oil, and dirt, etc,. and cleans the body.
2. Always wash your hands before and after meals.
3. Do not defecate in the open field.
4. Girls should take special care of cleanliness during the time of menstrual flow. They should keep track of their menstrual cycle and be prepared for the onset of menstruation.n

If personal hygiene (or cleanliness of body) is not maintained by adolescent boy’s and girls, chances are of catching bacterial infections. These infections can make a person ill.

Physical Exercise

Do physical exercise regularly to keep your body fit and fine. All young boys and girls should include brisk walking, running, jogging, cycling, climbing, skipping, and dancing in their routine to keep themselves fit and healthy.

Say ‘No’ to Drugs

Adolescence is a period of transition between childhood and adulthood. It is a period of hyperactivity in the body and mind, which is a normal process of growing up. Some adolescents feel confused or insecure about these changes. We should not worry about these changes as there is nothing abnormal about them.

If somebody suggests that you will get relief from confusion, tension, boredom, worries, etc., and feel better by taking some drugs, just say ‘No’ (unless the drugs have been prescribed by a doctor).

Drugs are very powerful chemical substances and should be used only under a doctor’s supervision. Adolescent boys and girls should avoid drugs to live a happy life. Some of the harmful effects of taking drugs are as follows.

1. Drugs are addictive. If you take drugs once, you feel like taking them again and again. Soon, the person feels he (or she) cannot live without drugs and becomes a drug addict.
2. Drug addicts become irritable and lose interest in their studies or jobs.
3. Drugs do physical harm to the body in the long run.
4. The sharing of syringes for injecting drugs spread diseases like AIDS among the drug addicts.

AIDS (Acquired Immune Deficiency Syndrome)

AIDS is a disease which is caused by HIV (Human Immunodeficiency Virus).

This virus can easily pass from an infected person to a healthy person through the ways given below

1. Sharing the syringes for injecting drugs.
2. Unprotected sex with an infected person.
3. Transfusion of blood into a healthy person from an infected person.
4. Through milk from an infected mother to an infant.

Myths, Taboos, Dos and Don’ts

After learning the scientific facts about human reproduction and adolescence, you should be able to discard many wrong notions that are prevalent in socie ty.

Some of these myths, with the facts in front of them, are given below.

Adolescent Pregnancy

You might know that in our country, the legal age for marriage is 18 years for girls and 21 years for boys. This is because teenage mothers are not prepared mentally or physically for motherhood.

Early marriage and motherhood cause health problems in the mother and the child. It also curtails employment opportunities for the young woman and may cause mental agony as she is not ready for the responsibilities of motherhood. To avoid such pregnancies, it is very important to create understanding among the youngsters about sex-related issues.

Adolescence Period of life, when the body changes, leading to reproductive maturity.

Adrenaline Hormone released by the adrenal gland. Adam’s apple Larynx in boys, visible as a protruding part of the throat.

Balanced diet: A diet that contains all the important constituents required by the body to grow.

Endocrine glands are Ductless glands that release their secretions, called hormones, directly into the blood.

Hormone: Chemical substances, secreted by endocrine glands.

Insulin Hormone released from the pancreas. Oestrogen: Female hormone produced by the ovaries. Puberty Time in life when an individual becomes sexually mature.

Pituitary gland: Master gland of the body. Reproductive health: Condition of the male and female reproductive systems during all life stages.

Secondary sexual characteristics are the characteristics that are governed by the hormones released at the time of puberty.

Sex chromosome Type of chromosome involved in sex determination.

Target site: Particular body part or organ that responds to a hormone.

Jestosterone Male hormone produced from the testes.

Jhyroxine Hormone is released by the thyroid gland.

The voice box produces sound when we are talking.

CBSE Class 8 Science Chapter 8 Chapter 8 Force And Pressure Force: A Push Or A Pull

To move an object, it has to be pushed or pulled. A push or a pull acting on an object is known as force. Thus, any object is in motion due to the application of force.

The direction in which the body is pushed or pulled is called the direction of force.

Actions like kicking, opening, shutting, lifting, hitting, etc., are examples of push or pull. These actions usually results in some kind of change in the state of motion of an object.

Best Short Notes For CBSE Class 8 Force And Pressure Chapter Force Is Due To An Interaction

The force between the objects is caused by an interaction of one object with another object. If there is no interaction between two objects, then no force can show its effect. Assume that a man is standing behind a stationary car where there is no interaction between the man and the car, so no force is acting on the car, therefore, the car does not move.

Note, consider the second diagram in which the man pushes the car and it starts moving. This is possible because the man interacts with the car.

From this example, we can conclude that at least two objects must interact for a force to come into play.

Magnetic Force And Electrostatic Force Class 8 Comparison Notes Exploring Forces

The strength of the force is expressed by its magnitude and direction. So, the change in the magnitude or direction of the applied force always results in a change in the effect produced by an applied force.

If forces are applied on an object in the same direction, then they add to one another. For example, two men apply a pushing force together in the same direction on the heavy box; the two forces add upto provide a much bigger force.

If the two forces act in opposite directions on an object, the net force acting on it is the difference between the two forces. For example, the box will move in the direction where the stronger push is applied.

Another example is the game ‘tug of war’ in which two teams pull at a rope in opposite directions. When the two teams pull the rope equally hard by applying equal forces, then the net force acting on the rope is zero, and hence, the rope does not move. In general, if more than one force is acting on an object, then the effect on the object will be due to the net force.

Free Printable Force And Pressure Notes For Class 8 CBSE Effects Of Force

If we apply force on an object, it can have the following effects, depending on the type of object.

• It can change the state of motion of an object.
• It can change the size or shape of an object.

Force can change the State of Motion

The state of motion is described by its speed and the direction of motion. The state of rest is considered to be the state of zero speed.

A force may bring a change in the state of motion of an object.

Force can increase the speed if the force applied on the object is in the direction of its motion and decrease the speed, if the force is applied in the opposite direction to the direction of motion. Even, it can stop the moving object or change the direction of motion ofan object.

For Example;

1. A goalkeeper dives or jumps up to save the goal. He applies force to the moving ball. This force can stop or deflect the ball.
2. In the following figure, the child applies force in the direction of motion of the tyre to increase its speed.

Sometimes the application of force does not change the state of motion. For example, a Heavy box may not move even if you apply the maximum force that you can exert. Similarly, you cannot displace a wall by pushing that.

Force can change the Shape of an Object.

The application of force on an object may change its shape. For Example

1. The shape of an inflated balloon changes when by press it.
2. The shape of the ball of dough changes when it is rolled to make a chapati.

We can summaries that applying force

1. May make an object move from rest.
2. May change the speed of an object if it is moving.
3. May change the direction of motion of an object.
4. May bring about a change in the shape of an object.
5. May cause some or all of these effects.

Types Of Force

There are different types of force which are categorised into two groups

1. Contact Force

These forces come into action only when two objects are in actual contact with each other. These are also of two types

Muscular force. This force is caused by the action of muscles in our body. It enables us to perform all activities involving movement or bending of our body.

For Example. In the process of digestion, the food gets pushed through the alimentary canal, expansion and contraction of lungs while breathing, physical activities done by animals, etc.

Frictional force. This force always acts opposite to the motion of any moving object, for Example. Ball rolling on the ground slows down, boat comes to a rowing, etc.

Non-Contact Forces

These forces came into action without any contact with one another, for Example, Magnetic force, electrostatic force, and gravitational force.

Magnetic force: The force exerted by the magnet is called the magnetic force. Like poles of two magnets repel each other and unlike poles attract each other. A magnet can attract iron nails without touching them, or a magnet exerts a force even without touching an object. So, it is considered a non-contact force.

Electrostatic force The force exerted by a charged body on another charged body is known as electrostatic force, Example; When a plastic comb is rubbed in dry hair, the comb gets electric charges by friction and if this electrically charged comb is brought near the small pieces of paper, then it exerts an electrostatic force of attraction on them (pieces of paper) even if they are not in touch with each other.

Gravitational force: The force due to which every body of the universe attracts another body, whether the object is small or large, is known as gravitational force. Objects or things fall towards the Earth because it pulls them. This force is called the force of gravity or gravity, and this is an attractive force. Example. The earth attracts leaves of trees, water begins to flow towards the ground as soon as we open a tap, etc.

Science Insight The Gravitational force of the moon is (l/6)th of the gravitational force of the Earth. Due to this fact, the moon does not have its atmosphere.

Pressure

The force acting on a unit area of surface is defined as pressure. It can be represented as,

⇒ \(\text { Pressure }=\frac{\text { Force }}{\text { Area }}\)

For example, if we push our thumb into a wooden plank, it does not go in. But, if we push a nail by its pointed end, it goes in easily. This is because in a nail, the force acting on per unit area of wood is very large.

Thumb produces a smaller pressure because it acts on a large area of wood, but the same force produces a much larger pressure when it is applied on a small area in a nail.

Applications of pressure in our daily life

A porter reduces pressure by placing a cloth on his head while carrying a heavy load. This increases the area of contact of the load with their head, due to which the pressure on the head decreases, and it becomes easier for a porter to carry the load.

So, the larger the area, the smaller the pressure on a surface for the same force.

School bags have broad straps, so that the weight of the bag falls over a larger area, producing less pressure over the shoulder.

The tip of the needle is sharp as it produces more pressure on a small area of the cloth. A sharp needle can pierce through the cloth while sewing.

Pressure Exerted By Liquids And Gases

Like solid, fluid also exerts pressure, and fluid can be liquids or gases.

Pressure Exerted By Liquids

1. Liquids exert pressure due to the weight that liquids possess.
2. Liquids always possess pressure in the downward direction.
3. The pressure exerted by liquids at the bottom depends on the height of the liquid column.
4. The pressure exerted by liquids increases with depth.
5. Liquids exert pressure in all directions on the walls of the container.
6. Liquids exert equal pressure at the same height.

Pressure Exerted By Gases

1. Gases also exert pressure in all directions on the walls of containers like liquids.
2. Gases exert pressure due to constant collisions of molecules

Common Mistakes In Force And Pressure Chapter Class 8 Atmospheric Pressure

The envelope of air around us is known as the atmosphere. It extends up to many kelometres above the surface of the Earth. The pressure exerted by the atmosphere is known as atmospheric pressure. It is the force that air exerts on per unit area. In other words, it is the weight of air present in a very tall column of unit area.

Science Insight: The maximum atmospheric pressure. The Earth is at sea level because the column of air is the largest above the sea. The pressure decreases as we go above the Earth’s surface because the length of the air column decreases. That’s why the atmospheric pressure above the mountain is less as compared to that on the Earth’s surface.

Some facts about atmospheric pressure are

1. The weight of air above our heads is about 225 kg. But, we are not crushed under the weight of this air because the pressure inside our body is equal to the atmospheric pressure outside.
2. A suction cup sticks to a flat surface. When we press the suction cup against a flat surface, it flattens and expels most of the air from under it.
3. This creates a partial vacuum inside the suction cup. The atmospheric pressure outside is much greater, and this holds the cup firmly to the surface.

Magdeburg Hemispheres

In the 17th century, a German scientist named to Von Guericke invented a pump to extract all the air from a vessel. This pump is called a vacuum pump. So, he demonstrated an experiment in the town of Magdeburg to show the large atmospheric pressure.

Force refers to a pull or push on an object.

Push It is moving an object away from the source of force.

Pull: It is moving an object towards the source of force. Contact Force Force that is exerted by one object on another object when they are in contact with each other.

Muscular Force: The force exerted by the muscles of a body.

Frictional Force: The force exerted by one object to oppose its motion over another object.

Non-contact Force: The force exerted by one object on another without being in contact.

Magnetic Force: The force exerted by a magnet.

Electrostatic Force: The force exerted by an electrically charged object.

Gravitational Force: The force of attraction (pull) exerted by the Earth.

Pressure refers to the force that is exerted over a unit area of a surface.

Atmospheric Pressure: It refers to the pressure exerted by the atmosphere.

CBSE Class 8 Science Conservation of Plants and Animals Chapter 5 Conservation Of Plants: Deforestation And Its Causes

A forest is a large area covered with trees and plants. It provides habitat for various plants and animals. The process of clearing away forests by cutting them down or burning them is called deforestation. Various causes that lead to deforestation are

1. Procuring land for cultivation, Forest trees are cut down to obtain more agricultural land for the cultivation of crops to feed the increasing population.
2. Building houses and factories. The trees are cut down to build homes, factories, roads, dams, etc.
3. Making various products Paper or wood for making doors, windows, etc., is obtained from trees.
4. Natural causes Forest fires, severe droughts, etc., also lead to deforestation.

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Consequences Of Deforestation

Deforestation harms the environment and various living organisms. Some major consequences discussed here are

1. Global Warming

Green plants use carbon dioxide for photosynthesis (preparation of food). When fewer trees are left due to deforestation, less carbon dioxide is absorbed, resulting in its increased amount in the atmosphere.

Carbon dioxide gas traps heat rays reflected by the surface of the Earth. Trapping of heat rays by carbon dioxide increases the temperature of Earth’s atmosphere, thus leading to global warming.

2. Disturbance In the Water Cycle

The consequent increase in the temperature of Earth disturbs the water cycle. This leads to a decrease in rainfall that lowers the groundwater level and could cause droughts.

3. Soil Erosion And Desertification

Roots of trees bind the particles of topsoil, preventing them from being carried away by wind and water. When the forest trees are cut, there are no roots, and the bare topsoil becomes loose quickly and erodes rapidly. This results in soil erosion.

Removal of topsoil exposes the lower layer, which is hard, rocky, and has less humus and is less fertile. Other properties like nutrient content, texture, etc. of the soil also change. Over time, continued soil erosion can thus convert the land into a desert. This is called desertification.

4. Floods

The roots of forest trees help in absorbing the rainwater and allow it to percolate into the ground. Due to deforestation, there is a decrease in the water-holding capacity of the soil. The movement of water from the soil surface into the ground (in filtration rate) is reduced. This results in floods (excess water overflowing from the banks of the river into the adjoining areas).

5. Shortage Of Wood And Other Forest Products

Deforestation will lead to a shortage of wood used for various purposes. Also, there will be a lack of other forest products such as honey, gum, lac, etc., if cutting down forest trees continues

Deforestation Causes And Effects Class 8 Science Notes: Conservation Of Forest And Wildlife

Now we know that deforestation leads to many harmful effects on the environment, so it is important to conserve the forests and wildlife present in them.

This can be accomplished by protecting specific areas, where plantation, cultivation, grazing, felling trees, hunting, and poaching are prohibited.

Such areas are marked by the government as protected areas. The government lays down rules, methods, and policies to protect and conserve these areas.

The three main types of protected areas are as follows

1. Biosphere reserves
2. Wildlife sanctuaries
3. National parks

Red Data Book explanation for Class 8 CBSE Science Biosphere Reserve

It is a large protected area of land meant for the conservation of wildlife, biodiversity, and the traditional lifestyle of the tribal people living in the area.

A biosphere reserve may also contain other protected areas within it, for example. The Pachmarhi biosphere reserve consists of one National park named Satpura and two wildlife sanctuaries, i.e., Bori and Pachmarhi.

The biodiversity found in the Pachmarhi biosphere reserve is similar to that of the upper Himalayan peaks and to those belonging to the lower Western Ghats.

Great Nicobar Biosphere Reserve and the Tiger Reserve are some examples of biosphere reserves located in India.

Afforestation Methods And Benefits Class 8 Science Notes Flora And Fauna

Some animals and plants typically belong to a particular area and are known as flora and fauna, respectively, of that area.

These are found naturally in that area, for example, cheetah, wolf, leopard, chinkara, blue bull, barking deer, and wild dog are examples of fauna of the Pachmarhi biosphere reserve. Teak, jamun, sal, mango, silver fern, and arjun are the flora of the Pachmarhi biosphere reserve.

Endemic Species

Endemic species are the species of plants and animals that are found exclusively in a particular area. They are not naturally found anywhere else. Examples: Sal and wild mango are examples of endemic flora of the Pachmarhi biosphere reserve. Bison, Indian giant squirrel, and flying squirrel are endemic fauna of this area.

The endemic species may vanish soon due to some factors, such as the destruction of their habitat, increasing population, introduction of new species, etc. Such factors affect the natural habitat of endemic species and endanger their existence.

Species

It is a group of population that is capable of interbreeding. This means that the members of a species can reproduce fertile offspring (one which can reproduce its kind) only with the members of their species and not with members of other species. Members ofa species have common characteristics.

Ecosystem And Its Components Class 8 CBSE Notes Wildlife Sanctuary

A wildlife sanctuary is a protected area for animals in their natural environment, like forests. There is no disturbance to animals and their habitat. Killing (poaching) and capturing ofwild animals is strictly prohibited and punishable by the law. It provides protection and suitable living conditions to wild animals.

Bharatpur wildlife sanctuary (Rajasthan) and Lockchao wildlife sanctuary (Manipur) are some examples of wildlife sanctuaries located in India.

Some of the threatened wild animals like black buck, white eyed buck, elephant, golden cat, pink-headed duck, gharial, marsh crocodile, python, rhinoceros, etc. are protected and preserved in wildlife sanctuaries. Indian sanctuaries have unique landscapes, i.e., broad level forests, mountain forests, and bush lands in deltas of big rivers.

Zoo

Zoos or zoological parks are large areas where many types of wild animals and birds brought from different parts of the world are kept in cages or enclosures, so that people can see them.

Though zoos and wildlife sanctuaries both protect wild animals, they are different from each other. Differences between the two are given below

CBSE Class 8 Biology Notes On Conservation Of Plants And Animals National Park

A national park is an area reserved for wildlife where they can freely use the habitats and natural resources.

It is a relatively large protected area and is maintained by the government to preserve flora and fauna, landscape, and historic objects of the area. These areas are diverse enough to protect whole sets of ecosystems.

National parks also provide human recreation and enjoyment. People are allowed to enter these areas, but exploitation of natural resources is strictly prohibited.

Satpura National Park

The first reserve forest of India is Satpura National Park in Madhya Pradesh. The finest Indian teak is found in this forest. The evidence of prehistoric human life in the I jungles, called rock shelters, is also found inside the I Satpura National Park, which gives us an idea of the life of primitive people.

Also, many rock paintings are found in these shelters, and the figures of animals and men fighting, hunting, dancing, and playing musical instruments are depicted in these paintings. Many tribals still live in this area. A total of 55 rock shelters have been identified in Pachmarhi Biosphere Reserve

Project Tiger

Project Tiger is a wildlife conservation project that was launched by the Government of India to protect tigers in the country. The objective of this project was to ensure the survival and maintenance of the tiger population in the country. There is a ‘Satpura Tiger Reserve’ in the Satpura National Park, which is a part of Project Tiger.

Tigers are one of the many species that are slowly disappearing from our forests. But the Satpura Tiger Reserve is unique in the sense that a significant increase in the population of tigers has been seen here. Once upon a time, animals like barasingha, wild buffalo, lions, and elephants were also found in the Satpura National Park. All these animals have now vanished from this area due to the illegal activities of people living in nearby areas

Endangered Animals and Extinct Animals The species that no longer exist anywhere on Earth are called extinct animals. These have died out completely. A species becomes extinct when the last living member of that species dies. For example. dinosaurs.

Animals whose numbers are diminishing to a level that they might face extinction shortly are called endangered animals. These are on the verge of vanishing from the Earth, and if appropriate actions to save them are not taken, they may become extinct.

The small animals are much more in danger of becoming extinct than the bigger animals. At times, we kill snakes, frogs, lizards, bats, and owls ruthlessly without realising their importance in the ecosystem. They might be small in size, but their role in the ecosystem cannot be ignored.

Hence, we are indirectly harming ourselves by killing them. These small animals form parts of food chains and food webs, which are essential for maintaining balance in nature. For instance, when the snake is killed for its skin. It disrupts the food chain by leading to an increase in the population of pests such as mice, which in turn can cause damage to crops.

Note: An ecosystem is made up of all the plants, animals, and microorganisms in an area, along with non-living components such as climate, soil, rivers, deltas, etc

Red Data Book

• A Red Data Book contains a list of species whose continued existence is threatened, i.e., endangered animals and plants.
• Red Data Book is maintained internationally by an organisation known as the International Union for Conservation of Nature (IUCN). India also maintains the Red Data Book for plants and animals found in India.

Ecosystem And Its Components Class 8 CBSE Notes Migration

The phenomenon of the movement of animals (or birds) to move from one place to another based on season is called migration.

It is an adaptation to escape the harsh and cold conditions of their normal habitat, to survive, and reproduce. Birds that cover long distances to reach another land are known as migratory birds.

Migratory birds fly to faraway areas every year during a particular time because of climate change. They fly to lay eggs as the weather in their natural habitat becomes very cold and inhospitable. For example, Siberian cranes, flamingoes, etc., that flock to India during the winter.

CBSE Class 8 Biology Notes On Conservation of Plants And Animals, Recycling Of Paper

Paper is made from wood pulp that is produced from the wood of forest trees. Therefore, it is one of the important products we get from the forests and thus, a major cause of deforestation. It is estimated that 17 full-grown trees are needed to make one ton of paper. So, we should save paper as much as possible.

We can save paper by filling every sheet of paper, by reusing and recycling paper (as paper can be recycled 5 to 7 times for use). ‘Recycling of paper’ means processing the waste paper to make new paper that can be used again. We should save, reuse, and recycle paper for the following reasons

1. To save forest trees from being cut.
2. To save the water used in paper making.
3. To save the energy (electricity) used in making paper.
4. To reduce the amount of harmful chemicals used in paper making

Migration Of Animals and Its Reasons Class 8 Science Notes Reforestation

Reforestation is known as the phenomenon of restocking the destroyed forests by planting new trees (generally of the same species that were cut). We should at least plant as many trees as have been cut down.

Reforestation can also take place naturally. If the deforested area is left undisturbed, it re-establishes itself. In natural reforestation, there is no role for human beings.

If we have to retain our green wealth, i.e., forests for future generations, the plantation of more trees is the only option.

In India, the Government has enacted the Forest Conservation Act (1980) to save forests. It aims at the presentation and conservation of natural resources, while meeting the basic needs of the people living in or near the forests at the same time.

Chipko Andolan was a 1970 environmental movement In India whore people hugged trees to prevent deforestation highlighting tho need for environmental conservation.

Biodiversity: Diversity of plants, animals, and microorganisms in a particular area.

Biosphere reserve Large protected area where plants and animals are protected along with natural resources.

Deforestation: Large-scale removal of trees from forests.

Desertification: Conversion of fertile land to desert due to deforestation.

Ecosystem: It consists of living and non-living components.

Endangered species are Species that are threatened and can become extinct if not conserved or protected.

Endemic species: Species of plants and animals that are found exclusively in a particular area.

Extinct Species: Those that no longer exist anywhere on Earth.

Flora: Plants present in an area:

Fauna: Animals living in an area.

Migratory birds: Are Birds that cover long distances to reach another land.

National park: Areas are reserved for wildlife where they can freely use the habitats and natural resources.

Reforestation: Restocking of destroyed forests by planting new trees.

Red Data Book: Official record of all endangered plants and animals.

Sanctuary: A Protected area for animals in their natural environment.

Binary Fission vs Budding Difference Explained For Class 8 Modes Of Reproduction

Depending upon the number of parents involved in the process, the phenomenon of reproduction is divided into the following two main types

1. Sexual reproduction
2. Asexual reproduction

Sexual Reproduction

In this type of reproduction, a new individual is produced by the fusion of a male and female gamete. During sexual reproduction, gametes from both parents fuse to form a zygote that develops into a new individual. It mainly occurs in multicellular organisms, such as humans, chickens, dogs, and fish.

Human Male And Female Reproductive System Diagram Notes Class 8 Sexual Reproduction In Humans

In human beings, reproduction involves mainly two sexes, male. male and female. Each sex has a different reproductive system with distinct organs performing different functions

Male Reproductive Organs

In males, the reproductive organs include a pair of testes (singular: testis), two sperm ducts and a penis.

Testes: These are paired, oval-shaped structures outside the abdominal cavity. These produce the male gametes called sperm.

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Sperm (male gamete) is a tiny, motile, single-celled structure with a head, a middle piece, and a tail. The long tail helps it to move through the female tract. Millions of sperm are produced by the testes in a single ejaculation

Sperm ducts (vas deferens) These connect the testes to the urethra.

Penis It is an external male genital organ that transfers sperms into the body of a female.

Female Reproductive Organs

In females, the reproductive organs include a pair of ovaries, oviducts, and a uterus.

Ovaries: These are paired, oval-shaped organs located in the lower part of the abdominal cavity. It produces female gametes called ova (eggs)

Ova/egg (female gamete) It is a microscopic, round single cell with a nucleus, cytoplasm, and a telomere

Oviduct (or Fallopian tube). It is a tube-like structure with a funnel-shaped opening near the ovary. In human beings, a single mature egg is released into the oviduct by one of the ovaries every month. It is also known as the site of fertilisation in human females.

Uterus (womb) It is a hollow, bag-like structure, where the growth and development of fetus takes place.

Most Important Questions From Reproduction In Animals Class 8 CBSE Fertilisation

The process of fusion of the egg and the sperm is called fertilisation. It is the first step of sexual reproduction.

During fertilisation, when the nuclei of the sperm come in contact with an egg, it fuse to form a single nucleus. This results in the formation of a fertilised egg or zygote. The zygote marks the beginning ofa new individual.

The process Involves The meeting of egg cell from the mother and a sprm cell the father. So, the new Individual Inherit’ some characteristics from the mother and some front the father. You must have noticed that some of your siblings characters are similar to those of your mother and some to those of your father.

There are various methods by which fertilisation takes place in different organisms. These are

1. External Fertilisation

Fertilisation that takes place outside the body of a female is called external fertilisation. For example, in aquatic animals such as Fishes, starfish and frogs fertilisation takes place mainly in water.

They lay their eggs and sperm together on the surface of the water. The sperms fuse with the eggs in water to form zygote. The zygote develop into new individuals in the water itself. Animals living in or near ponds and lakes also undergo external fertilisation.

During the spring (or rainy) season, frogs and toads move to the water in ponds and slow-flowing streams.

When the male and female come together in water, the female lays hundreds of eggs. The eggs are delicate and do not have a shell, unlike the hen’s eggs. A layer of jelly-like substance holds the eggs together and protects them.

As the eggs are laid, the male ejects millions of sperm directly on them. Each sperm swims randomly in water with the help of its long tail. The sperms come in contact with the eggs floating in water.

This results in fertilisation. As the gametes are released in water, many of them may be carried away by the movement of water, wind, etc., or may be eaten by other aquatic animals, so a large number is released to compensate for the loss.

2. Internal Fertilisation

Fertilisation that takes place inside the female body is called internal fertilisation. For example, in higher animals such as reptiles, birds, and mammals, examples; humans, cows, dogs, and hens.

In Vitro Fetilisation

Some women are unable to have babies because their oviducts are blocked and sperm cannot reach the egg for fertilisation.

This prevents the ova from being fertilised. The problem can be overcome through a test-tube baby technology or in vitro fertilisation.

In this technique, freshly matured ova arc collected from a woman’s ovaries (by using a special syringe) and sperm trom the man. They are kept together for few hours in an incubator (in vitro).

The sperms fertilise the ova to form a zygote. After a week, the embryo thus formed is then inserted into the woman’s womb, where the embryo develops into a baby in about nine months. The baby thus born is called a test-tube baby.

This technique is called In Vitro Fertilisation (IVF) (Fertilisation outside the body). The success rate is only about 30-40%.

This term is misleading because babies cannot grow in test tubes

Development of Embryo in Humans

Development of the embryo in human beings takes place through the following steps

In human beings, soon after fertilisation, the zygote travels down the oviduct and divides repeatedly to form a ball of cells by cell division.

The cells then begin to form groups that finally develop into different tissues and organs of the body. This developing structure is called the embryo. It gets embedded in the wall of the uterus for further development.

1. Zygote formation and development of an embryo from the zygote (b) Ball of cells (enlarged)
2. Embedding of the embryo in the uterus (enlarged)
3. The embryo continues to develop in the uterus and gradually develops different body parts such as the brain. hands, legs, eyes, ears, etc.
4. The stage of he embryo in which all body parts can be recognised is called a foetus. When the development of the foetus is complete, the mother gives birth to the baby

Handwritten Notes For Class 8 Science Chapter 6 Reproduction Development Of The Embryo In The Hen

Hen is a bird that also undergoes internal fertilisation like humans. But it does not give birth to a young one. You will be wondering how chicks are born. Let’s discuss in the section given below

1. Soon after fertilisation (inside the female hen), the zygote divides repeatedly and travels down the oviduct.
2. Many protective layers are formed around it as it makes its way down. The hard shell that we see in a hen’s egg is one such protective layer (outermost).
3. After the hard shell is formed around the developing embryo, the hen finally lays the egg.
4. The hen then sits on the eggs to provide sufficient warmth for the development of eggs. The embryo takes about 3 weeks to develop into a complete chick. The development of the chick takes place inside the egg shell during this period.
5. After the chick is completely developed, it bursts open the eggshell and comes out of it.

Development of Embryo In other Animals

In animals such as frogs, fish, etc., which undergo external fertilization, development of the embryo takes place outside the female body.

The embryos continue to grow within their egg coverings. After the embryos develop, the eggs hatch, and the young ones come out of them.

During development, the embryo gets its food from the egg yolk and the albumin in egg helps in protection.

Viviparous And Oviparous Animals

The animals that give birth to young ones are known as viviparous animals. Example dogs, cows, cats, etc. The animals that lay eggs are called oviparous animals. Example: birds, fish, etc.

Young Ones To Adults

The new individuals which are born or hatched from the eggs continue to grow till they become adults. In some animals such as amphibians (Example, frog) and certain insects (Example, silkworm), the young ones may look very different from the adults.

This process of change by which a larva is transformed into an adult is called metamorphosis

Life Cycle of the Frog

A frog lays eggs in large numbers (spawning). The baby that hatches out of the frog’s eggs is called a tadpole. The tadpole undergoes several changes before it becomes an adult frog.

egg →tadpole → adult

A tadpole undergoes metamorphosis to become an adult frog. These adult frogs are capable of jumping and swimming.

Differences between tadpole and frog

Exceptions of Metamorphosis

Human beings also exhibit changes in body appearance as they grow, but these changes are not drastic. In human beings, the body parts in adults are similar to those which are present in babies from the time of birth.

So, human beings do not undergo metamorphosis. Some common animals, such as cats, dogs, hens, tigers, lions, deer, horses, cows, etc, also do not undergo metamorphosis during their life cycle

Asexual Reproduction

In this method anew organism is produce from a single parents without the fusion of gamestes. The offspring producing are identical to the parents There are various moderns of the ascual reproduction . some of the are discussed below.

1. Budding

It is a type of asexual reproduction in which a bud(or a bulb develops from the parent body. This bud enlarges and gets detached from the parent. It later develops into a similar identical organism that can live independently, for Example, Hydra, yeast, etc.

Budding in Hydra

It is a multicellular animal that reproduces by budding. In this, one or more small, bulb-like outgrowths called buds are formed on the side of its body by repeated division of cells. The nucleus of the parent body divides, and one goes into the bud.

2. Binary Fission

The process of reproduction in which a parent organism divides (both nucleus and cytoplasm divide), into two daughter cells each of which grow and mature into two separate new individuals is called binary fission, example. Amoeba, Paramecium.

Fission in Amoeba

It is a single-celled organism. It reproduces by this method under favourable conditions. Firstly, the nucleus divides into to it has then, followed by the division of the cytoplasm A constriction develops in the body, which gradually depens to from two daughter amoebae

Besides these two processes, there .ire oilier methods of asexual reproduction (such as fragmentation, spore formation, vegetative propagation). These methods will he studied by you in higher classes.

Differences between sexual and asexual reproduction

Difference Between Zygote And Embryo Class 8 Notes Cloning

Cloning is defined as the production of an exact copy ofa cell or any other living part or a complete organism. Story of Dolly, the Clone

Story of Dolly, the Clone

Cloning ofan animal was successfully performed for the first time by Ian Wilmut and his colleagues at the Roslin Institute

Institute in Edinburgh, Scotland. They successfully cloned a sheep named Dolly. Dolly was born on 5th July, 1996, and was the first mammal to be cloned

Steps Followed in Cloning.

1. During the process of cloning Dolly, a cell was collected from the mammary gland of a female Finn Dorset sheep.
2. Simultaneously, an egg was obtained from a Scottish blackface ewe.
3. The nucleus was removed from the egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep was inserted into the egg of the Scottish blackface ewe whose nucleus had been removed.
4. The egg thus produced was implanted into the Scottish blackface ewe.
5. Development of this egg followed normally, and finally Dolly was born.

Though Dolly was given birth by a Scottish blackface ewe, it was found to be identical to the Finn Dorset sheep from which the nucleus was taken. Since the nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not show any characteristics of the Scottish blackface ewe.

Dolly was a healthy clone of the Finn Dorsett sheep and produced several offspring of her own through normal sexual means. Unfortunately, Dolly died on 14th February, 2003 due to a certain lung disease.

Since Dolly, several attempts have been made to produce cloned mammals. However, many die before birth or die soon after birth. The cloned animals are often found to be born with severe abnormalities.

Asexual reproduction: The type of reproduction in which only a single parent is involved.

Budding In this, the new individual develops from the bud.

Eggs are Female reproductive cells produced by the ovaries.

External fertilisation The fertilisation that takes place outside the female body is called external fertilisation. Embryo The zygote divides repeatedly to give rise to an embryo. It gets embedded in the wall of the uterus for further development.

Fertilisation: The fusion of ovum and sperm.

Foetus: The stage of the embryo in which all body parts are identifiable.

Internal fertilization: The fertilisation that takes place inside the female body is called internal fertilisation.

Metamorphosis: The transformation of a larva into an adult through drastic change.

Oviparous animals are Animals that lay eggs.

Sexual reproduction: Reproduction resulting from the fusion of male and female gametes.

Sperm: Male reproductive cells produced by the testes. Viviparous animals are Animals that give birth to live young ones.

Zygote Fertilisation results in the formation of a fertilised egg or zygote.

CBSE Class 8 Science Chapter 1 Crop Management Revision Notes Crop Management Crops

When plants of the same type are grown and cultivated in a field on a large scale, it is called a crop (fasal in Hindi). Examples include wheat, paddy, and other crops.

India is a vast country, where the climatic conditions like temperature, humidity, and rainfall vary from one region to another. Accordingly, a rich variety of crops is grown in different parts of the country.

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Despite this diversity, two broad cropping patterns can be identified particular season in which they grow.

These are as follows

1. Kharif Crops

The crops which are sown in the rainy season are called Kharif crops or summer crops, for example, paddy (rice), maize, soyabean, cotton, groundnut, millets (bajra), etc. These crops are generally grown from June to September.

2. Rabi Crops

The crops that are grown in winter season are called Rabi crops or winter crops, for example, wheat, gram, pea, mustard, and linseed.

These crops are generally grown from October to March.

Note: Paddy or rice crops are grown in the rainy season because they require a lot of water. It cannot be grown in the winter season.

Class 8 Science Chapter 1 Crop Management Agricultural Practices

Cultivation of crops involves several activities undertaken by farmers over time. activities or tasks are referred to as agricultural practices.

The various agricultural practices of crop production involve

1. Preparation of soil
2. Sowing
3. Adding manure and fertilisers
4. Irrigation
5. Protecting from weeds
6. Harvesting
7. Storage

Class 8 Science Chapter 1 Crop Management Notes For self-study Preparation Of Soil

The first step for crop production is soil preparation. Soil is the uppermost layer of the Earth where plants are grown.

Only a few centimeters of the top layer of soil support plant growth. It provides water, air, minerals, humus, etc. to the plants. Soil is prepared for sowing seeds by three major methods, which are as follows

1. Ploughing

The process of loosening and turning the soil is called ploughing or tilling. It is done by using a plough. The loosening and turning of soil by ploughing is helpful because of the following Reasons

Plant ixsois can pencil ale lively mul ileepci Inin the soil and plants are belli lb inly to the ground.

Loose soil helps the plant roots to breathe easily. Ploughing of soil brings the nutrient-rich soil to the top so that the plant can use these nutrients.

Loosened soil helps in the growth of earthworms and microbes present in the soil. These organisms are friends of the farmers since they further turn and loosen the soil and add humus to it.

2. Levelling

Levelling of soil is done with the help of a leveller. The ploughed field may have big pieces of soil called crumbs. Levelling helps to break these crumbs with a plank and prevents loss of moisture from the loose soil. It also helps in the uniform distribution of water in the fields during irrigation.

3. Manuring

Continuous growing of crops makes the soil deprived in certain nutrients. Therefore, farmers have to add manure to the fields to replenish the soil with nutrients. This process is called manuring. Sometimes, manure is added to the soil before tilling. This helps in the proper mixing of manure with soil.

Agricultural Implements

The above-mentioned practices are done by using various tools. These tools are called agricultural implements. These are discussed below.

Plough It is the most ancient tool used for tilling the soil, adding manure and fertilisers, removing weeds, and turning the soil. This is made up of wood and is drawn by a pair of bulls and other animals (horses, camels, etc). It comprises of strong triangular iron strip called a ploughshare

The Main part of the plough is a long log of Wood Wich is called plough shaft it has a handle at one end and the other end is attached to a beam which is placed on buld necks one pair of bulls which is placed on bull necks one pair of bulls and a man can easy operate the plough Now a day the indigenous wooden plough is inceresigly being replaced by iron plough.

It is used for removing weeds and for loosening the soil. It is simple in structure, a long rod of wood or Iron. A strong, broad, and iron. The iron plate is fixed; all of its codes will work like a blade. It is pulled by the animals.

Cultivator nowadays, ploughing is done by a tractor-driven cultivator. It is a modern form of plough which can dig into a considerable area of soil at the same time loosen it and turn it. A cultivator saves both labour and time.

1-4 Sowing

It is the process of scattering or planting seeds into the soil to grow a new crop plant. Showing is the most important part of crop production before sowing, claiming healthy and good quality of seeds are selected to produce healthy plants and high yields of crops.

Methods of Sowing

Sowing is done either manually or by using machines, as described below.

Traditional Tools

Traditional tools are used with the help of bulls. It was used by farmers earlier in their fields for sowing seeds. Its upper funnel-shaped part is filled with seeds. The seeds pass down through two or three pipes having sharp ends. These ends pierce into the soil and place seeds in that space

2. Seed Drill

It is a mechanical method of sowing by using a tractor. The seeds are sown uniformly at equal distance and depth. The soil is covered with soil after sowing.

This method protects the seeds from the damage caused by birds. It also saves time and labour. The advantage of sowing seeds at an appropriate distance and depth is that it prevents overcrowding of plants and each plant gets sufficient sunlight, nutrients, and water from the soil.

The National Seed Corporation (NSC) in India plays a key role in producing high-quality agricultural seeds and has contributed to the establishment of seed¬ testing laboratories across various regions in the country.

Adding Manure And Fertilisers

Manure and fertilisers are substances that are rich in nutrients and are added to the soil for the healthy growth of plants.

The addition of manure or fertiliser to the soil is called manuring. Farmers add manure to the fields to replenish the soil with nutrients. Improper or insufficient manuring makes plants weak. For replenishing soil with nutrients following methods are used.

Manures

These are organic (natural) substances derived from the decomposition of plant and animal wastes, like cow dung, etc. Plant and animal wastes are dumped in pits in open places and allowed to decompose by some microorganisms. The decomposed matter is used as organic manure.

Fertilisers

These are the chemicals that contain the necessary plant nutrients like nitrogen, phosphorus, etc. Examples- Urea, ammonium sulphate, super phosphate, potash, NPK (Nitrogen, Phosphorus, Potassium).

Fertilisers are used to increase soil fertility, which helps farmers to get better yields of crops such as wheat, paddy, and maize. The excessive use of fertilizer is harmful because it makes the soil less fertile and also causes water pollution.

Advantages of Manure Over Fertilisers

The organic manure is considered better than fertilisers for the following reasons

1. It enhances the water-holding capacity of the soil.
2. It makes the soil porous, due to which the exchange of gases becomes easy.
3. It increases the number of friendly microbes.
4. It improves the texture of the soil.

Crop Rotation

The practice in which different types of crops, especially leguminous and non-leguminous crops, are grown alternately in the same field is called crop rotation.

This procedure is followed to improve the fertility of the soil. For example, crops like cereals, vegetables, etc., deplete the soil of the soil. Therefore, leguminous crops are grown alternatively after these crops to maintain the nitrogen level of the soil

The leguminous plants possess nitrogen-fixing bacteria called Rhizobium in their roots, which can directly fix atmospheric nitrogen into soluble nitrogenous compounds. Rotation of crops also saves a lot of nitrogenous or chemical fertilisers.

CBSE Class 8 Science Chapter 1 Crop Management Revision Notes Irrigation

The process of supplying water to crop plants at regular intervals is called irrigation. Water Is Important for the proper growth and development of all living organisms, including plants. A plant contains nearly 90% of water.

Importance of Water to Crop Plant

1. Water is absorbed by the plant roots. Along with water, minerals and fertilisers are also absorbed.
2. Water is essential because germination of the seed does not take place under dry conditions.
3. It also protects the crop from both frost and hot air currents.
4. For healthy crop growth, fields have to be watered regularly to maintain the moisture of the soil

Frequency and Timing of Irrigation

The frequency and timing of irrigation differ from crop to crop, soil to soil, and season to season. For example, in summer, the frequency of irrigation is required at a higher rate than in rainy or winter seasons. This is because the rate of evaporation of water from soil and leaves is high in the summer season

Sources of Irrigation

Water supplied for irrigation can be taken from different sources, Example rain, wells, tubewells, ponds, lakes, rivers, dams, and canals

Traditional Methods of Irrigation

The water from different sources, such as wells, lakes, and canals, is lifted by different methods to take it to the fields. Cattle or human labour is used in these methods

The various traditional ways of irrigation are moat (pulley system), dhckli, chain pump, and rahat (lever system). Pumps are commonly used for lifting water. They use diesel, biogas, electricity, and solar energy

Modern Methods of Irrigation

Traditional methods use manpower and animal power, and there is a wastage of water that also occurs during irrigation. To overcome these problems, modern methods of irrigation are used. The main methods used are as follows

1. Sprinkler System

In this system, there is a main pipeline laid in the field joined with perpendicular pipes, having rotating nozzles on the top.

When water is allowed to flow through the main pipe under pressure with the help of a pump, it escapes from rotating nozzles and is sprinkled like rain over the crops. Sprinklers are very useful for lawns, coffee plantations, and several other crops. This system of irrigation is useful on uneven land where sufficient water is not available.

2. Drip System

In this system, water flows through the narrow pipes and falls drop by drop just near the roots of plants.

In this method, there is no wastage of water as water falls drop by drop, therefore, this system of irrigation is very useful in regions where the availability of water is poor. It is the best technique for watering fruit plants, gardens, and trees.

Protection From Weeds

The unwanted, wild plants that grow along with the cultivated crops are called weeds.

1. The process of removing weeds is called weeding. It is necessary to remove weeds from time to time because
2. These unwanted plants are harmful because they compete with the crop plant for nutrients, fertiliser, space, light, and water. Thus, they reduce crop yield.
3. Some weeds interfere with even in harvesting and may be poisonous for animals and humans.

Weeding Methods

Farmers adopt many ways to remove weeds and control their growth. Tilling before sowing crops helps in uprooting and killing weeds, which may then dry up and get mixed with the soil. The best time for the removal of weeds is before they produce flowers and seeds. Weeding can be done either manually or by spraying weedicides as discussed below

Manual Weeding

In this method, the weeds are removed from the crop field by pulling them up by hand and throwing them away. Another method of manual weeding is done by using a khurpi. A seed drill is also used to uproot weeds.

Spraying Weedicides

Certain chemicals called weedicides (herbicides) like 2,4-D are sprayed in the fields to kill the weeds. The crops remain unaffected by weedicides.

It should be sprayed during the vegetative growth of weeds before flowering and seed formation, otherwise, weeds become difficult to control. Precautionary measures such as covering the nose and mouth with cloth should be taken while spraying weedicides, as they pose health hazards.

Harvesting

The process of cutting and gathering mature (ripened) crop is called harvesting. When crops get mature, they usually turn yellow, golden, or brown. In harvesting, crops are pulled out or cut close to the ground. 1 Harvesting in our country is either done manually by a sickle or by a machine called a harvester.

To obtain seed grains from harvested crops, the following two methods are used

Threshing

The process by which the grain seeds are separated from the chaff is called threshing. This is carried out with the help of a machine called a combine, which is a combination ofa harvester and a thresher.

Winnowing

After threshing, the grains are separated from chaff by a process called winnowing. In this process, the mixture of grain, hay, and chaff is dropped from a height into blowing wind.

The heavier seed falls straight to the ground, whereas the chaff and hay are much lighter and are carried away by the wind. The grains form a separate heap and can be collected and packed in gunny bags.

Harvest Festivals

After three or four months of hard work, there comes the day ofthe harvest. The sight of golden fields of standing crop, laden with grain, fills the hearts of farmers with joy and a sense of well-being.

The period is one of great joy and happiness. It is celebrated with great zeal. Special festivals associated with the harvest season are Pongal, Baisakhi, Holi, Diwali, Nabanya, and Bihu.

Storage

The harvested and cleaned grains are dried to evaporate moisture from them before storage. Since the crops/grains have to be kept for a longer time, they should be safe from moisture, insects, rats, and microorganisms. Also, insecticides, rodenticides are sprayed to protect them from rats and insects.

The grains are stored in jute bags or metallic bins by tanners. At large scale, grains are stored in silos and granaries to protect them from pests. Dried neem leaves are used for storing food grains at home, as neem is a natural insecticide

Class 8 Science Chapter 1 Crop Management Short Notes For Quick Revision Food From Animals

Besides plants, some animals also provide us with different kinds of food. The food obtained by animals is rich in protein.

The food derived from animals and the sources are tabulated below

Animal Husbandry

The process of rearing animals, by providing them proper food, shelter, and care, is called animal husbandry. Fish are an important source of animal food in coastal areas. We get cod liver oil from fish, which is rich in vitamin D

Animal husbandry Agricultural practice of breeding and raising livestock.

Agriculture practices: Tasks carried out by farmers for the cultivation of crops.

• Crop Plants of the same kind, cultivated in a field on a large scale.
• Fertilisers are Chemical substances used to increase the fertility of soil.
• Granaries are Places where a large amount of grain can be stored.
• Harvesting Process of cutting and collecting the mature crop.

Irrigation: Water supply to crops at regular intervals.

• Kharif Crops grown in the rainy season

Manure: Organic substances derived from the decomposition of plant and animal wastes.

• A plough Agricultural implement used for tilling or plowing. ploughing the soil.
• Rabi Crops are grown in the winter season.

Sowing: Planting the seeds in the soil.

• Seeds fertilised and matured ovules.
• Storage Process of protecting the grains, by keeping them in closed containers.
• Silo Tall and cylindrical structures used for storage of grains.
• Threshing Method of separating seed grains from the chaff.
• Winnowing Method of separating seed grains from the chaff by blowing air.
• Weeds are Undesirable plants that grow along with crops. Weedicide Chemicals used to kill weeds

CBSE Class 8 Science Chapter 12 Some Natural Phenomena: Lightning

A short burst of fire, which occurs due to the sudden flow of electric current between two objects, is known as an electric spark.

It can be observed in our daily life in many situations, when a plug is loose in its socket, when the wires become loose on an electric pole, etc. In nature, lightning is caused by the accumulation of charges in the clouds.

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Initially, people are afraid of lightning and consider it the wrath of God. But we need not be scared of lightning; we should take precautions to protect ourselves from the sparks.

CBSE Class 8 Science Chapter 12 Charging By Rubbing

When an object is rubbed with an appropriate object (like polythene, dry hair), it acquires a small electric charge, said to be a charged object. So, we can say that an electric charge can be produced by rubbing one object with another object of a specific kind. An object with no electric charge is said to be an uncharged object.

Some common examples of charging by rubbing are

1. When a plastic refill is rubbed with polythene, it acquires a small electric charge.
2. When a plastic comb is rubbed with dry hair, it acquires a small electric charge

CBSE Class 8 Science Chapter 12 Types Of Charges And Their Interaction

When two objects are rubbed together, then both the objects get charged with opposite charges, i.e. positive charge (+ sign) and negative charge (- sign) Example; When we rub a glass rod with a silk cloth, then the charge acquired by a glass rod is positive and silk cloth gets a negative charge.

The nature of the object determines which object gets a positive charge and which one gets a negative charge.

Depending on the type of electric charge on the objects, they will either attract or repel each other.

Like charges repel each other. It means, if both bodies have the same type of charges, they will repel each other.

Unlike charges attract each other. It means, if both bodies have different types of charges, they will attract each other.

The electrical charges generated by rubbing are known as static charges. Example: by rubbing a plastic comb with dry hair. Static charges do not move themselves. If these charges move, then they constitute electric current.

Transfer of Charges

Electric charge can he transferred from a charged object to an uncharged object, it is called transfer of charge. On the basis ot transfer of charges, we can construct a device called an electroscope. It is a device that is used to test whether an object is carrying a charge or not.

Detection of Charge with Electroscope

To detect the electric charge on a body, we touch the metal top ot the electroscope with it and notice the aluminium leaves.

1. If the body is electrically charged, then the leaves diverge.
2. If the body is electrically uncharged, then the leaves do not diverge.

An electroscope proves that electric charge can be transferred from a charged object to another through a metal conductor.

Earthing

The process of transferring an electric charge from a charged object to the earth is known as earthing. When a body loses charge to the earth or any other body, then it is said to be discharged.

Note: Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electric current.

CBSE Class 8 Science Chapter 12 Story Of Lightning

1. Lightning is bright streaks or flashes of light seen in the dark cloud sky. Due to the transfer of charges from cloud to cloud or from cloud to the earth. It is usually followed by a loud cracking sound known as thunder.
2. During the development of a thunderstorm, there is always an upward movement of air while the water droplets move downward.
3. These vigorous movements cause the separation of charges. Therefore, the top of the cloud becomes positively charged while the bottom of the cloud becomes negatively charged.
4. The quantity of opposite electric charges on the top and bottom of storm clouds becomes very large, so the air, which is normally a poor conductor of electricity, will no longer be able to resist their flow. Therefore, the electric charges start flowing at high speed.
5. When both the positive and negative charges of the cloud meet, they produce an intense spark of electricity. We observe this electric spark as the flash of lightning in the sky. This process is also called an electric discharge.
6. The process of electric discharge can occur between two or more clouds or between clouds and the Earth.

CBSE Class 8 Science Chapter 12 Lightning Safety

During thunderstorms and lightning, a house or a building is always a safe place to be in. Any open place is not a safe place.

If you are travelling by car or by bus, then you are safe inside with the windows shut. The following precautionary measures should be taken to protect ourselves from lightning strikes during a thunderstorm.

CBSE Class 8 Science Chapter 12 Lightning Safety Outside the House

Dos During a Thunderstorm

1. We should stay away from tall trees.
2. If we are in a park or forest, we should take shelter under shorter trees.
3. If we are in a closed car, we should stay inside with the windows and doors closed.
4. Squat on the Hoot. Thin position makes us the smallest target to he struck.

Don’ts During a Thunderstorm

1. Do not stand in open ground during a lightning storm,
2. Do not carry an umbrella as its metal rod is a good conductor of electric charge.
3. Always stay away from poles or other metallic objects.

Lightning Safety Inside the House

Dos During a Thunderstorm

1. We should switch OFF the TV as antennas fixed on the roof of a building are prone to lightning strikes,
2. We should avoid touching the metallic pipes and electric equipment connected by electric wires in the house.
3. We should unplug electrical appliances like the refrigerator, computers, etc.

Don’ts During a Thunderstorm

1. We should not use a wired telephone,
2. We should not take a bath as water is a good conductor of electricity.

Lightning Conductor

A device that is used to protect buildings from the effects of lightning is known as a lightning conductor.

Lightning Conductor

A device that is used to protect buildings from the effects of lightning is known as a lightning conductor. end of the lightning or is fixed to a vital plate, which is deep into the pound, is shown in the figure.

Lightning Conductor Protects the Building From Thunderstorms

1. If the lightning strikes on building, it will first strike the top end of the lightning conductor.
2. The lightning conductor provides  an easy route for the runnier of electrical charge to discharge to ground,
3. The electrical energy of lightning, which passes through a tin metal strip, gel discharged safely into the ground, and the hurled metal plate.
4. Since the lightning charge does not pass through the Imilding, no damage is caused.

CBSE Class 8 Science Chapter 12 Earthquake

A sudden shaking or trembling of the earth that lasts for a very short time is an earthquake. It can cause immense damage to buildings, bridges, dams, and people.

A sudden shaking of the earth causes tremors, and these tremors produce waves on the surface of the earth. Tremors can be caused by to volcanic eruption or a meteor hitting the Earth.

Causes of Earthquakes

The tremors are caused by the disturbances inside the Earth. Before learning about the causes, we should know that the Earth consists of three layers.

1. Core The innermost part of the Earth is called its core.
2. Mantle The central region of the Earth is called the mantle.
3. Crust The outermost layer of the Earth is called the crust.

The outermost layer ofthe most is fragmented, due to which the Earth’s crust is divided into many plates. These plates are in Continuous motion

When these plates collide with one another or brush past one another then they cause disturbance in the Earth’s crust. This disturbance results from an earthquake on the Earth’s surface.

The place inside the Earth’s crust where the earthquake is generated is called the focus of the earthquake.

Seismic or Fault Zones

As earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. These weak zones are also known as seismic or fault zones. In India, Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic plain are the most threatened areas.

Richter Scale

It is a scale that is used to measure the intensity or magnitude of an earthquake. Destructive earthquakes have magnitudes higher than 7 on the Richter scale.

Seismograph

The seismic waves are those waves that are produced by tremors on the Earth’s surface. These waves are recorded by an instrument called the seismograph.

• This instrument is simply a vibrating rod or a pendulum that starts vibrating when tremors occur. A pen is attached to the vibrating system, and this pen records the seismic waves on paper that moves under it.
• By studying waves, scientists can construct a complete map of the earthquake, as shown in the figure. They can also calculate its power to cause destruction.

Protections Against Earthquake

Some of the important precautions that can be taken by people living in seismic zones are as follows

1. All the quake-proof houses should be constructed using modern building technology. For which, we should consult a qualified architect and structural engineers.
2. Even in highly seismic areas, it is advisable to use mud or timber for building houses, as these are better than heavy materials, which keep roofs as light as possible.
3. There should be a tight fixation of cupboards and shelves to the wall, so that they do not fall when shaking occurs.
4. The heavy mirrors, photo frames, water heaters, etc., should be mounted to the wall at low levels, so that in the event of an earthquake, they do not fall on the people.
5. Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have firefighting equipment in working order.

In the event of an earthquake, follow these steps to protect yourself.

If you are at home

1. Try to get a shelter under a table and stay there till the shaking stops.
2. Stay away from tall and heavy objects as they may fall on you.
3. If you are in bed. Then do not get up. Immediately, protect your head with a pillow.

If you are outside your home

1. Observe a clear spot away from buildings, trees, and overhead power lines. Drop to the ground.
2. Besides this, if we are in a car or a bus, we should not get out. The car should be driven slowly to a clear spot.

CBSE Chapter 3 Coal And Petroleum Natural Resources

The various resources which are obtained from nature are called natural resources, for example, air, water, soil, plants, petroleum, etc. These are necessary for the survival of all living beings. Based on their availability in nature, natural resources can be classified into the following two types

1. Inexhaustible Natural Resources

Those natural resources which are present in unlimited quantity in nature and are not likely to be exhausted (used up) by human activities are called inexhaustible natural resources, Example; sunlight, air, water, etc.

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2. Exhaustible Natural Resources

Those natural resources which are present in a limited quantity in nature and can be exhausted (used up) by human activities are called exhaustible natural resources, for example, coal, petroleum, natural gas, minerals, forests, wildlife, etc. Coal, petroleum and natural gas are collectively known as fossil fuels. As they were formed from the remains of dead organisms (fossils).

Class 8 Science Chapter 3 Coal And Petroleum Revision Notes Coal

It is a hard, black combustible fuel. Coal is found in deep coal mines under the surface of the Earth. It burns in the presence of air and produces CO2, gas and heat. Earlier, it was used in railway engines to produce steam to run the engines.

Formation Of Coal

About 300 million years ago, the Earth had dense forests in low-lying wetland areas. Natural processes like flooding, earthquakes, volcanoes, etc., buried these forests under the surface of the Earth.

As more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. Under high pressure and temperature dead plants got slowly converted into coal.

This slow process of conversion of dead vegetation buried deep under the Earth into coal is called carbonisation.

Coal As A Source Of Energy

Coal contains mainly carbon, and when heated in air, it burns and produces mainly carbon dioxide gas and heat. The reaction can be written as

Carbon Oxygen Carbon dioxide Heat
(Coal) (From air)

Uses of Coal

Some of the uses of coal are given below

1. Coal is one of the fuels used to cook food.
2. It is used in thermal power plants to produce electricity.
3. It is used as a fuel in various industries.
4. It is used for the manufacture of coke, coal tar and coal gas.
5. Products Obtained from Coal
6. Coal is processed in the industry to get some useful products. Which are coke, coal tar and coal gas.

1. Coke

It is a tough, porous and black substance. Coke is an almost pure form of carbon. When coal is heated in the absence of air, then coal gas and coal tar are eliminated and ok is left behind as a black residue.

Uses of Coke

• Coke is mainly used
• In the extraction of metals such as iron, zinc, etc.
• In the manufacture of steel.
• As a fuel

2. Coal Tar

It is a thick, black liquid having an unpleasant smell. It is a mixture ofabout 200 different carbon compounds.

Uses of Coal Tar

Some of the uses of coal tar are as follows

1. The various compounds obtained from coal tar are used as starting materials for manufacturing substances used in everyday life and in industries. For example, perfumes, photographic materials, roofing materials, drugs, explosives, plastics and paints etc.
2. Naphthalene balls are obtained from coal tar and are used to repel moths and other insects.
3. Coal tar has been used for metalling the roads. But, these days bitumen (a petroleum product) is being used for metalling the roads.

3. Coal Gas

It is a gaseous fuel which is obtained during the processing of coal in the absence of oxygen to get coke. It is mainly a mixture of methane and hydrogen with some carbon monoxide.

Uses of Coal Gas

1. Some of the uses of coal gas are as follows
2. It is used as a fuel in many industries situated near the coal processing plants.
3. It has also been used for lighting purposes.
4. Science Insight Coal gas was used for street lighting for the first time in London in 1810 and in New York around 1820. This is because when it burns, it produces a good amount of light. Nowadays, it is used as a source of heat rather than light.

CBSE Class 8 Science Chapter 3 Coal And Petroleum Notes For Competitive Exams Petroleum

It is a dark coloured, thick crude (raw) oil having an unpleasant odour. The name petroleum means ‘rock oil (petra = rock, oleum = oil). It is found deep below the Earth’s crust, trapped in rocks in certain areas.

Petrol and diesel are obtained from petroleum. Petrol is used as a fuel in light automobiles, for example, motorcycles, scooters and cars, whereas diesel is used in heavy motor vehicles, for example, trucks and tractors.

Formation of Petroleum

Petroleum was formed by the decomposition of the remains of plants and animals buried under the sea millions of years ago.

The plants and animals which lived in the sea. died and their bodies settled at the bottom of the sea. The lead remains covered with mud, sand and clay. Due to high temperature, high pressure and the absence of air, these were slowly converted into petroleum and natural gas as shown in the figure.

The layer containing petroleum oil and gas is found above the water. It is because oil and gas are lighter than water and do not mix with it.

Petroleum is extracted by drilling holes (oil wells) in the Earth’s crust. In the drilling process, natural gas comes out first with a great pressure and for a time, the crude petroleum oil comes out by itself due to gas pressure.

The world’s first oil well was drilled in Pennsylvania, USA, in 1859. In 1867, oil was struck at Makum in Assam. In India, oil is found in Assam, Gujarat, Mumbai (offshore area) and the river basins of the Godavari and Krishna rivers.

Refining of Petroleum

Petroleum is a mixture of petroleum gas, petrol, diesel, lubricating oil, paraffin wax, etc.

Before the use of petroleum for specific purposes, it has to be refined (or purified). The process ofseparating the various useful constituents (fractions) of petroleum is known as refining. It is carried out in a petroleum refinery.

Various Constituents of Petroleum and Their Uses

The various constituents of petroleum and their uses are given below in the table.

Petrochemicals

The useful chemicals which are obtained from petroleum and natural gas are called petrochemicals. They are very important as these are used to manufacture a wide range of useful materials such as detergents, fibres (polyester, nylon, acrylic, etc), plastics (polythene, etc). Hydrogen gas obtained from natural gas is used in the production of fertilisers such as urea. Thus, due to its great commercial importance, petroleum is also called ‘black gold’.

Natural Gas

It mainly consists of methane (95%). Other constituents are ethane and propane. It occurs deep under the Earth’s crust either alone or along with oil above the petroleum.

In India, natural gas has been found in Tripura, Rajasthan, Maharashtra and in the Krishna Godavari delta.

It is a very important fossil fuel as it is easy to transport through pipes. When it is compressed by applying pressure, it is called Compressed Natural Gas (CNG).

This gas is used for power generation. It is now being used as a fuel for transport vehicles as it is a clean fuel.

Natural gas is used as a starting material to manufacture many chemicals (petrochemicals) and fertilisers.

CBSE Class 8 Science Chapter 3 Coal And Petroleum Notes For Quick Learning: Conservation Of Natural Resources

Coal, petroleum and natural gas are fossil fuels and will last only a few hundred years. Fossil fuels, forests and minerals, etc., are exhaustible natural resources. If consumed at a rate faster than the rate at which these are formed in nature, they will get exhausted very soon.

Once the present stock of these gets exhausted, no new supplies of these fossil fuels will be available to us shortly. This is because it requires millions of years to convert the dead organisms into these fuels.

Moreover, the burning of these fuels is a major cause of air pollution. Their use is also linked to global warming. So, we should use these fuels only when necessary as

1. It will reduce air pollution and thus lead to a better environment.
2. It will reduce the risk of global warming.
3. It will ensure their availability for a longer period.
4. How to Save Petrol or Diesel while Driving?
5. Petrol and diesel are the main fuels used for driving vehicles. So, we should make every effort to save these precious fuels.
6. In India, the Petroleum Conservation Research Association (PCRA) advises people how to save petrol or diesel while driving their vehicles.
7. The various tips to avoid the wastage of these fuels while driving are as follows:
8. Drive at a constant and moderate speed as far as possible.
9. Switch off the engine of the vehicle at traffic lights or at a place where you have to wait.
10. Ensure the correct tyre pressure of the vehicle. Low tyre pressure consumes more fuel.
11. Ensure regular maintenance of the vehicle.

Coal is A fossil fuel obtained from deep under the Earth’s crust.

Coal gas is obtained during the processing of coal to get coke. It can be used as fuel.

Coke, A tough, porous and black substance, almost pure form of carbon.

Coal, A black, thick liquid obtained by processing coal.

Fossil fuels are Fuels obtained from the decomposition of dead remains of plants and animals, for Example, petrol, coal, etc.

Natural gas is a fossil fuel which mainly contains methane and can be transported through pipes.

Petroleum is a dark coloured crude oil having an unpleasant odour.

Petroleum refining: Petroleum is separated into different fractions by this process.

Petrochemicals are useful chemicals which are obtained from petroleum and natural gas.

CBSE Class 8 Science Chapter 11 Chemical Effects Of Electric Current Conduction Of Electricity In Liquids

Like solids, liquids may also conduct electricity. Some liquids are good conductors of electricity, and some are poor conductors of electricity.

To examine whether a liquid permits electric current to pass through it or not, we can use the tester. When the liquid like lemon juice, vinegar etc., is between the two ends of the tester, the bulb glows.

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On the other hand, when the liquid, like distilled water, is between the two ends of the tester, the bulb does not glow.

From the above experiments, we can say that lemon juice and vinegar are the liquids that allow electricity to pass through them.

i.e. They are conducting liquids, while distilled water does not allow the electricity to pass through it, i.e. it is not a conducting liquid.

CBSE Class 8 Science Chapter 11 Detecting Weak Electric Current Through A Liquid

In some situations, even if the liquid is conducting but the bulb does not glow.

It may be due to the low conductivity of the liquid that it allows only a weak current to pass through it. This weak current is not sufficient to heat the filament of the bulb to a high temperature.

If the electrical conductivity of liquid which we are testing, is very small, then the current in the circuit may be weak. The weak electric current flowing through liquids can be detected in two ways.

With the help of LED (Light Emitting Diode)

With the help of a compass surrounded by turns of circuit wire.

By using LED, LED can be used in the tester in place of a bulb. It lights up even when a very weak current passes through the circuit.

l.F.n has two wires, one wire is longer than the other. The longer wire is connected to the positive terminal, and the shorter wire is connected to the negative terminal of the battery.

With the help of a compass We know that, electric current produces a magnetic effect. So, when a weak electric current flow’s in a wire, this produces a magnetic effect due to w’hich a compass needle starts deflecting. So, this test circuit shows the presence ofelectric current.

Poor Conductors can be Good Conductors

Poor conductors are those materials which allow electricity to pass through them under certain conditions.

As we know, air is poor conductor of electricity but during lightning, an electric current pass through it. Similarly, distilled water does not conduct electricity but when we add some salts, acids, bases to it, this solution starts conducting electricity.

So, we can say that liquids like solutions of acids, bases and salts are good conductors ofelectricity. The water that we get from sources like handpumps, taps, tube wells, etc., contains a small amount of natural salts. These natural salts make it a good conductor of electricity.

CBSE Class 8 Science Chapter 11 Chemical Effects Of Electric Current

When electric current is passed through conducting liquids, it causes chemical changes in the liquids. This is called the chemical effect of electric current.

The passage of an electric current through a conducting solution may cause the following chemical effects

1. Change in colour of the conducting liquid.
2. Deposits of metal on the electrodes.
3. Bubbles of gas near the electrodes.

Electrodes

The conducting rods are generally made up of carbon. These rods are known as electrodes.

The electrode connected to the positive terminal of the battery is called the anode, while the electrode connected to the negative terminal of the battery is called the cathode.

The chemical effect produced by an electric current depends on the nature of the conducting solution and the nature of the electrode used.

When electric current is passed through conducting water, it breaks the water into hydrogen gas and oxygen gas.

This breaking of water through electric current is called chemical decomposition. Such chemical decomposition of conducting liquids through electric current is called electrolysis.

CBSE Class 8 Science Chapter 11 Electroplating

The process of depositing a thin layer of desired metal over a metal object with the help of electric current is called electroplating.

Example: Wheel rims, handle of a bicycle are made shiny by coating them with chromium metal. It is one of the most common applications of the chemical effects of electric current.

The objective of electroplating is

1. To protect metal from rusting or corrosion.
2. To make metal look shinier and attractive.

Note: Carbon is only a non-metal, which can be electroplated with copper.

Process Of Electroplating

1. When an electric current is passed through the copper sulphate solution, the solution dissociates into copper and sulphate.
2. Now, the free copper gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it.
3. An equal amount of copper gets dissolved into the solution from the other electrode (Copper plate).
4. Hence, the loss of copper from the solution is restored and the process of transferring copper from one electrode to other, continues.
5. Here, the copper transfers from anode to cathode.

Uses of Electroplating

Chromium has a shiny appearance, and it does not corrode and resists scratches. So, the objects made from a cheaper metal get a coating of chromium over them.

The following are the uses of electroplating in our day-to-day activities.

1. The electroplating of silver and gold on the less expensive metals like iron, copper, etc is done by jewellery makers. So, these kind of ornaments have the appearance of silver or gold but arc much less expensive.
2. Tin cans used for storing food are made by electroplating tin onto iron. Tin is less reactive than iron. Thus, by electroplating the iron with tin, the food does not come into contact with iron directly, so it is protected from getting spoiled.

Some Major Concerns of Electroplating

The conducting solutions like copper sulphate in the electroplating process are the pollutants for environment. It contains various types of salts which may be poisonous.

It may affect aquatic as well as human life, so the disposal of the used conducting solution is a major concern for us.

Good Conductors: Materials which allow an electric current to pass through it is known as good conductors. Examples: Salt water.

Poor Conductor: Materials which allow only a very small amount of current to pass through them are known as poor conductors.

Examples: Acetic acid /vinegar.

Electrolyte: A liquid which conducts electricity is known as an electrolyte.

Electrode: The carbon rods whose metal caps are covered by copper wires and joined to a battery are known as electrode.

Electrolytic cell: An arrangement in which a cell having two electrodes is kept in a conducting liquid in a vessel is called an electrolytic cell.

Electrolysis: The chemical decomposition produced by passing an electric current through an electrolyte is called electrolysis.

Electroplating: The process of depositing a thin layer of desired metal over a metal object with the help of electric current is called electroplating.