It is convenient to represent electric components by symbols.
Using these, an electric circuit can be represented by a circuit diagram.
When an electric current flows through a wire, the wire gets heated.
It is the heating effect of current. This effect has many applications. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
Wires made from some special materials melt quickly and break when large electric currents are passed through them.
These materials are used for making electric fuses which prevent fires and damage to electric appliances.
When an electric current flows through a wire, it behaves like a magnet.
Electric Circuit: A complete pathway of the flow of electric current.
Component of Electric Circuit: Component of electric circuit are following :
Cell: Provides energy for the current to flow.
Bulb: Lights ups when an electric current flows through it.
Switch: Keeps the circuit off or on.
Connecting wires: Help to conduct the electric current and complete the circuit.
1.Heating Effect: The wire gets hot when an electric current passes through it.
This is the heating effect of the electric current.
Electric heater contains a coil of wire called element which becomes red hot when current passes through it.
The amount of heat produced in a wire depends on its material, length and thickness.
Fuse: It is a safety device which prevents damage to electric circuit.
It is made by inserting a short wire into porcelain or insulating material.
MCB: Stands for Miniature Circuit Breakers. These are switches which automatically turn off when current in a circuit exceeds the safe limit.
Effects of Electric Current:
Electric Current and its Effects
2. Magnetic Effect: When electric current passes through a wire, it behaves like a magnet. This is the magnetic effect of the electric current. This was first observed by Hans Christian Oersted.
A current carrying coil of an insulated wire wrapped around a piece of iron is called an electromagnet.
Electromagnet: An electromagnet is a coil of wire wound on a soft iron core, used to separate magnetic material from the junk. Doctors use tiny electromagnets to take out small pieces of magnetic material that have accidentally fallen in the eye.
Many toys also have electromagnets inside them.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 7 – Science – Study – Premium” posts_per_page=”25″]
Light: It is the natural agent that stimulates sight and makes things visible.
Light travels along straight line.
Any polished or a shining surface acts as a mirror.
An image which can be obtained on a screen is called a real image. (Scroll down to continue …).
Study Tools
Audio, Visual & Digital Content
It is formed by light rays that actually pass through the screen.
An image which cannot be obtained on a screen is called a virtual image.
It is formed by light rays that seem to pass through the screen.
The image formed by a plane mirror is erect.
It is virtual and is of the same size as the object.
The image is at the same distance behind the mirror as the object is in front of it.
In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image.
A concave mirror can form a real and inverted image.
When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.
A Convex mirror is the mirror that curves out; the reflecting surface is convex.
Image formed is virtual, upright and diminished. Image formed by a convex mirror is erect, virtual and smaller in size than the object.
A Concave lens is the lens that is thinner at the center than at the edges.
It is a diverging lens.
Image formed is virtual, erect and diminished.
A convex lens can form real and inverted image.
When the object is placed very close to the lens, the image formed is virtual, erect and magnified.
When used to see objects magnified, the convex lens is called a magnifying glass.
White light is composed of seven colors.
Properties of Light:
1. Rectilinear Propagation of Light: It is the property of light by which it travels in a straight line in any direction.
The direction of path in which light make a ray.
2. Reflection of Light: It is the bouncing back of light after striking the surface of an object.
Shiny smooth surfaces reflect almost all the light.
3. Dispersion: It is the phenomenon of splitting of white light into its seven colors. White
light is mixture of: Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR) colors.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 7 – Science – Study – Premium” posts_per_page=”25″]
Forest: Large area of land thickly covered with trees, bushes, etc.
We get various products from the forests around us.
Forest is a system comprising various plants, animals and micro-organisms.
In a forest, trees from the uppermost layer, followed by shrubs, the herbs to
the lowest layer of vegetation. (Scroll down to continue …).
Study Tools
Audio, Visual & Digital Content
Different layers of vegetation provide food and shelter for animals, birds and insects.
The various components of the forest are interdependent on one another.
The forest keeps on growing and changing, and can regenerate.
In the forest, there is interaction between soil, water, air and living organisms.
Forests protect the soil from erosion.
Soil helps forests to grow and regenerate.
Deforestation: Cutting down of trees is known as deforestation.
Importance of Forests:
Forests:
Provide timber,
Purify air,
Provide shelter,
Prevent soil,
Absorbs noise. Independence of Plants and Animals in Forest: Plants and animals depends on each other to remain alive.
All organisms interact with each other and their physical environment to derive and survive.
Effects of deforestation: Amount of carbon dioxide in air will increase, resulting in the increase of earth’s temperature. (Global Warming) Animals will not get food and shelter.
Soil will not hold water, which will cause floods.
Endanger lives and environment. Conservation of Forests: Do not allow overgrazing.
Promote afforestation.
Protect wildlife.
Food Chain:
Interdependence between producers and consumers studied in form of various linkage that appears as a chain or Interdependence of organisms which shows who eats whom.
Food Web: A system of interdependent food chains used to represent various relationships in organisms.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 7 – Science – Study – Premium” posts_per_page=”25″]
Wastewater: Black-brown water which is rich in lather , mixed with oil that goes down the drains from skins, showers, toilets, laundries is called wastewater.
sewage: Wastewater is generated in homes, industries, agricultural fields and in other human activities. This is called sewage. (Scroll down to continue …).
Study Tools
Audio, Visual & Digital Content
Sewage is a liquid waste which causes water and soil pollution.
Wastewater is treated in a sewage treatment plant.
Treatment plants reduce pollutants in wastewater to a level where nature can take care of it.
Where underground sewerage systems and refuse disposal systems are not available, the low cost on-site sanitation system can be adopted.
By-products of wastewater treatment are sludge and bio gas.
Open drain system is a breeding place for flies, mosquitoes and organisms which cause diseases.
We should not defecate in the open. It is possible to have safe disposal of excreta by low cost methods.
Sewage Treatment:
Aeration: Air is bubbled through the wastewater while it is continuously stirred.
Filtration: Aerated water passes through a deep filter of layered sand, fine gravel and medium gravel.
Chlorination: Chlorine is added and mixed to the filtered water until water is clear.
Wastewater Treatment Plant (WWTP):
Wastewater passes through screens to remove large objects.
To go to a grit and sand removal tank at low speed.
Water is allowed to settle in large tank.
Floating solids are removed with skimmer.
Settled solids (sludge) are removed with scraper.
Clear water is called clarified water.
Water is then decomposed by anaerobic bacteria in a tank and air is passed.
Microbes settled at bottom as activated sludge and water from top is removed.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 7 – Science – Study – Premium” posts_per_page=”25″]
In order to provide food for a large population – regular production, proper management and distribution of food is necessary. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
Crop : When plants of the same kind are grown and cultivated at one place on a large scale, it is called a crop.
In India, crops can be broadly categorised into two types based on seasons – Rabi and Kharif crops. Sowing of seeds at appropriate depths and distances gives good yield.
Good variety of seeds are sown after selection of healthy seeds.
Sowing is done by seed drills.
Soil needs replenishment and enrichment through the use of organic manure introduction of new crop varieties.
Basic practices of crop production: (i) Preparation of Soil: One of the most important tasks in agriculture is to turn the soil and loosen it.
The process of loosening and turning of the soil is called tilling or ploughing.
(ii) Sowing: Sowing of seeds at appropriate depths and distances gives good yield.
Good variety of seeds is sown after selection of healthy seeds. Sowing is done by seed drills.
(iii) Adding Manure and Fertilisers Soil needs replenishment and enrichment through the use of organic manure and fertilisers.
Use of chemical fertilisers
fertilisers has increased tremendously with the introduction of new crop varieties.
Fertiliser: The inorganic compounds containing nutrients such as nitrogen, potassium and phosphorus. They are made in the factories.
Example: ammonium sulphate, potash, etc.
Manure: A natural substance prepared from decomposition of plant and animal wastes (cow dung, animal bones, dead leaves, dead insects and vegetable wastes) by t(he action of microbes.
iv) Irrigation : Supply of water to crops at appropriate intervals is called irrigation. Method of Irrigation: (a)Tradition methods of Irrigation: Moat, Chain pump, Dheki, Rahat.
(b) Modern methods of Irrigation: Sprinkler system, Drip
(v) Protection from Weeds: Weeding involves removal of unwanted and uncultivated plants called weeds.
(vi) Harvesting: Harvesting is the cutting of the mature crop manually or by machines.
(vii) Storage Proper storage of grains is necessary to protect them from pests and microorganisms.
Harvested food grains normally contain more moisture than required for storage.
Large scale of storage of grains is done in silos and granaries to pest like rats and insects.
Farmers store grains in jute bags or metallic bins.
Food is also obtained from animals for which animals are reared.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Micro-organisms: Micro-organisms are too small and are not visible to the un aided eye.
They can survive under all types of environment, ranging from ice cold climate to hot springs and deserts to marshy lands.
They are also found inside the bodies of animals including humans. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
Micro-organisms are found in air, water and in the bodies of plants and animals.They may be unicellular or multicellular.
Micro-organisms are classified into four major groups. These groups are bacteria,fungi, protozoa and some algae.
Viruses are quite different from other micro-organisms. They reproduce only inside
Micro-organisms: Friend and Foe
Based on the significance, micro-organisms can be useful or harmful.
Uses Of Microorganisms
Protozoan cause serious diseases like dysentery and malaria.
Some bacteria and blue green algae present in the soil fix nitrogen from the atmosphere and convert into nitrogenous compounds.
Certain bacteria convert compounds of nitrogen present in the soil into nitrogen gas which is released to the atmosphere.
Pathogens: Some of the microorganisms cause diseases animals. Such disease causing microorganisms are called pathogens.
Cleaning of Environment: The microorganisms decompose dead organic waste of plants and animals converting them into simple substances. These substances are again used by other plants and animals.
Microorganisms can be used to degrade theharmful and smelly substances and thereby clean up the environment.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Natural Resources: Resources include everything provide by the nature. They form the wealth of a country.
Types of Natural Resources: (i) Inexhaustible: There are some resources that are present unlimited in nature and will not be exhausted even if used continuously.
Example: Sunlight, Air. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
(ii) Exhaustible: These resources are limited and can soon get exhausted because of Coal, petroleum and natural gas are fossil fuels.
Fossil fuels were formed from the dead remains of living organisms millions of yearsago.
Fossil fuels are exhaustible resources.
Coke, coal tar and coal gas are the products of coal.
Petroleum gas, petrol, diesel, kerosene, paraffin wax, lubricating oil are obtained by refining petroleum.
Coal and petroleum resources are limited.
We should use them judiciously.
Petroleum is mixture of various constituents such as petroleum gas, petrol, diesel, lubricating oil, paraffin wax, etc.
Refining: Refining is the process of refracting the various constituents / fractions of petroleum.
It carried out in a petroleum refinery.
Natural Gas: A very important fuel as it easy to transport through pipes and can be compressed and stored under high pressure as Compressed Natural Gas (CNG).
It causes no pollution and has high calorific value.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Combustion: The process of burning a substance in the presence of air (oxygen) andundergoes a chemical reaction to produce heat and light. The substances which burn in air are called combustible.
Oxygen (in air) is essential for combustion. During the process of combustion, heat and light are given out. Ignition temperature is the lowest temperature at which a combustible substancecatches fire.
Types of combustion: The type of combustion differs depending on the type of fuel. (Scroll down to continue)
Study Tools
Audio, Visual & Digital Content
Based on nature and intensity combustions are classified into three types. They are: (i) Rapid combustion (ii) Spontaneous combustion (iii) Explosion
Flame: It is a zone or burning vapour. The substances which vaporise during burning give flames.
Example: Kerosene oil and molten wax. Inflammable substances have very low ignition temperature. Fire can be controlled by removing one or more requirements essential for producing fire. Water is commonly used to control fires. Water cannot be used to control fires involving electrical equipments or oils.
There are three different zones of a flame – dark zone, luminous zone and non-luminous zone.
Fuel is any material that is burned to obtain energy that can be used to heat or A good fuel must:
Be readily available. Be cheap. Burn easily at a moderate rate.
Produce a large amount of heat. • Not leave behind any undesirable Fuels differ in their efficiency and cost. Fuel efficiency is expressed in terms of its calorific value which is expressed in units of kilo joule per kg.
Types of Fuels: (i) Solid Fuels: Combustible substances which are solid at room temperature.Example: coal, coke, wood, charcoal, etc. (ii) Liquid fuels: Volatile liquids which produce combustible vapour. Example: Petrol,kerosene, alcohol, diesel, etc. (iii) Gaseous fuels: Combustible gases or mixture of combustible gases. Example: Effects of Burning of Fuels: (i) Carbon fuels like wood, coal petroleum release un burnt carbon particles. Theseare dangerous pollutants causing respiratory diseases, such as asthma.
(ii) Incomplete combustion of carbon fuels gives carbon monoxide which is apoisonous gas.
(iii) Increased concentration of carbon dioxide in the air is believed to cause globalwarming.
(iv) Oxides of Sulphur and nitrogen dissolve in rain water and form acids. Such rain is Un burnt carbon particles in air are dangerous pollutants causing respiratoryproblems.
Incomplete combustion of a fuel gives poisonous carbon monoxide gas. Increased percentage of carbon dioxide in air has been linked to global warming.
Oxides of sulphur and nitrogen produced by the burning of coal, diesel and petrol cause Acid rain which is harmful for crops, buildings and soil.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Biodiversity: Occurrence of innumerable types of organisms and the whole range of their varieties (biotypes) adapted to different climates, environments and areas. Wildlife sanctuary, national park and bio-sphere reserve are names given to the areas meant for conservation and preservation of forest and wild animals. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
Biodiversity refers to the variety of living organisms in a specific area.
Plants and animals of a particular area are known as the flora and fauna of that area.
Endemic species are found only in a particular area.
Endangered species are those which are facing the danger of extinction.
Red Data Book contains a record of endangered species.
Migration is the phenomenon of movement of a species from its own habitat to some other habitat for a particular time period every year for a specific purpose like breeding.
We should save, reuse and recycle paper to save trees, energy and water.
Reforestation is the restocking of destroyed forests by planting new trees.
Flora: Different types of plants belonging to an area.
Example: Silver ferns, sal, teak, mango, etc.
Fauna: All animals found in an area.
Example: dog, frog, insects, bull, jackal, etc.
Endemic Species: Species of plants and animals found exclusively in a particular area.
These are not naturally found anywhere else.
Extinct Species: Species of plants and animals which have already been lost.
Type of Threatened Species:
(ii) Vulnerable Species: A vulnerable species is a species of animals or plants which are likely to become endangered unless something changes.
Example: Chinkara deerand black buck, golden langur, etc (iii) Rare Species: Species whose population are originally small and scattered in the National Parks: Protective areas reserved exclusively for the betterment of the wildlife. These are established at the approval of legislature.
Example: Hazaribagh National Park in Jharkhand, Desert National Park in Rajasthan.
Sanctuaries: Hunting prohibited areas set up by government are known as sanctuaries. These are only for the protection of wild animals.
Example: Jaldapara in Madarihat (West Bengal), Keoladeo Ghana in Bharatpur (Rajasthan) igration: The regular, periodic, two way movements of birds and some animals from their place of residence to some other place along well defined routes. It is linked to seasonal factors, breeding, shortage of foods, etc.
The sanctuary is known for the migratory birds.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Reproduction is a process in which the organisms produce the young ones of their own kind. There are two modes by which animals reproduce.
These are: (i) Sexual reproduction, and (ii) Asexual reproduction (Scrolldown to continue …)
Study Tools
Audio, Visual & Digital Content
Sexual Reproduction
Reproduction resulting from the fusion of male and female gametes is called sexual reproduction.
The reproductive organs in the female include ovaries, oviducts and uterus.
The reproductive organs in male include testes, sperm ducts and penis.
The ovary produces female gametes called ovum and the testes produce male gametes called sperms. The fusion of ovum and sperm is called fertilization.
zygote: The fertilized egg is called a zygote.
internal fertilization: Fertilization that takes place inside the female body is called internal fertilization. This is observed in human beings and other animals such as hens, cows and dogs.
external fertilization: Fertilization that takes place outside the female body is called external fertilization. This is observed in frogs, fish, starfish, etc.
The zygote divides repeatedly to give rise to an embryo. The embryo gets embedded in the wall of the uterus for further development.
The stage of the embryo in which all the body parts are identifiable is called foetus.
Animals such as human beings, cows and dogs which give birth to young Ones.
Asexual Reproduction: The type of reproduction in which only a single parent is involved is called asexual reproduction. The transformation of the larva into adult through drastic changes is called Asexual Reproduction
budding: In hydra, new individuals develop from buds. This method of asexual reproduction is called budding.
binary fission.: Amoeba reproduces by dividing itself into two. This type of asexual reproduction is called binary fission.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Humans become capable of reproduction after puberty. Puberty sets in between the ages of 11 years and 19 years. These children are called adolescents.
The onset of puberty brings about growth of the reproductive organs. Hairs grow at various places on the body.
Breasts develop in girls.
Facial hair (moustache and beard) appear in boys.
Voice of boys become hoarse as voice box enlarges during adolescence.
Children gain height during adolescence.
Puberty: It is the time when sex organs begin to work.
It brings about growth in reproductive organs and changes in the body.
Puberty starts at the beginning of adolescence.
Study Tools
Audio, Visual & Digital Content
Reaching the age of Adolescence
The onset of puberty starts much earlier in girls, between 8 and 13 years of age, whereas it starts between 10 and 14 years of age in boys and lasts upto 2 to 4 years.
Changes during Puberty: (i) Increase in Height: The bones of the legs and arms elongate and the individual becomes tall and lanky.
(ii) Changes in Body Shape: In boys, the body becomes more muscular, the shoulder becomes wider.
In girls, the body shows curves in certain parts and the region below the waist become wider and broader.
(iii) Change in Voice: In boys, the voice becomes hoarse and heavy due to the extra growth of larynx (voice box).
In girls, the voice becomes high pitched and shriller.
(iv) Sweat and sebaceous glands: They become more active and secrete more.
(v) Emotional aspects develop.
Periodic cycle of 28 days. The onset of puberty and maturity of reproductive parts are controlled by hormones.
Hormones are secretions of endocrine glands which pour them directly into the blood stream.
Pituitary gland secretes hormones which include growth hormone and hormones that make other glands such as the testes, ovaries, thyroids and adrenals, secrete hormones.
Pancreas secretes insulin, thyroid produces thyroxin and adrenals produce adrenalin.
Testosterone is the male hormone and estrogens, the female hormone.
The uterine wall in females prepares itself to receive the developing fertilised egg.
Menstruation: In case there isno fertilisation, the thickened lining of the uterine wall breaks down and goes out of the body along with blood.This is called menstruation.
Sperm and Ovum fuses together to form a zygote.
Sex chromosomes in sperms determine the baby’s gender. Humans have 23 pairs of chromosomes in each cell.
A pair of chromosome forms the sex chromosomes. Males carry XY and females carry XX chromosomes.
If a sperm carrying X fertilises the ovum with X chromosome, then the resulting baby is a girl.
If a sperm carrying Y chromosome fertilises the ovum with X chromosome, then the resulting baby is a boy.
Hence, males are responsible for the gender of the newly formed babies, Not Women.
Reproductive Health: It is important to eat balanced food and maintain personal hygiene during adolescence.
Cleaning of private parts is important to prevent possible infections.
The activity of sweat glands increases in puberty and releases bad odour from the body.
Thus frequent bathing is necessary. Physical exercise keeps the body fit and healthy as it releases lot of sweat.
Peer pressure is best addressed by self-confidence and a healthy lifestyle.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Force: A push or a pull, that changes or tends to change the state of rest or of uniform motion of an object or changes its direction or shape.
A force arises due to the interaction between two objects.
Force has magnitude as well as direction.Therefore force is a vector quantity.
The SI unit of force is newton. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
A change in the speed of an object or the direction of its motion or both implies a change in its state of motion.
Force acting on an object may cause a change in its state of motion or a change in its shape.
Contact Non Contact Forces:
A force can act on an object with or without being in contact with it. Based on Contact the forces are classclassified as Contact Forces and Non Contact Forces.
Contact Forces: The forces act on a body when the source of force touches the body directly.
The point where the force is applied on an object is called the point of application of force (or point of contact).
Examples of Contact Forces:
(i) Muscular Force: The force exerted by the muscles of the body.
We use force acted by muscles of animals like Humans, bullocks, horses and camels to get our activities done.
(ii) Mechanical Force: The force acted by a machine.
Non-Contact Forces:
Non-Contact Forces: Forces which do not involve physical contact between two bodies on which they act.
Examples of Non-Contact Forces:
(i) Magnetic Force: A magnet exerts a non-contact force on objects made of iron, steel, cobalt or nickel.
(ii) Electrostatic Force: The force which result due to repulsion of similar charges or attraction of opposite charges.
(iii) Gravitational Forces: The force that exists between any two bodies by virtue of
Pressure
Pressure: Thrust acting per unit surface area is called pressure.
Thrust
Thrust is the force acting on an object perpendicular to its surface.
In SI system, pressure is measured in newton per square metre which is equal to 1 pascal (Pa).
Like solids, fluids (liquids and gases) also exert pressure.
A solid exerts pressure only in the downward direction due to its weight, whereas liquids and gases exert pressure inall directions.
Hence liquids and gases exert pressure on the walls of their container.
Atmospheric Pressure
Ttmosphere: The thick blanket of air that covers the earth is termed atmosphere.
The pressure exerted by the atmosphere is called atmospheric Pressure.
The tremendous atmospheric pressure surrounding us is not felt by us because the fluid pressure inside our bodies counter-balances the atmospheric pressure around us.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Friction is the force that resists the motion of one object against another. It is the force felt between two surfaces when one attempts to slide against the other.
Causes of Friction: Friction exists between two surfaces due to irregularities on the surfaces of the objects in contact, interlocking of micro-level irregularities of the two surfaces and ploughing of harder surfaces into smoother surfaces.
Factors Affecting Friction:
(i) Roughness of the surface.
(ii) The extent to which the two surfaces press against each other.
(iii) Nature of the surface· (Scroll doen to continue …)
Study Tools
Audio, Visual & Digital Content
Types of Friction: Friction is classified into three types as: Static Friction, Sliding Friction, Rolling Friction. (i) Static Friction: When a body is at rest, the force of friction is called the static friction and is always equal and opposite to the applied force.
limiting friction: The force of friction which acts when the body is just at the verge of sliding on the surface is called limiting friction.
Motion between two contacting surfaces
(ii) Sliding friction: The friction force which opposes the actual relative sliding motion between two contact surfaces.
Sliding friction is smaller than static friction.
(iii) Rolling Friction: The frictional force that exists between two surfaces when a body rolls over the other.
Rolling friction is smaller than sliding friction.
Static friction > Sliding friction > Rolling friction
Effects of Friction
(i) Friction produces heat.
(ii) Friction causes wear and tear.
(iii) Friction opposes motion.
Advantages of Friction
(i) Friction between pen and paper enables us to write on the paper.
(ii) Friction between our feet and the ground allows our movements like standing, walking and running.
(iii) Friction between the surface of the road and tyres of our vehicles allow the vehicles to move without slipping.
Disadvantages of Friction:
(i) Friction causes moving objects to stop or slow down.
(ii) Friction produces heat causing wastage of energy in machines.
(iii) Friction causes wear and tear of moving parts of machinery, soles of shoes, etc.
Friction is a necessary Evil:
As friction is advantageous to us it is considered as a friend but, due to its disadvantages it is a foe.
Depending on the circumstance, friction can be a help or a hindrance.
Thus it is a necessary evil.
Increasing Friction: By pressing the surfaces together more strongly.
For example, when brakes are applied on a bicycle or car, the brake pads press against a moving part of the wheel and the force of friction increases.
Friction can be increased by increasing the roughness of the surfaces in contact.
For example, treading of shoes and tyres is done to increase friction.
Reducing Friction: Friction between the sliding surfaces of two objects can be reduced by making the surfaces in contact smooth by polishing them.
Sliding friction between the moving parts of vehicles and machinery can be reduced by introducing oil, grease, graphite or any other lubricant.
Lubricant: A lubricant is a substance that reduces friction between surfaces in contact. This reduces the heat generated when the surfaces move. Lubricants can also transmit forces, transport foreign particles, or heat or cool the surfaces.
Rolling friction is less than sliding friction. Hence, sliding friction is replaced by rolling friction by using rollers, like ball bearings between the hub and the axles in the moving parts of machines and vehicles.
Friction is reduced by providing wheels,
Examples: suitcase, school bags of kids, etc.
Streamlined shape: Aeroplanes, boats, fishes and birds which move through fluids have bodies of special shape, called streamlined shape, The streamlined shape reduces the friction due to fluid and reduces the energy usage.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Sound: Vibrations that travel through the air or another medium and can be heard when they reach a person’s or animal’s ear. Musical Sound: The sound which produce a pleasing effect on the ear.
Noise: The sounds which produce a jarring or unpleasant effect. (Scroll down to contue …)
Study Tools
Audio, Visual & Digital Content
Types of Sound: (i) Audible Sound: Vibrations whose frequency lies between 20 Hz to 20,000 Hz (20 kHz) are heard by human ear.
(ii) Inaudible Sound: The sounds having frequencies above 20,000 Hz and below 20 Hz cannot be heard by the normal human ear.
Low frequency sounds which cannot be heard are called infrasonics.
High frequency sounds which cannot be heard are called ultrasonics. In human beings, the vibration of the vocal cords produces sound.
Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.
The eardrum senses the vibrations of sound. It sends vibrations to the inner ear. From there, the signal goes to the brain.
That is how we hear. Higher the frequency of vibration, the higher is the pitch, and shriller is the sound. Unpleasant sounds are called noise.
Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings. Lack of sleep, hypertension (high bloodpressure), anxiety and many more health disorders may be caused by noise pollution.
A person who is loud sound continuously may get temporary or even permanent impairment of hearing.
Attempts should be made to minimise noise pollution. Silencing devices must be installed in air craft engines, transport vehicles, industrial machines and home appliances. Plantation on the roadside and elsewhere can reduce noise pollution.
Amplitude: The maximum extent of vibration of the vibrating body from its mean position is known as its amplitude.
Time Period: One complete to and fro movement of the pendulum around its mean positionis called one oscillation.
The time taken by the vibrating particle to complete one oscillation is called time period.
Frequency: The number of vibrations made by the vibrating body in one second is known as its frequency. The SI unit of frequency is the hertz (Hz).
Characteristics of Sounds: (i) Loudness: The sensation produced in the ear which enables us to distinguish between a loud and a faint sound. Larger the amplitude of vibration, the louder is the sound produced. It is proportional to square of the amplitude.
(ii) Pitch: The characteristics of sound which distinguishes between a shrill sound and a soft sound.
Higher the frequency of vibration, higher is the pitch and shrillness.
(iii) Quality: Characteristic which enables us to distinguish between musical notes emittedby different musical instruments or voices even though they have the same.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Materials through which electric current can pass through easily are called electricconductors or conductors of electricity.
Electrical conductivity Or Electric Conductivity: Electrical conductivity is a measure of the ability of a substance to allow the flow of electric current.
Among solids metals and graphite are good conductors which have high electrical conductivity.
Some liquids are also good conductors.
Study Tools
Audio, Visual & Digital Content
Pure water or distilled water is a poor conductor of electricity. But the presence of even small amount of impurities(salts and minerals) makes water a good conductor as it contains ions through which conduction takes place.
Hence water from taps, wells, lakes, etc. conduct electricity as they contain impurities.
Most liquids that conduct electricity are solutions of acids, bases and salts.
When electricity is passed through a conducting solution, the molecules of the solution dissociate into ions.
Ions are atoms or group of atoms with a positive or a negative charge.
These ions cause electrical conduction through the liquid.
Electrolyte: A liquid That conducts electricity is called an electrolyte.
Electrolysis: The process of chemical decomposition compound in a solution when an electric current passes through it is called electrolysis.
Electrolysis, is due to the chemical effect of electric current.
electrolytic cell: Two electrodes are inserted in the solution and are connected to the terminals of a battery with a switch in between them.
This arrangement is called an electrolytic cell.
anode: The electrode that is connected to the positive terminal of the battery is called the anode,
cathode: The electrode that is connected to the negative terminal of the battery is called the cathode.
Electrolysis is used in refining, electroplating and extraction of metals from impure samples.
electrorefining: This process of refining and extraction of metals from impure samples is called electrorefining.
electroplating: electroplating is the process of coating a useful metal with another metal.
chemicaleffect of electric current: The process of passing an electric current through a conducting solution to cause chemical reactions is known as the chemicaleffect of electric current.
Chemical effects of electric current: (i) Formation of bubbles of a gas on the electrodes.
(ii) Deposition of metal on electrodes.
(iii) Change in colour of solutions. Electroplating: The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating. The object to be electroplated is made the cathode (negative electrode) by connecting it to thenegative terminal of the battery.
The metal which has to be deposited is made the anode (positive electrode) by connecting it to the positive terminal of the battery. Usually a salt solution of the metal to be coated is made as anode.
Application of Electroplating: (i) Metals that rust are often coated with other metals to prevent rusting.
(ii) Chromium plating is found on bath taps, car bumpers, etc. to give a bright attractive appearance and resist scratches and wear.
(iii) Silver plating is done on cutlery and jewellery items.
(iv) Tin cans, used for storing food, are made by electroplating tin onto iron.
Tin is less reactive than iron. Thus, food does not come into contact with iron and is protected from getting spoilt.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Some objects can be charged by rubbing with other objects. There are two kinds of charges — positive charge and negative charge Like charges repel and unlike charges attract each other.
Study Tools
Audio, Visual & Digital Content
Static Electric Charges: The electric charges produced due to rubbing are called static electric charges. The electric charges constitute electric current during their motion.
An electroscope may be used to detect whether a body is charged or not.
An electroscope is a deviced used to detect the presence of electric charge.
earthing: The process of transfer of charge from a charged object to the earth is called earthing.
Lightning
Lightning is a sudden, natural and violent phenomenon of creating bright flashes during a thunderstorm.
Lightning occurs due to a high-voltage electric discharge between two clouds or a cloud and the ground, or within a single cloud during a thunderstorm.
The formation of clouds involves friction between water droplets in the atmosphere.
The friction charges the particles in the atmosphere. The negative charges accumulate at the bottom of the cloud and the positive charges at the top.
As the accumulation of the charges increases, the cloud will induce positive charges on the ground nearby.
As the amount of charge increases. This causes an imbalance of charges between a region of a cloud and gorund or another cloud (or in a cloud itself). This imbalance is significant enough to break through air resistance.
Lightning releases an average of one gigajoule of energy.
Lightning strike could destroy life and property.
Protective measures are of utmost importance during lightning strikes.
Taking shelter in interiors (house or other closed places) and vehicles (closed e.g. cars) are the most preferred measures.
Lightning conductors can protect buildings from the effects of lightning.
Earth Quake
A natural phenomenon that cannot be predicted is an earthquake.
The earth consists of three major layers, called the crust, the mantle and the core.
The core is further divided into the inner core and the outer core.
The mantle consists of semi-solid material above which the crust floats.
The crust consists of oceans and continents.
The crust is divided into several parts, called tectonic plates.
The regions where one tectonic plate slides against another are referred to as fault zones, and these are the regions where an earthquake is likely to occur.
Hence, these zones are referred to as seismic zones.
The place in the interior of the earth where an earthquake occurs is the focus, and the region on the surface of the earth that is the closest to focus is likely to experience the largest damage.
This region is called the epicenter of the earthquake.
The instrument that measures the severity of an earthquake is a seismograph.
It basically consists of a drum that rolls and a pendulum with a stylus that traces the waves of an earthquake on a sheet like a graph paper.
The energy released at the focus propagates outwardly in form of waves known as seismic waves. Destructive energy of an earthquake is measured on the Richter scale.
It is a logarithmic scale, ranging from 1 to 10 for indicating the intensity of an earthquake.
The earthquake measuring 7 or more on Richter scale can cause severe damage to life and property. Protective measures for earthquake:
If you are at home: Take shelter under a table and stay there till shaking stops.Stay away from tall and heavy objects that may fall on you. If you are in bed, do not get up. Protect your head with a pillow.
If you are outdoors: Find a clear spot, away from buildings, trees and overhead power lines. Drop to the ground. If you are in a car or a bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop. Moreover, it is advisable to make the structure of buildings simple so that they are ‘Quake Safe’.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Light is the natural agent that stimulates sight and makes things visible. Light is reflected from all surfaces. It is a form of energy.
Reflection of Light: Bouncing back of light after striking any surface such as a rough surface, Smooth surfce, shiny surface or polished surface, into the same medium, is called reflection of light. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
luminous objects: Objects that give out light on their own are called luminous objects.
Examples: Flame, Sun, Glowing bulb
Non-luminous objects: Objects that do not give out light on their own are called non-luminous objects.
Examples: Rocks, Plasic Toys, Walls of room.
The Non – Luuminous objects, reflect light that falls on them.
When the reflected light enters into our eyes from the objects we could see them.
Types of Reflection: (i) Regular Reflection: When a beam of parallel light rays is incident on a smooth and plane surface, the reflected rays will also be parallel. This type of reflection is called Regular Reflection.
The reflection from a plane mirror is an example of regular reflection.
(ii) Diffused or Irregular Reflection: When a parallel beam of light is incident upon a rough or uneven surface, it is reflected in many directions due to presence of irregularities on that.
multiple reflections: If a reflected light ray is reflected again on being incident on another surface, it is termed as multiple reflections.
Multiple reflections are used in periscope, kaleidoscope etc.
Periscopes are used in submarines, war tanks and by soldiers in bunkers to see objects.
In a kaleidoscope, beautiful patterns are formed due to multiple reflections.
Laws of reflection of light:
(i) The angle of incidence is equal to the angle of reflection.
(ii) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.
Lateral inversion: Lateral inversion is the effect produced by a plane mirror in reversing images from left to right.
Example: Our left hand will appear as right and vice versa.
Characteristics of the image formed by a plane mirror
Size the image formed by a plane mirror is equal to the siz of the object.
The image formed by the plane mirror is left-right inverted.
The image formed by the plane mirror erect and virtual formed behind the mirror.
Since the distance of the object in front of the mirror two mirrors inclined to each other give multiple images.
Sunlight: Sunlight, called white light, consists of seven colours.
dispersion: Splitting of light into its constituent colours is known as dispersion.
Human Eye:
A normal eye can see nearby and distant objects clearly.
Visually challenged persons can read and write using Braille system.
Visually challenged persons develop their other senses more sharply to improve their interaction with their environment.
Parts of Human Eye: (i) Cornea: Transparent bulge on the front surface of the eyeball which protects the eye andhelps in refraction of light.
(ii) Iris: Coloured diaphragm behind the cornea which controls the amount of light entering the eye.
(iii) Pupil: Dark hole in the middle of iris through which light enters the eye. (iv) Eye lens: Transparent, crystalline structure behind pupil and iris. (v) Ciliary muscles: Hole the eye lens in position and control the focal length of the eye lens.
(vi) Retina: Surface of the rear part of the eyeball where the light entering the eye is focused.
(vii) Rods and Cones: Rod cells respond to the brightness of light while cone cells.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 8 – Science – Study – Premium” posts_per_page=”25″]
Anything (Physical Material not emotions, feelings etc.) which has mass and volume (occupy space) is called matter.
We feel the presence of matter by one or more of our five sense organs.
Matter is made up of particles. (Scroll down to ontinue …)
Study Tools
Audio, Visual & Digital Content
Particles:
Particles are very small in size. Therefore we cannot see particles with our naked eye.
Characteristics of the particles of matter:
(1) All matter (elements or compounds) consists of very small particles which can exist independently and are called particles.
(ii) The particles of matter are in a state of continuous motion and possess kinetic energy.
(iii) There are intermolecular spaces in between the particles (molecules) of matter.
(iv) The particles (molecules) of matter attract each other with a force called intermolecular force.
Intermolecular force is maximum in solids and least in the gases.
These material particles can be touched, moved by changing temperature or attracted by decreasing or increasing forces of attraction or repulsion.
2. States of Matter
Matter exists in three different physical states namely solid, liquid and gas.
One substance such as water can exist in all the three states such as, ice in solid state, water in liquid state and steam or vapours in gaseous state.
The state of matter depends on temperature, forces of attraction between their constituent particles etc.
3. Interconversion of Matter
All these three different states of matter are interconvertible depending upon temperature and pressure.
The state of matter can be changed by changing temperature or pressure.
Due to change in temperature and pressure there will be a change in inter-particle space as well as force between them, resulting in change in physical state.
Examples:
Applying pressure and reducing temperature can liquefy gases.
Solid CO₂ gets converted directly to a gaseous state on decrease of pressure to 1 atmosphere without changing into a liquid state. Due to this fact solid CO₂ is also known as DRY ICE.
4. Plasma: It is the fourth state of matter consisting of super energetic and super excited particles. These particles are in the form of ionised gases.
Examples:
The plasma in stars is formed due to high temperature.
Glowing plasma formed in fluorescent tubes and neon sign bulbs.
These devices contain inert gases which get ionised due to the passage of electric current. The colour of the glowing plasma depends upon the nature of the gas.
5. Sublimation: The process in which a solid state directly changes into a gaseous state on heating or vice-versa on cooling.
6. Melting or Fusion: The process of changing a solid into a liquid state by absorbing heat at a constant temperature is known as Melting or Fusion.
7. Freezing or Solidification: The process of changing a liquid into solid state by losing heat at a constant temperature is known as Freezing or Solidification.
8. Condensation: The process of changing a gas into a liquid state by giving out heat at constant temperature is known as Condensation .
Boiling or Vaporisation : The process of changing a liquid into a gaseous state by absorbing heat at constant temperature is known as Boiling or Vaporisation .
Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into a vapour state.
Evaporation: The phenomenon of changing the physical state from liquid to vapour, at any temperature is called evaporation.
Evaporation is a surface phenomenon. Particles from the surface gain required energy to overcome the forces of attraction present in the liquid and change into the vapour state.
The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed.
Evaporation causes cooling.
Evaporation takes place at all temperatures, below the boiling point of a liquid
Factors affecting evaporation:
• Rate of evaporation increases with increase in surface area.
• Rate of evaporation increases with increase in temperature.
• Rate of evaporation increases with decrease in Humidity.
• Rate of evaporation increases with increase in wind speed.
Latent heat of boiling or Latent heat of Vaporisation: Latent heat of boiling or Latent heat of Vaporisationis the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.
Kelvin is the SI unit of temperature.
0°C = 273.16 K.
For convenience, we take 0°C = 273 K after rounding off the decimal.
To change a temperature on the Kelvin scale to the Celsius scale you have to subtract 273 from the given temperature, and to convert a temperature on the Celsius scale to the Kelvin scale you have to add 273 to the given temperature.
Conversion Formula: t°C = (t+273) K
Boiling point or Vaporisation point: Boiling point or Vaporisation point is the fixed temperature at which a liquid converts into a gaseous state at atmospheric pressure.
Melting point or Fusion point: Melting point or Fusion point is the temperature at which a solid starts converting into a liquid state at atmospheric pressure.
Evaporation Causes cooling: During evaporation the particles at the surface of the liquid gain energy from the surroundings and change into vapour.. Therefore Evaporation Causes cooling effect.
Sponge can be compressed although it is solid: Sponge contains minute holes in which air is trapped.So when it is pressed, the air gets expelled and the sponge gets compressed. Also,the material of the sponge is not rigid.
Temperature does not change during change of state: The temperature remains constant at its melting and boiling points (during change of state) until all the substance melts or boils.
Because the heat supplied is continuously used up in changing the state of the substance by overcoming the force of attraction between the particles.
There is no increase in the kinetic energy of the particles and thus, temperature does not change.
This heat energy absorbed without showing any rise in temperature is given the name latent heat of fusion/latent heat of vaporisation.
Effect of pressure on physical state of a substance:
If pressure is applied, melting point decreases and boiling point increases
When pressure is increased, the particles come closer and the force of attraction increases between them and this results in a change of state.
Example: When high pressure is applied to a gas by reducing its temperature, the particles of gas come close and get converted to a liquid. This is also known as liquefaction.
The amount of heat energy required in changing a 1 kg of solid into liquid at atmospheric pressure and its melting point is known as the latent heat of fusion.
[ Lice = 80 cal/g = 3.34 × 105 J/kg].
• The amount of heat which is required to convert 1 kg of the liquid (at its boiling point) to vapours of gas without any change in temperature is known as latent heat of vaporisation.
[Lwater =540 cal/g= 22.5 × 105 J/kg].
• The amount of heat absorbed or liberated , Q = mL.
• The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.
• Q = m.s. t, where m = mass of the body, s = specific heat of the body and t is temperature difference and m.s is called thermal capacity.
• Change of liquid into vapours at any temperature below the boiling point.
Takes the latent heat from the body. Thus, the body cools when evaporation takes place.
Evaporation:
(1) Evaporation is a slow process.
(ii) Evaporation takes place at the surface mass of the liquid.
(iii) Evaporation takes place at all temperatures.
(iv) The substance becomes cool due to evapora- tion process.
(v) Heat is absorbed from the surroundings due to Evaporation. Absorption of heat from the surroundings causes cooling effect.
Boiling:
(1) Boiling is a rapid process.
(ii) Boiling takes place throughout the mass of a liquid.
(iii) Boiling takes place at a definite temperature called the boil- ing point.
(iv) The substance remains hot during the boiling process.
(v) Heat is required from an external source such as a burner for boiling to take place.
Scales of temperature
• Three scales are commonly used for measuring temperature, namely, the Celsius scale, the Fahrenheit scale and the Kelvin scale.
• The relation between the Celsius and the Kelvin scale can be expressed as:
C + 273 = K
• The relation between the Celsius and the Fahrenheit scale can be expressed as follows.
Property
Solid
Liquid
Gas
Inter particle space
Very less
Larger than solid butlesser than gas
Very large
Inter particle force
Very strong
Weaker than solidbut stronger than gas
Very weak
Nature (Rigidity)
Very hard and rigid
Fluid
Highly fluid
Compressibility
Negligible
Very small
Highly compressible
Shape
Definite shape
Indefiniteshape
Indefinite Shape
Volume
Definite Volume
Indefinite shape
Indefinite volume
Density
high
Less than solid
Very low
Kinetic energy
low
high
Very high
Diffusion
Negligible
Slow
Very high
Specific Heat
11.8 NATURAL PHENOMENA AND CONSEQUENCES OF HIGH SPECIFIC HEAT CAPACITY OF WATER
Some consequences of high specific heat capacity of water are given below.
(i) The climate near the seashore is moderate :
The specific heat capacity of water is very high (= 1000 cal kg-1 °C-1 or 4200 J kg-1 K-¹). It is about five times as high as that of sand. Hence the heat energy required for the same rise in temperature by a certain mass of water will be nearly five times that required by the same mass of sand.
Similarly, a certain mass of water will give out nearly five times more heat energy than that given by sand of the same mass for the same fall in temperature.
As such, sand (or earth) gets heated or cooled more rapidly as compared to water under similar conditions.
Thus, a large difference in temperature is developed between the land and the sea due to which land and sea breezes are formed”. These breezes make the climate near the seashore moderate.
(ii) Hot water bottles are used for fomentation: The reason is that water does not cool quickly due to its large specific heat capacity, so a hot water bottle provides heat energy for fomentation for a long time.
(iii) Water is used as an effective coolant: By allowing water to flow in pipes around the heated parts of a machine, heat energy from such parts is removed (e.g. radiators in car and generator are filled with water). Water in pipes extracts more heat from surroundings without much rise in its temperature because of its large specific heat capacity.
(iv) In cold countries, water is used as a heat reservoir for wine and juice bottles to avoid their freezing: The reason is that water due to its high specific heat capacity can impart a large amount of heat before reaching up to the freezing temperature. Hence bottles kept in water remain warm and they do not freeze even when the surrounding temperature falls considerably.
(v) Farmers fill their fields with water to protect the crops from frost: In the absence of water, if on a cold winter night, the atmospheric temperature falls below 0°C, the water in the fine capillaries of plants will freeze, so the veins will burst due to the increase in volume of water on freezing. As a result, plants will die and the crop will be destroyed. In order to save crop on such cold nights, farmers fill their fields with water because water has a high specific heat capacity, so it does not allow the temperature in the surrounding area of plants to fall up to 0°C.
(vi) All plants and animals have a high content of water in their bodies: All plants and animals have nearly 80% to 90% of water in their bodies so it helps in maintaining the body temperature nearly same in all seasons due to high specific heat capacity of water.
SOME EXAMPLES OF HIGH AND LOW THERMAL CAPACITY
(1) The base of a cooking pan is made thick : By making the base of the cooking pan thick, its thermal capacity becomes large and it imparts sufficient heat energy at a low temperature to the food for its proper cooking. Further it keeps the food warm for a long time, after cooking.
(2) The base of an electric iron is made thick and heavy: By doing so, the thermal capacity of the base becomes large and it remains hot for a long duration even after switching off the current.
(3) The vessel used for measurement of heat (i.e., calorimeter) is made of thin sheet of copper:
The reason is that the specific heat capacity of copper is low and by making the vessel thin, its thermal capacity becomes low so that it takes a negligible amount of heat from its contents to attain the temperature of the contents.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 9 – Science – Study – Premium” posts_per_page=”25″]
Matter: Anything that occupies space is called matter.
Example: Air, water, rock etc.,
Matter exists in our surroundings in both pure and impure forms. (Scroll down to continue …).
Study Tools
Audio, Visual & Digital Content
Mixture: A mixture is a matter that contains more than one pure substance in any ratio/proportion.
A mixture is an impure form of matter.
Example:
Water in milk, lemon juice, Ginger Garlic paste, etc.,
The mixture may or may not be separated into its constituent particles by physical processes.
Substance: A matter that cannot be separated into its constituent particles by any physical process is known as a substance.
Example:
Solution: A homogeneous mixture of two or more substances is called a solution.
Example:
Tea, sugar, and common salt are dissolved in water.
Alloy: A homogeneous mixture of metals is called an alloy.
Properties of the Solution:
A solution is a homogeneous mixture
Particles are extremely small, not visible to the naked eye
The light path is invisible in solution.
Solute particles cannot be separated by filtration
Concentration of solution: The concentration of a solution is the amount of solute present in a given quantity of the solution.
Unsaturated and Saturated Solutions: a solution in which a larger quantity of solute can be dissolved without raising its temperature, is called an unsaturated solution.
• A solution in which no more solute can be dissolved at a certain temperature, is called a saturated solution.
Solubility: The maximum amount of a solute that can be dissolved in 100 grams of a solvent at a specified temperature is known as the solubility of the solute in that solvent.
Suspension: a heterogeneous mixture of solids and liquids where the solid particles are suspended throughout the medium.
Example: Mixture of chalk powder and water
Properties of Suspension
• Particles are visible to the naked eye
• Light path in a suspension is visible
• Particles settle down
Colloidal Solution: Colloidal Solution Is a heterogeneous mixture, but appears to be homogeneous.
Examples: Milk, soap lather, soda water, pumice stone, rubber, bread, fog, cloud, insecticide spray, butter, etc.
Properties of colloidal solutions
• Heterogeneous mixture
• Particle size is small, not visible to the naked eye
• Light path can be visible;
• Particles do not settle down
• Substances cannot be separated by filtration
Tyndall Effect: Scattering of light beam by suspended particles in the solution.
Physical and Chemical changes:
Physical and change: The changes in which no new substances are formed are called physical changes.
Chemical change: The changes in which new substances are formed are called chemical changes.
SEPARATION OF MIXTURES
The method of separation depends on both the type of mixture and the physical properties of its constituents.
These are :
(i) The physical state of the constituents.
(ii) The differences in the physical properties
of the constituents, such as:
(a) boiling point
(b) melting point
(c) density
(d) magnetic properties
(e) ability to sublime
(f) volatility
(g) solubility in various solvents.
• Evaporation: Used for separating mixtures of volatile solvents and non-volatile solutes.
Working Principle:
One component should be non-volatile. It may or may not be soluble in water.
Example: Separating salt from its solution
• Centrifugation used for separating components based on the difference in their weights.
Working Principle:
Difference in the densities of two liquids.
Example: Separating mixtures of cream from milk
• Separating Funnel: Used for separating two or more immiscible liquids.
Working Principle:
Immiscible liquids with different densities get separated into different layers if they are in the same container.
Example: Separating oil and water
Sublimation:
Sublimation is the process of converting a solid into vapour and returning it to the solid state without passing through the liquid state.
Sublimation is used to separate sublimable solids from their mixtures.
Working Principle:
One of the components can be sublime.
Example: Separating ammonium chloride from a mixture
Chromatography:
The process of separating the different dissolved constituents of a mixture by their adsorption (adsorption refers to the collection of one substance on the surface of another substance.) over an appropriate adsorbing material is called chromatography.
Chromatography is used to separate those solutes that dissolve in the same solvent.
Working Principle:
Adsorption/partition
Example: Separating the components of a dye
Distillation:
Distillation is the process of heating a liquid to convert it into vapours and then condensing the vapours back into a liquid.
Distillation is used to separate two miscible liquids that boil without decomposition.
Working Principle:
One component should be a soluble solid in a liquid.
Example: Separating a mixture of acetone and water
Fractional distillation
Fractional distillation is a process that involves the distillation and collection of fractions or different liquids boiling at different temperatures.
Fractional distillation is used to separate a mixture of liquids when their boiling temperatures differ by less than 25 K.
Example: Separating different components of petroleum
Crystallization: Used to separate pure solids from a solution by forming crystals.
Working Principle:
A solid dissolved in a liquid is separated by evaporating the solvent completely by heating the mixture.
Example: Obtaining pure crystals of copper sulphate from an impure sample.
Differences Between Mixture And Compound
Property
Mixture
Compound
Nature
When two or more elements or compounds or both are mixed together, such that they do not combine chemically, a mixture is formed.
When two or more elements unitechemically, a compound is formed.
Structure
Mixtures are generally heterogeneous. However, some mixtures can be homogeneous.
Compounds are always homogeneous.
Composition
In case of mixtures their constituents can be present in any ratio, i.e., mixtures havevariable composition.
In case of compounds, the constituents arepresent in a fixed ratio by weight.
Properties
The constituents of a mixture retain theirindividual chemical and physical properties.
The properties of a compound are entirelydifferent from the properties of itsconstituents
Separation of constituents
The constituents of a mixture can beseparated by applying physical methods likesolubility, filtration, evaporation, distillation,use of magnet, etc.
The constituents of a compound cannot beseparated by applying physical methods.However, constituents of a compound can beseparated by chemical means.
Energy change
There may or may not be energy changeduring the formation of mixture.
During the formation of a compound eitherthe energy is absorbed or given out.
Different kinds of matter contain different kinds of atoms present in them.
Protons were discovered by Ernest Rutherford, in his famous gold foil experiment.
Electrons were discovered by J.J. Thomson, in his cathode ray tube experiment.
Neutrons were discovered by James Chadwick. (Scroll down to continue …).
Study Tools
Audio, Visual & Digital Content
Laws of Chemical Combination: Antoine Laurent Lavoisier, is known as ‘Father of Modern Chemistry.
Lavoisier put forward the law of conservation of mass, which laid the foundation of chemical sciences.
Law of Conservation of Mass:Law of Conservation of Mass states that, “mass is neither created nor destroyed in a chemical reaction.
In other words, the mass of the reactants must be equal to the mass of products.
Law of Constant Proportions or Definite Composition:Law of Constant Proportions or Definite Composition states that, in a pure chemical substance, the elements are always present in definite proportions by mass.
Dalton’s Atomic Theory:
(i) Every element is composed of extremely small particles called atoms.
(ii) Atoms of a given element are identical, both in mass and properties.
(iiii) Different chemical elements have different kinds of atoms; in particular, their atoms have different masses.
(iv) The atoms neither be created nor be destroyed or transformed into atoms of other elements.
(v) Compounds are formed when atoms of different elements combine with each other in small whole number ratios.
(vi) The relative number and kinds of atoms in a given compound are constant.
Drawbacks of Dalton’s Atomic Theory:
(i) According to modern theory, an atom is not the ultimate indivisible particle of matter. Today, we know that atoms are divisible, they are themselves made-up of particles (protons, electrons, neutrons,etc.).
(i) In the case of isotopes of an element, the assumption that the atoms of the same element have the same mass does not hold good.
Atom: It is the smallest particle of an element that maintains its chemical identity throughout all chemical and physical changes.
The smallest unit of a substance which can exist independently is called a molecule.
Atomicity: It is defined as the number of atoms present in a molecule of an element or a compound.
Mono atomic: Molecule having only one atom is called mono atomic,
e.g., He, Ne, Ar.
Diatomic: Molecules made-up of two atoms are called diatomic, e.g., H₂, Cl₂, O₂, N2.
Triatomic: Molecules made-up of three atoms, called triatomic.
e.g., O3, H₂O, NO2.
Tetraatomic : Molecules made-up of four atoms, called tetra atomic.
e.g., P4, NH3, SO3
Polyatomic: Molecules made-up of five or more atoms, called polyatomic/
e.g., CH4.
Polyatomic: Any molecule which is made-up of more than four atoms is called polyatomic,
e.g., Sg.
Relative Atomic Mass: It is defined as the number of times one atom of an element is heavier than
(1/12)th of the mass of an atom of Carbon – 12.
Relative Atomic Mass (RAM) = Mass of an atom of an element/
¹/12 th mass of C-12
Molecular Mass: The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule of a substance,
e.g., molecular mass of water is 18 u.
The mole (or mol) is the SI unit of the amount of a substance. One mole is equal to the amount of substance that contains as many elementary units as there are atoms in 12 g of the carbon-12 isotope.
The elementary units may be atoms, molecules, ions, radicals, electrons, etc., and must be specified.
This number is called Avogadro’s number (No) or Avogadro’s constant
[NA = 6.0221367 x 1023]. Generally,
Avogadro’s Number is rounded to 6.022 x 1023.
For better understanding we can compare avogadro number with a dozen as:
One dozen oranges contain 12 oranges, similarly, 1 mole of hydrogen atoms contain 6.022 x 1023 H atoms.
H₂O = 2 x H + 1 × O
= 2 x 1+1 x 16 = 2+16
= 18 amu or u.
By : 1 mole of a compound has a mass equal to its relative molecular mass expressed in grams.
1 mole = 6.022 × 1023 number
= Relative mass in grams.
A molecule is the smallest particle of an element or a compound capable of independent existence under ordinary conditions. It shows all the properties of the substance.
A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element.
Clusters of atoms that act as an ion are called polyatomic ions. They carry a fixed charge on them.
The chemical formula of a molecular compound is determined by the valency of each element.
In ionic compounds, the charge on each ion is used to determine the chemical formula of the compound.
Scientists use the relative atomic mass scale to compare the masses of different atoms of elements. Atoms of carbon-12 isotopes are assigned a relative atomic mass of 12 and the relative masses of all other atoms are obtained by comparison with the mass of a carbon-12 atom.
The Avogadro constant 6.022 × 1023 is defined as the number of atoms in exactly 12 g of carbon-12.
The mole is the amount of substance that contains the same number of particles (atoms/ions/ molecules/formula units, etc.) as there are atoms in exactly 12g of carbon-12. Mass of 1 mole of a substance is called its molar mass.
The relative atomic mass of the atom of an element is the average mass of the atom as compared to 1/12th mass of one carbon-12 atom.
Hint: We know that chemical formulas can also be written using a criss-cross method. In the criss-cross method, the numerical value of the ion charge of the two atoms is crossed over, which becomes the subscript of the other ion. Using this technique, we will write the chemical formula of the given compounds.
Complete step by step answer:
Let’s us discuss about the given compound as,
A.Magnesium chloride
We have to remember that the atomic number of Magnesium is 12 and has a valency of 2.
It means it has two electrons in the outermost shell for bonding.
The atomic number of chlorine is 17 and has 7 electrons in the outermost shell.
It means it just needs one more atom for bonding.
Hence, we will use atoms of chlorine to bond with one atom of magnesium.
We can apply the criss-cross method for this compound as,
Therefore, the chemical formula of magnesium chloride is MgCl2
B.Calcium oxide
We have to know that the atomic number of calcium is 20 and has a valency of 2, it means it has 2 two atoms in the outermost shell for bonding.
The atomic number of Oxygen is 8
8 and has a valency of 2, it has 6 atoms in the outermost shell, it needs 2 more to complete the octet.
Hence, we need one calcium atom to bond with one oxygen atom.
We can apply the criss-cross method for this compound as,
Therefore, the chemical formula of magnesium chloride is CaO
C. Copper nitrate
We have to know that the atomic number of copper is 29 and has two atoms in the outermost shell for bonding. While a nitrate molecule has only one valence electron.
We need 2 nitrate molecules to satisfy the valency of 1 copper atom.
We can apply the criss-cross method for this compound as,
Therefore, the chemical formula of magnesium chloride is
Cu(NO3)2
D.Aluminium chloride
We have to know that the atomic number of aluminium is 13 and has a valency of 3 atoms and chlorine atom has a valency of 1. Since it has 7 electrons in the outermost shell.
Thus, we need 3 chlorine atoms to satisfy the valency of 1 aluminium atom.
We can apply the criss-cross method for this compound as,
Therefore, the chemical formula of magnesium chloride is AlCl3.
E.Potassium nitrate
We have to remember that the atomic number of potassium is 19 and has a valency of 1 and nitrate also has a valency of 1, since it needs one more atom to complete its octet. Hence, we need only one molecule of nitrate for one atom of potassium.
We can apply the criss-cross method for this compound as,
Therefore, the chemical formula of magnesium chloride is KNO3.
Note: As we know that the criss-cross method is the most efficient way to write the correct chemical formula of the molecule. It is generally used for finding out the formula of a bonding of a metal with a non-metal to form ionic bonds. Signs of the two ions are dropped, the ion value is crossed which becomes the subscript of the crossed atoms.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 9 – Science – Study – Premium” posts_per_page=”25″]
Different kinds of matter contain different kinds of atoms present in them.
Protons were discovered by Ernest Rutherford, in his famous gold foil experiment.
Electrons were discovered by J.J. Thomson, in his cathode ray tube experiment.
Neutrons were discovered by James Chadwick. (Scroll down to continue …)
Study Tools
Audio, Visual & Digital Content
Charged Particles in Matter
Whenever we rub two objects together, they become electrically charged.
This is because atoms contain charged particles in them.
Therefore, atoms can be divided further into particles i.e proton, electron and neutron.
Atoms consist of an equal number of protons and electrons.
Protons exist in the interiors of the atom and electrons exist in the exteriors of the atom. Therefore, electrons can be removed from an atom.
Since electrons exist in the exteriors of the atom they can be removed from an atom.
Dalton’s Atomic Theory
The postulates of the atomic theory by John Dalton
The matter is made up of tiny particles called Atoms that cannot be divided.
Atoms are never formed or destroyed during a chemical reaction.
Atoms of an element exhibit the same nature.
Atoms of the same element have equal size, mass and they exhibit similar chemical properties.
Atoms of different elements exhibit variant chemical properties.
Atoms form compounds by combining in a ratio of whole numbers.
A compound contains molecules in which a constant number and types of atoms are present.
Failure of Dalton’s Atomic Theory
Dalton suggested that atoms can neither be created nor destroyed and are indivisible.
But the discovery of electrons and protons in atoms disproved this aspect of Dalton’s theory.
Thomson’s Model of an Atom
According to J.J. Thomson, the structure of an atom can be compared to Christmas pudding.
According to this model the electrons are present inside a positive sphere.
An atom is composed of a positively charged sphere in which electrons are embedded.
Atoms are neutral as the positive and negative charges are equal in number.
Rutherford’s Model of an Atom
Rutherford’s Experiment
Rutherford experimented by passing alpha rays through a thin gold foil.
He expected that the gold atoms would deflect the Alpha particles.
Observations
Inferences
Alpha particles which had high speed moved straight through the gold foil
Atom contains a lot of empty space
Some particles got diverted a by small angles
Positive charges in the atom are not occupying much of its space
Only one out of 12000 particles bounced back
The positive charges are concentrated over a particular area of the atom.
Based on his experiment Rutherford gave the nuclear model of an atom as the following.
Rutherford’s Atomic Model
Rutherford’s Atomic Model is known as Planetary Atomic Model and Nuclear Atomic Model.
According to Rutherford’s Atomic Model:
Atoms contain a lot of unoccupied space
The center of the atom is highly positive , Rutherford named it as nucleus
The atom contains an equal amount of positive and negative charges.
Nucleus of Atom
The nucleus is located at the center of the atom.
All the mass of the atom is because of the nucleus.
The electrons revolve around the nucleus in circular parts which called Orbits
The size of an atomic nucleus is much smaller than its atom.
Drawbacks of the Nuclear Atomic Model
The Rutherford’s Atomic Model failed to explain how an atom remains stable despite having positive and negative charges present in it.
Maxwell’s theory of radiation if any charged particle moves in a circular motion it radiates energy.
So, if electrons move in a circular motion around the nucleus they should radiate some energy as a result this decreases at the speed of the electrons. As a result, they would fall into the nucleus and the nucleus should collapse because of its high positive charge.
But it is not happening because the matter is not collapsing.
Nucleons: The subatomic particles present in the nucleus are collectively called Nucleons. Protons and Neutrons are nucleons.
Bohr’s Model of an Atom
Bohr Atomic Model states as the following:
Electrons revolve around the nucleus in particular circular paths, called orbits.
The electrons do not emit any energy while moving in their orbits.
The orbits are also called Energy Levels.
Energy Levels or Orbits are represented by using letters or numbers as shown in the figure.
Neutron:
J. Chadwick discovered Neutron, a subatomic particle of an atom.
Neutron carries no charge.
Subatomic Particles of Atom
Electrons
Electron carry a negative charge
Protons
Protons carry a positive charge
Neutrons
Neutrons are neutral
Electronic Configuration:The distribution of electrons in different shells or orbits is called Electronic Configuration.
If Orbit number = n
Then number of electrons present in an Orbit = 2n2
So, for n =1
Maximum electrons present in shell – K = 2 * (1)2 = 2
The outermost shell can contain at most 8 electrons.
The shells in an atom are filled in sequence.
Thus, until the inner shells of an atom are filled completely the outer shells cannot contain any electrons.
Valency
Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.
The atoms which have completely filled the outermost shell are not very active chemically.
The valency of an atom or the combining capacity of an atom is given by the number of elements present in the outermost shell.
For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.
What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?
Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.
Representation Element:
Atomic Number of an Element
Atomic Number (Z) = Number of protons in an atom
Mass Number of an Element
Mass Number = Number of protons + Number of neutrons
Isotopes
The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.
Isotopes are pure substances.
Isotopes have a similar chemical nature.
Isotopes have distinct physical characteristics.
Use of Isotopes:
1. The fuel of Nuclear Reactor – Isotope of Uranium
2. Treatment of Cancer – Isotope of Cobalt
3. Treatment of Goiter – Isotope of Iodine
Example: Consider two atomic species namely U and V. Are they isotopes?
U
V
Protons
5
5
Neutrons
5
6
Mass Number
5 + 5 = 10
5 + 6 = 11
Atomic Number
5
5
From the above example, we can infer that U and V are isotopes because their atomic number is the same.
Isobars
The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars.
Hindi Version
Dig Deep
Topic Level Resources
Sub – Topics
Select A Topic
Topic:
Chapters Index
Select Another Chapter
[display-posts category=”CBSE 9 – Science – Study – Premium” posts_per_page=”25″]
