PSEB Notes

This PSEB 6th Class Science Notes Chapter 15 Air Around Us will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 15 Air Around Us

→ All living things require air to live.

→ Air is present everywhere around us however we cannot see the air.

→ Our earth is surrounded by a thin layer of air that extends upto many kilometres above the surface of the earth. This is known as the atmosphere.

→ Air is made up of water vapours, Oxygen, Nitrogen, Carbon dioxide, dust, and smoke.

→ Nitrogen and Oxygen are the main constituents of air and are present in large quantities.

→ In the air, nitrogen is 78% and oxygen is only 21%.

→ Oxygen is required for burning.

→ Carbon dioxide is a greenhouse gas.

→ The balance of Oxygen and Carbon dioxide in the atmosphere is maintained by the presence of living organisms (man, animals) and plants.

→ Carbon dioxide is used by plants for producing food by photosynthesis. They release oxygen.

→ Oxygen is consumed by animals and humans for respiration. They produce carbon dioxide.

→ Carbon dioxide is used as a fire extinguisher because carbon dioxide does not support g burning.

→ Air can be compressed and it also exerts pressure.

→ Moving air is called wind.

→ Moving air has the power to rotate a windmill which produces electricity.

→ Windmills are very useful as they produce electricity.

→ Air helps in the dispersal of seeds, pollens, and movement of sailing boats, yachts, gliders, aeroplanes, etc.

→ Dispersal of pollens is known as pollination.

→ Ozone present in the upper layer of the atmosphere protects us from the UV rays of the sun.

→ Air: It is a mixture of many gases like nitrogen, oxygen, carbon dioxide, water vapours, etc.

→ Wind: The moving air is wind.

→ Windmill: A device that is being rotated by air and is used for producing electricity.

→ Atmosphere: The thin blanket of air surrounding the earth constitutes the atmosphere.

This PSEB 6th Class Science Notes Chapter 14 Water will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 14 Water

→ Water is a vital resource and is considered a boon to live.

→ Water is present all over the earth.

→ Around the 3/4th part of the earth, the surface is covered with water.

→ The human body consists of 70% of water.

→ We need water for our daily activities and for producing other things.

→ Two main sources of water are surface water and groundwater.

→ Water exists in three states i.e. solid, liquid, and gas.

→ We get water from ponds, lakes, rivers, wells, etc.

→ Sea or Ocean water is salty. It is not fit for drinking and for domestic, agricultural, and industrial needs.

→ As water is a vital resource so it must be conserved.

→ Judicious use of water and saving it is conservation of water.

→ Rainwater is a pure form of water and is safe for drinking.

→ Rainwater harvesting is a technique that conserves water by storage of rainwater. This water can be reused for different useful purposes.

→ The water cycle is a cyclic process in which water is circulated between the earth and the atmosphere.

→ Ocean water supplies water to the water cycle and thus plays an important role.

→ Water from oceans, lakes, wet clothes and plants gets evaporated.

→ Clouds are just the condensation of evaporated water in the air.

→ Clouds cause rains and water returns to earth, oceans, lakes, etc.

→ Snow on mountains melts and water runs into rivers.

→ Water is also absorbed by the ground and becomes available to us as groundwater.

→ Heavy rains and no rains cause too many problems.

→ Heavy rains cause floods which destroy lives and properties.

→ Overflow of water in a large amount beyond its normal limits is known as a flood.

→ A flood is a natural disaster.

→ No rain or very little rain in a region is caused by the drought.

→ In drought conditions availability of food, fodder, and water become negligible.

→ With the increase of population, industries, and agriculture also increase which lay stress on the use of water.

→ Melting: The process of conversion of solid into liquid is called melting.

→ Freezing: The process of converting liquid into solid is called freezing.

→ Evaporation: The process of converting liquid into its vapours is called evaporation.

→ Transpiration: The process by which plants lose their water in the air is called transpiration.

→ Condensation: The process of conversion of water vapours into water is called condensation.

→ Precipitation: Falling of water in the form of rain and snow is called precipitation.

→ Clouds: The condensed water droplets join together to form clouds.

→ Water cycle: The evaporation of water to air, formation of clouds by condensation, and falling of rain on the earth constitutes the water cycle in nature.

→ Rainwater harvesting: Collection of rainwater for use at a time when it is not available otherwise is rainwater harvesting.

This PSEB 6th Class Science Notes Chapter 13 Fun with Magnets will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 13 Fun with Magnets

→ Magnetite is a natural magnet.

→ Some rocks have the property of attracting pieces of iron and these are called natural magnets.

→ Magnet has the property of attracting iron, cobalt, and nickel. Such substances are called magnetic substances.

→ Every magnet has two poles:

• North Pole
• South pole

→ A freely suspended magnet sets itself in a North-South direction.

→ Unlike (opposite) poles of two magnets attract each other whereas like poles repel each other.

→ Man-made magnets are called artificial magnets.

→ Magnets made from iron pieces are called artificial magnets.

→ Magnets are of different shapes such as Bar magnet, Horseshoe shaped (U-shaped) magnet, cylinderical magnet, or spherical magnet.

→ Compass is used to find direction.

→ When a magnet is brought near a compass, its needle gets deflected.

→ On heating a magnet, striking it with a hammer, or dropping it from a height it loses its property of magnetism.

→ If a magnet is not properly stored then it loses its property with time.

→ To store bar magnets with safety the opposite poles of two magnets are placed facing each other.

→ The magnet should be kept away from cassettes mobiles, TVs, CDs, and computers, etc.

→ Compass: Compass is a device that is needed to determine direction.

→ Magnet: Such substances which have the property of attracting iron are called magnets.

→ Magnetite: Magnetite is a natural magnet that was first discovered by a Shepherd named Magnus.

→ North pole: That end of a freely suspended magnet that points towards the geographical north direction is called its North pole.

→ South pole: That end of a freely suspended magnet that points towards the geographical south direction is called its South pole.

This PSEB 6th Class Science Notes Chapter 12 Electricity and Circuits will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 12 Electricity and Circuits

→ An electric cell is a source of electric energy.

→ The electric cell has two terminals: one positive terminal (+) and the other negative terminal (-).

→ The electric bulb has one filament which is connected to its terminals.

→ On passing the electric current, the bulb begins to glow.

→ In a closed electric circuit, the electric current passes from one terminal of the electric cell to the other terminal.

→ The switch is a simple device that is used to either break the electric circuit or to complete it.

→ Materials that allow an electric current to pass through them, are called conductors.

→ Materials that do not allow an electric current to pass through them are called insulators.

→ Light-emitting thin wire of the bulb is called filament of the bulb.

→ On the breaking of filament, the bulb becomes fused.

→ The complete path of electric current is called an Electric circuit.

→ Bulb: It is a simple device that gives out light on passing an electric current through it.

→ Electric conductor: Materials that allow current to pass through them are called electric conductors.

→ Electric cell: Electric cell is a source of electricity. The chemicals stored in it produce an electric current.

→ Electric circuit: The path of current from one terminal to another terminal, through a switch, bulb, and switch, is called an electric circuit.

→ Filament: A small thin metallic wire (Tungsten) inside a bulb that glows when current is allowed to pass through it, is called a filament.

→ Insulators: Materials that do not allow current to pass current through them are called insulators.

→ Switch: A simple device that is used to break or complete a circuit is called a switch.

→ Terminal: The point of entering or exit of electric current is called terminal.

→ An electric cell has two terminals:

• Positive terminal
• Negative terminal.

This PSEB 6th Class Science Notes Chapter 11 Light Shadows and Reflections will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 11 Light Shadows and Reflections

→ Light is a form of energy that enables us to see the things around us.

→ A source of light can be either natural or artificial as for example Sun, Moon, Stars CFL, Candle, and LED.

→ Light ordinarily travels along a straight path.

→ Opaque objects do not allow light to pass through them and the objects lying on the other side of it cannot be seen.

→ Transparent objects allow light to pass through them and we can see through these objects clearly.

→ Translucent objects allow light to pass through them partially and the objects lying on another side of these objects are not seen clearly.

→ Shadows are formed when an opaque object comes in the path of light.

→ A pinhole camera can be made with simple materials and can be used to obtain the image of the Sun and other bright (luminous) objects. This image is inverted and small in size.

→ Reflection from the mirror gives us clear images.

→ Luminous Object: Such objects have light of their own and give out light.

→ Non-Luminous Object: Such object which has no light of its own and becomes luminous from light emitted by other luminous objects.

→ Light: It is a form of energy that enables us to see the surrounding objects but light itself is not seen.

→ Source of light: Such luminous bodies which have the light of their own and which emit light as for example candle CFL etc. The source light may either be a Natural or Artificial source.

→ Transparent objects: Such objects through which light can pass and on the other side of which we can see clearly are called transparent objects. Examples are Air, water, glass, etc.

→ Opaque objects: Those objects which do not allow light to pass through them and the objects on the other side of which cannot be seen are called opaque objects. Examples are Cardboard sheets, wood, metal, and rubber, etc.

→ Translucent objects: Those objects through which light can pass but partially and the objects lying on the other side of which are not seen clearly are called translucent objects. Examples are Tissue paper, thin cloth, oiled paper, etc.

→ Shadow: When an opaque object is placed in the path of the light rays coming from a source of light then light cannot pass through and a black region is formed on the other side of the opaque body. This black region where no light is received is called the shadow. The size of the shadow can be small or large.

→ Sundial: It is an instrument that shows time with the help of shadows formed by sunlight.

→ Solar Eclipse: When the moon while revolving around the earth comes in such a situation that the moon is in between the earth and the sun and all the three are in a straight line then the shadow of the sun is formed on the earth, a solar eclipse is said to occur.

→ Lunar Eclipse: When the earth comes in between the sun and moon and all the three are in a straight line and the shadow of the moon is formed on the earth; then the lunar eclipse is said to occur.

→ Pinhole camera: It is an instrument in which the inverted and small-sized image of a stationary object such as a building or tree is formed. It is constructed on the principle of propagation of light in a straight line.

→ Mirror: Any smooth and polished surface which changes the path of light rays falling on it, is called a mirror.

→ Reflection of light: When light falls on a mirror and light falling on it is sent back into the same medium in a particular direction, then this phenomenon of change of direction of the path of light is called reflection of light. Reflection of light is of two types:

• Regular reflection
• Irregular reflection

→ Regular reflection: When light falls on a smooth plane mirror or a polished surface then a reflection of light takes place in a regular manner. This reflection of light is called regular reflection.

→ Irregular reflection: When light falls on any rough or uneven surface than light rays after reflection gets scattered. This reflection of light is called irregular reflection. We can see the various objects around us due to this reflection.

This PSEB 6th Class Science Notes Chapter 10 Motion and Measurement of Distances will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 10 Motion and Measurement of Distances

→ Measurement is a comparison of an unknown quantity with a known fixed quantity of the same kind.

→ Different modes of transport are used to move from one place to another.

→ Earlier cubit, hand span, or foot were used to measure the lengths or distances. Measurements made with them were not accurate.

→ The system of units used is known as the International System of Unit (S.I. Units) and is accepted all over the world.

→ The S.I. unit of length is meter.

→ Change in the position of an object with respect to things in the surroundings is called motion.

→ The motion of an object in a straight line is called linear motion.

→ In a circular motion, the object moves in a circular path and its distance from a fixed point (centre) always remains the same.

→ A motion that repeats itself after a fixed interval of time is called Periodic motion.

→ The fixed amount of known quantity is called unit.

→ The standard unit of length is a meter.

→ Distance: It is a measurement of length between two points.

→ Measurement: The comparison of an unknown quantity with some known quantity of the same kind.

→ Unit: The known fixed quantity taken as standard for comparison.

→ Odometer: A device fitted in motor vehicles to measure the distance covered by the vehicle.

→ Motion: When an object changes its position as compared to the surroundings with the passage of time, then the object is said to be in motion.

→ Linear Motion: The motion of an object in a straight line.

→ Circular motion: The motion of an object in a circular path, is called circular motion.

→ Periodic motion: When an object repeats its motions after a fixed interval of time, its motion is called periodic motion.

This PSEB 6th Class Science Notes Chapter 9 The Living Organisms and Their Surroundings will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 9 The Living Organisms and Their Surroundings

→ Different types of living organisms are present everywhere.

→ All living beings have some common characteristics.

→ They need food, respond to stimuli, respire, excrete, grow, reproduce and show movements.

→ The surroundings where organisms live are called a habitat.

→ Habitat may be terrestrial or aquatic or aerial/arboreal. Deserts, mountains, and grasslands are examples of terrestrial habitats. Oceans, ponds, and lakes are examples of aquatic habitats.

→ Organisms having terrestrial habitat are called terrestrial organisms whereas organisms having aquatic habitat are known as aquatic organisms.

→ Plants having terrestrial habitat are called terrestrial plants whereas plants having aquatic habitat are known as aquatic plants.

→ Animals having terrestrial habitats are called terrestrial animals whereas animals having aquatic habitat are known as aquatic animals.

→ All types of habitats have two main components: biotic and abiotic.

→ Biotic components are all living things like plants, animals, human micro-organisms.

→ Abiotic components include all non-living things like rocks, soil, air, water, sunlight.

→ Abiotic factors or components change over thousands of years.

→ The ability of the living things to adjust themselves to their surroundings is called Adaptation.

→ Organisms i.e. plants and animals adapt differently to live better in different habitats.

→ Rats and snakes stay in burrows deep in the sand to stay away from the intense heat.

→ Cactus is a desert plant that has spines to reduce the loss of water from leaves through transpiration.

→ Trees in the mountain regions are normally cone-shaped having sloping branches and needle-like leaves.

→ Animals have thick skin or fur to protect them from the cold of mountainous regions.

→ Lion, deer, or other animals are adapted to live in grasslands.

→ Squids and octopuses are sea animals. Their habitats are deep-sea (sea bed). They make their body streamlined while swimming.

→ Aquatic plants can be floating, totally submerged, or partially submerged type.

→ A frog is an animal that can live both on land and in water. Its body is adapted to both types of abiotic factors.

→ A cloud grows in size, moves from one place to another but it is a non-living thing.

→ Some animals go into a state of long sleep during winter for survival and this winter sleep is called Hibernation.

→ Cell: The basic unit of life is called a cell.

→ Unicellular Organisms: Organisms that are made of only one cell.

→ Multicellular Organisms: Organisms that are made of many cells.

→ Stimulus: Living things respond to immediate and long-term changes in their environment.

→ Adaptations: The presence of specific features and habits, which enable a plant or an animal to live in a particular habitat, is called adaptation.

→ Habitat: The surroundings where plants and animals live, are called their habitat.

→ Biotic Component: The living components of habitat are biotic components, e.g. plants, animals.

→ Xerophytes: The plants which grow and propagate in deserts, e.g. cactus.

→ Hydrophytes: The plants which grow and propagate in water.

→ Photosynthesis: The process of manufacturing food by green plants with the help of water, carbon dioxide, sunlight, and chlorophyll.

→ Breathing: Taking in and releasing out air from the body is called breathing.

→ Respiration: The process of exchange of carbon dioxide and oxygen is called respiration.
(or)
The process of taking oxygen in, combining it with food, and throwing out carbon dioxide and water vapour is called Respiration.

→ Excretion: The process by which waste products are removed from the body of organisms.

→ Producers: Organisms that can manufacture their own food. Examples are all types of green plants.

→ Consumers: Organisms that cannot manufacture their own food but consume food prepared by green plants or other organisms.

→ Predators: Some strong animals that make weaker animals their prey are called predators.

→ Decomposers: Microorganisms that feed on dead plants and animals and break them into simpler substances.

→ Scavengers: Animals that feed on dead animals.

This PSEB 6th Class Science Notes Chapter 8 Body Movements will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 8 Body Movements

→ Movement is a change in the position of any part of the body of an organism.

→ Locomotion is the movement of the whole body of an organism from one place to another.

→ Animals show locomotion and other types of movements but plants do not locomotion though they show some other types of movements.

→ Walking by man, swimming by fishes, running by horse, crawling by a snake, jumping by grasshopper, and flying by birds, etc. are different modes of locomotion.

→ The purpose of locomotion by animals is to find water, food, shelter, and protect themselves from enemies.

→ The framework made up of bones that give support to the body is known as the skeleton.

→ Bones are hard and rigid whereas cartilage is smooth and flexible.

→ The human skeleton comprises bones and cartilages.

→ The human body has 300 bones at the time of birth.

→ The adult human body has 206 bones.

→ The rib cage is made up of ribs, backbone, and chest bone. It protects the internal parts of the body.

→ Skull encloses protects the brain.

→ A Joint is a place where bones meet together.

→ The ligament is an elastic tissue that connects bones together.

→ The movement of the body depends upon the contraction of muscles. These muscles always work in pairs.

→ Gait is the pattern of movement of limbs of animals.

→ The earthworm moves by contraction and extension of the muscles of its body.

→ A snail moves with the help of a large sticky muscular foot.

→ A cockroach can walk, run. climb and fly.

→ Forelimbs of birds are modified into wings that help in flight.

→ The body of fish is streamlined and it moves by fins placed laterally on its body.

→ Birds have streamlined bodies and hollow lightweight bones which help them during g flight.

→ Snakes move by creeping on their belly.

→ Different types of joints allow movements in different directions.

→ Our body has many joints – The ball and socket joint, the Hinge joint, the Fixed joint, and the Pivotal joint.

→ The ball and socket joint allows the movement in a circular form or in all directions.

→ The hinge joint allows back and forth movement.

→ The pivotal joint allows the movement forward and backward, right or left. The joint of the neck and head is an example.

→ Fixed joints are immovable.

→ X-ray helps in counting the number of bones and studying the shape of bones in all bodies.

→ Movement: is a change in the position of any part of the body of an organism.

→ Locomotion: is the movement of the whole body of an organism from one place to another.

→ Bone: It is the part of the skeleton which is hard in nature.

→ Joint: A joint is a part of the body where two or more bones meet to allow some type of movement.

→ Cartilage: Smooth, thick, and flexible tissue found in joints.

→ Fixed Joints: The joints at which bones cannot move is known as fixed joint.

→ Moveable Joints: The joints where the movement of the bones is possible are called movable joints.

→ Skeleton: The framework of the body that gives support and shape to the body.

→ Streamlined body: A body that tapers at both ends is called a streamlined body.

→ Tendon: The strong, fibrous tissue which attaches muscles to bones is called a tendon.

→ Ligaments: Strong, flexible tissue which connects two bones.

→ Pelvis: The large bony bowl formed by the fusion of hip bones and the last two parts of the backbone is known as the pelvis.

→ Pectoral Girdle: The girdle that supports the bones of the arm is called the pectoral girdle.

→ Pelvic Girdle: The girdle that supports the bones of the legs is called the pelvic girdle.

→ Vertebrae: These are the small ring-shaped bones present in the backbone.

→ Dislocation: The dislodging of bones at the joints from their normal positions is called dislocation.

This PSEB 6th Class Science Notes Chapter 7 Getting to Know Plants will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 7 Getting to Know Plants

→ We have a large variety of plants around us. These differ in size, shape, colour of leaves, flowers, etc.

→ Plants around us are usually grouped inter herbs, shrubs, and trees

→ Herbs are small-sized plants with soft green stems which generally lack branches.

→ Shrubs are medium-sized plants with hard stems and branched near the base or near the ground. Trees are tall and long with strong stems called trunks.

→ Parts of a plant consist of two types of system – root system and shoot system.

→ The root system is the underground part of the plant. It anchors the plant to the soil.

→ Roots absorb water and minerals from the soil.

→ The shoot system is the portion of the plant above the ground.

→ Two types of roots are taproots and fibrous roots.

→ Stem conducts water to leaves and other parts of the plant.

→ Stem conducts water to leaves and other parts of the plant.

→ The green colour of leaves is due to the presence of a pigment called chlorophyll.

→ Leaves perform photosynthesis and transpiration.

→ The leaf is made up of two parts-petiole and lamina.

→ The lines on the leaf lamina are called veins and in the centre is called midrib.

→ Venation is of two types: Parallel venation and Reticulate venation.

→ A flower is the reproductive part of a plant.

→ The flower has sepals, petals, stamens, and pistils.

→ Stamen is the male reproductive organ.

→ The pistil is the female reproductive organ.

→ Creepers: Herbs having a weak stem that cannot stand upright and spread on the ground are called creepers.

→ Climbers: Plants having a weak stem that cannot stand erect and take support of nearby objects to climb up are called climbers.

→ Taproot: The root which has a main root and branched smaller roots is called the taproot.

→ Fibrous root: This root has no main root. It is like a bundle of thread-like growth. All roots seem similar and are known as fibrous roots.

→ Weeds: The unwanted plants grown with the main crop are weeds.

→ Venation: The design made by veins in a leaf is called venation.

→ Parallel Venation: When the lines/veins are parallel to one another then it is known as parallel venation.

→ Reticulate Venation: When there is a network of veins then the venation is reticulate venation.

→ Herbs: The plants with small size, green and tender stems are known as herbs.

→ Shrubs: Plants having strong stem branched at the base are known as shrubs.

→ Trees: Trees are tall-sized plants in which the main stem is hard, broad, and thick at the base, and branches are found on the upper part.

→ Trunk: The main step of a tree which is woody, hard, broad, and thick at the base, and bare branches is known as the trunk.

→ Photosynthesis: The process by which plants prepare food using carbon dioxide and water in the presence of chlorophyll and sunlight.

→ Transpiration: Loss of water vapour from plants through stomata.

This PSEB 6th Class Science Notes Chapter 6 Changes Around Us will help you in revision during exams.

PSEB 6th Class Science Notes Chapter 6 Changes Around Us

→ Change is an act by which a thing becomes different from its previous one.

→ We see many changes around us and every change is important in a positive or negative way.

→ Changes can be grouped together by finding similarities and differences between them.

→ All the changes can be broadly divided into two types i.e. natural and manmade.

→ Changes that occur in nature and do not require our involvement are called natural changes. These are never ending changes.

→ Examples of natural changes include melting of snow, falling of leaves from trees, etc.

→ Changes occurring due to the efforts of human beings are called artificial or manmade changes.

→ Examples of manmade changes include preparing chapattis from wheat flour, cooking vegetables, etc.

→ Based on the speed, we can classify changes into two categories. These are slow changes and fast changes.

→ Slow changes are those which take more time to happen. For example, growing of tree, a baby becoming an adult, etc.

→ Fast changes are those that happen very fast. For example, burning of match sticks, bursting of crackers, etc.

→ Out of all the changes around us, only a few changes can be reversed. These are called reversible changes.

→ The changes which cannot be reversed are called irreversible changes.

→ A change in a substance is called reversible if we can get the substance in its original form by changing the conditions.

→ For example, ice changes to water on melting and water can be changed into ice by cooling, is a reversible change.

→ A change in a substance is called irreversible if we cannot get the substance in its original form by changing the conditions.

→ For example, roti once prepared on Tawa, cannot be changed back to dough again.

→ Some changes are periodic whereas others are non-periodic.

→ Changes that are repeated after regular intervals of time are called periodic changes.

→ For example, change of day and night, swinging of pendulum of a clock, the beating of heart, change of seasons.

→ Changes that are not repeated after a regular interval of time are called non-periodic changes.

→ For example, the occurrence of earthquakes, the occurrence of rain, etc.

→ We have classified changes into physical and chemical changes.

→ Any temporary change in which no new substance is formed and the chemical composition of the original substance remains the same is known as physical change.

→ During physical changes, physical properties like colour, shape, size, state, etc. may change. Therefore, we can say that Physical change is a reversible change.

→ Any permanent change in which new substances are formed. These have physical and chemical properties completely different from those of the original substance.

→ Physical changes are mostly reversible in nature whereas chemical changes are mostly irreversible changes.

→ Expansion and contraction are physical changes that are very useful in our daily life.

→ In the expansion the dimensions of the substance increase and in contraction the dimensions of the substance decrease.

→ Change: An act by which a thing becomes different from its previous one.

→ Natural changes: Changes that are occurring naturally and are never-ending are called natural changes.

→ Manmade changes: Changes that are occurring due to the efforts of human beings are called manmade changes.

→ Periodic change: Changes that are repeated after regular intervals of time are called periodic changes.

→ For example, change of day and night, swinging of pendulum of a clock, beating of heart, change of seasons.

→ Non-periodic changes: Changes that are not repeated after regular intervals of time are called non-periodic changes.

→ For example, the occurrence of earthquakes, the occurrence of rain, etc.

→ Reversible change: The changes which take place in a substance and can be reversed to their original state are called Reversible changes.

→ Irreversible change: The changes which cannot be reversed to the original substances are called Irreversible changes.

→ Physical Change: Physical change is a temporary change in which no new substance is formed and the chemical composition of the original substance remains the same.

→ Chemical change: Chemical change is a permanent change in which new substances are formed whose physical and chemical properties are completely different from those of the original substance.

→ Expansion: When a substance increases its size on heating then the change is called expansion.

→ Thermal expansion: When the expansion is due to an increase in temperature it is called thermal expansion.

→ Contraction: When a substance decreases its size on cooling then the change, is called contraction.

→ Evaporation: When a liquid changes into the gaseous form on heating or on decreasing pressure then the process is known as evaporation.

→ Melting: When a solid changes into liquid form on heating or on increasing pressure then the process is known as melting.