Class 8 Science

Age of Adolescence

All of your classmates are in the phase of life which is marked by various physical and psychological changes. The changes during adolescence are so sudden and dramatic that many of you must be feeling puzzled. This lesson will help you in proper understanding about various changes during the age of adolescence.


Adolescence is the period of life when many changes take place in the body, leading to reproductive maturity. Adolescence begins at around 10 years of age and lasts till 18 or 19 years of age.

Teenage: The years during adolescence period are counted as teen, .e.g. thirteen, fourteen and fifteen. Hence, this period is also called teenage.

Puberty: The process of changes during adolescence period is called puberty. Beginning of puberty marks the beginning of adolescence. The end of adolescence or puberty marks the completion of reproductive maturity.

Changes at Puberty

Increase in Height

The most apparent change during puberty is the sudden increase in height. Long bones grow rapidly and hence height of a person increases rapidly during this period. Initially, girls grow faster than boys but ultimately boys are usually taller than girls. The maximum height is attained by 18 years of age.

Change in Body Shape

The body of a boy becomes more muscular. His shoulders become wide and waist becomes narrow. The body of a girl becomes wider below waist and narrower at shoulder.

Voice Change

A boy's voice becomes deep, and the child like voice is first replaced by a hoarse voice. This happens because the voice is said to crack. Gradually, the voice becomes deep. The change in voice happens because voice box in a boy becomes larger. Girl’s voice is usually high pitched.

Increased Activity of Sweat and Sebaceous Glands

Secretion from sweat and sebaceous glands increases during adolescence. Due to this, some teenagers may suffer from acne and pimples.

Development of Sex Organs

In boys, testes begin to produce sperms. Testis and penis develop completely. In girls, ovaries enlarge and eggs begin to mature. Ovaries start releasing mature egg.

Reaching Mental, Intellectual and Emotional Maturity

These changes also have a profound effect on a person’s way of thinking. Teenagers spend more time on thinking than earlier. So many rapid changes in the body may create a sense of insecurity in most of the teenagers. But teenagers should not worry because these are natural changes.

Adam's Apple: Enlargement of voice box is apparent as a prominent protrusion in the neck. This protrusion is called Adam's apple.

Secondary Sexual Characters

Characters which help in distinguishing a male from a female are called secondary sexual characters. Marked difference between male and female of a species because of apparent physical traits is called sexual dimorphism.

  • Secondary Sexual Characters in Boys: Facial hairs, deep voice and hair on chest.
  • Secondary Sexual Characters in Girls: Enlargement of breasts and high pitched voice.
  • Some common characters in girls and boys: Hair under armpits, hair in pubic region.

Hormonal Control

The changes during this period are controlled by hormones. Hormones are substances secreted by endocrine glands. Hormones play important role in various metabolic activities.

In boys, testis secretes the hormone testosterone. This hormone is responsible for many changes in boys. In girls, ovaries secrete the hormone estrogen. This is responsible for development of breasts.

Action of these hormones is under the control a hormone secreted by pituitary gland. Another hormone from pituitary gland stimulates the maturation of eggs and production of sperms. Endocrine glands are ductless glands, which means that a hormone reaches the target site through bloodstream.

Reproductive Phase of Life in Humans

The reproductive phase begins with the onset of puberty and it happens at around the same age in boys and girls. But reproductive phase in men lasts much longer than in females. Reproductive phase in women lasts till about 50 years of age. Reproductive phase in men lasts even beyond 60 years of age.


In females, one egg is released by either of the ovaries during each month. After that, the uterus develops a thickening to support a foetus to prepare for a possible pregnancy. If fertilization does not take place, egg and thickening in uterus are discarded in small fragments. This results in bleeding through the vagina for some days. The bleeding (through vagina) at the end of menstrual cycle is called menstruation.

Menstrual Cycle

The cycle of events which begin from release of an egg and end in shedding off egg is called menstrual cycle. A menstrual cycle is usually of 28 to 30 days.


The first menstrual bleeding in a girl's life is called menarche. Menarche marks the beginning of puberty in girl.


Cessation of menstrual cycle is called menopause. This happens at around 45 – 50 years of age. Menopause marks the end of reproductive phase.

Crop Production

The science of farming is called agriculture. Farming includes cultivation of plants and rearing of animals for food and other beneficial items.

Microorganisms: Friend or Foe

An organism which cannot be seen by naked eyes is called microorganism or microbe. We need a microscope to see a microbe.

Synthetic Fibres

ibres which are man-made are called synthetic fibres, e.g. nylon, acrylic, etc. A synthetic fibre is made of multiple units of a chemical substance.

Metals Non-metals

Metals are lustrous, sonorous, ductile, and are good conductor of heat and electricity.

Coal & Petroleum

Resources which from nature are called natural resources. Examples: coal, petroleum, air, water, soil, etc.

Combustion & Flame

Combustion is a chemical process in which a substance reacts with oxygen to give off heat. Some substances may also give light along with heat during combustion.

Conservation of Plants and Animals

The part of the earth where all living organisms are present is called biosphere. Biosphere is the narrow zone between hydrosphere, troposphere and atmosphere.

Cell: The Fundamental Unit of Life

The basic structural unit of a living organism is called cell. All living beings are made up of cells.

Reproduction in Animals

The process by which an organism produces its offspring is called reproduction. Reproduction is necessary to continue the lineage of an organism.

Reaching the Age of Adolescence

Adolescence is the period of life when many changes take place in the body, leading to reproductive maturity. Adolescence begins at around 10 years of age and lasts till 18 or 19 years of age.

Force & Pressure

A push or pull on an object is called force. Pressure is the force acting on a unit area on an object.


When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction.


Sound is a kind of wave which gives us the sense of hearing. Sound is produced by a vibrating body.

Chemical Effects of Electric Current

When electric current is passed through a conducing solution, some chemical reaction takes place in the solution. This is called chemical effect of electric current.

Natural Phenomena

Transfer of charge from one object to another is called charging. The process of transfer of charge to the earth is called earthing.


When light falls on a shiny surface, most of it bounces back. This phenomenon is called reflection of light.

Solar System

The solar system is made up of the sun, its planets, satellites, asteroids, comets, dwarf planets, etc. All other members of the solar system keep on revolving around the sun.


When air is contaminated with unwanted substances and becomes harmful for the living and non-living, this is called air pollution.