Living things are found in water, deserts, snowy regions, forests and grasslands. Over a long time, their body parts and behaviour slowly change so that they can live in their surroundings. These changes are called adaptations. This chapter also explains why and how we classify living things and give them scientific names.
Key points
Adaptation (अनुकूलन) is gradual and continuous. It does not happen suddenly.
Aquatic plants (lotus): waxy layer on leaves, air spaces in stems and petioles for floating, short fibrous roots, ribbon-like leaves to bear fast currents.
Desert plants (cactus, acacia): leaves are absent, needle-like or turned into thorns, so very little water is lost. The stem is green, fleshy and stores water. Roots go deep or spread wide.
Snowy region plants (pine, deodar): conical shape and sloping branches so snow slides off; thick bark against cold.
Forest plants grow tall for sunlight; climbers use tendrils. Grass roots stop soil erosion.
Special feeding in plants: Cuscuta (dodder) is a parasite with haustorial (sucking) roots. Drosera, Venus flytrap and pitcher plant eat insects to get nitrogen. Fungi absorb food from bread or bhakri.
Fish: spindle-shaped body, scales, fins, gills, air bladder for floating.
Frog and duck: webbed toes work like oars; frog breathes through skin in water.
Carnivores have sharp canines, claws, eyes in front; herbivores have eyes on the sides (wide vision), long legs with hooves, movable ears.
Desert animals: thick skin, long eyelashes, nostrils with skin folds, flat cushioned soles; many live in burrows and come out at night.
Birds: hollow bones, feathers, wings, spindle-shaped body. Bats fly with a skin fold called patagium.
Camouflage: colour blends with surroundings (butterflies, grasshoppers, lizards).
Important terms
Adaptation – slow changes in body and life processes that help an organism live, feed, reproduce and protect itself in its habitat.
Survival of the fittest – Darwin's first principle: organisms that adapt best to a changing environment survive.
Natural selection – Darwin's second principle: a useful new character that helps survival is passed to the next generation.
Hierarchy of classification – Kingdom → Phylum → Class → Order → Family → Genus → Species.
Binomial nomenclature – two-part scientific name (Genus + species) given by Carl Linnaeus's method.
Species – organisms so similar that they can reproduce among themselves.
Formulas
No formulas. Remember these scientific names:
Organism
Scientific name
Human
Homo sapiens
Mango
Mangifera indica
Cow
Bos taurus
Dog
Canis lupus familiaris
Hibiscus
Hibiscus rosa-sinensis
Jowar
Sorghum bicolor
29th April is observed as World Frog Protection Day.
5 important questions
Q1. How are desert plants adapted to live with very little water?
Their leaves are absent, very small, needle-like or changed into thorns, so less water evaporates. The green fleshy stem stores water and does photosynthesis. A waxy layer covers the stem. Roots go deep or spread far in search of water.
Q2. Why is the camel called the 'Ship of the desert'?
The camel can walk easily on hot sand with its long legs and flat cushioned soles. It can live for many days without water. Its long eyelashes and closable nostrils protect it from sand. So it carries people and goods across deserts like a ship on water.
Q3. Why do insect-eating plants catch insects?
They grow in soil that has very little nitrogen. To get nitrogen they trap and digest insects. Examples: Venus flytrap, drosera (sundew), pitcher plant.
Q4. Explain binomial nomenclature with an example.
Each organism gets a scientific name of two parts – first the genus, then the species. Example: human is Homo sapiens (genus Homo, species sapiens). This avoids confusion caused by local names.
Q5. How are birds adapted for flying?
Spindle-shaped body reduces air resistance. Hollow bones and feathers make the body light. Forelimbs are changed into wings.
Exam tip
Learn adaptations habitat-wise (water, desert, snow, forest, air) with one example each, and remember the classification hierarchy in correct order.
A plant has a root, stem, leaves, flowers, fruits and seeds. Each part has its own structure and job. This chapter explains the types of roots, parts of a stem and leaf, venation, the parts of a flower, pollination, and monocot and dicot seeds.
Key points
From a sprouting seed, the radicle grows down into the root and the plumule grows up into the shoot.
Tap root: one main root with side (secondary) roots – found in dicot plants (mango, pea).
Fibrous roots: many thread-like roots from the stem base – found in monocot plants (onion, maize, wheat).
Root hairs near the tip absorb water. The root cap protects the delicate root tip.
Maize, sugarcane and jowar also have adventitious roots growing from the stem above the soil. Banyan has prop roots.
After pollination and fertilization, ovules become seeds and the ovary becomes the fruit.
Important terms
Radicle – part of the embryo that grows into the root.
Plumule – part that grows into the shoot.
Internode – stem portion between two nodes.
Stipules – small leaf-like parts near the leaf base.
Pollination – pollen grains falling on the stigma.
Dicotyledonous seed – seed that splits into two equal halves (pea, groundnut).
Monocotyledonous seed – seed that does not split into two (wheat, rice, jowar).
Formulas
No formulas. Remember this comparison:
Feature
Dicot
Monocot
Root
Tap root
Fibrous roots
Venation
Reticulate
Parallel
Seed
Two cotyledons
One cotyledon
Examples
Mango, pea, peepal
Maize, onion, jowar
5 important questions
Q1. Distinguish between tap root and fibrous roots.
A tap root has one thick main root that tapers and gives side roots; it is found in dicots like mango. Fibrous roots are many thin roots of equal size arising from the stem base; they are found in monocots like onion and maize.
Q2. What is the function of the root cap and root hairs?
The root cap covers and protects the soft growing root tip from injury. Root hairs increase the surface area and absorb water and minerals from the soil.
Q3. Name the parts of a flower and give the function of androecium and gynoecium.
Calyx, corolla, androecium and gynoecium. The androecium (stamens) is the male part that makes pollen. The gynoecium (carpels) is the female part; its ovary contains ovules that become seeds.
Q4. What are the similarities and differences between jowar and moong?
Both are flowering food crops with seeds. Jowar is a monocot – fibrous roots, parallel venation, seed does not split. Moong is a dicot – tap root, reticulate venation, seed splits into two halves.
Q5. Why are prop roots useful to a banyan tree?
Prop roots grow down from the branches into the soil. They support the heavy, spreading branches and also absorb water. A 250-year-old banyan in Kolkata's Botanical Garden is supported by thousands of prop roots.
Exam tip
Practise labelled diagrams of a leaf and the vertical section of a hibiscus flower – they are often asked.
Air, water and soil are our main natural resources. This chapter explains their properties – air has weight and pressure, water expands on freezing and shows anomalous behaviour, and soil differs in colour, texture and structure – and how we use these properties.
Key points
Air is a mixture of gases, so it occupies space and has mass and weight.
Moving gas molecules strike objects and create atmospheric pressure (वातावरणीय दाब). It acts equally in all directions.
At sea level, atmospheric pressure is about 1,01,400 N/m². It is measured with a barometer and decreases with height.
Bernoulli's principle (1726): when the speed of air increases, its pressure decreases. The straw fountain works on this.
Winds blow from high pressure to low pressure. Air carries sound and keeps the earth warm.
Dew forms when cool air at night cannot hold all its water vapour.
Fine particles in air spread light in all directions – scattering of light.
Water has no shape but a definite volume. It expands on freezing, so ice is lighter (less dense) and floats.
Anomalous behaviour: water has maximum density at 4°C; below 4°C it expands. So lakes freeze at the top and fish survive below.
Salt water is denser than pure water, so a potato floats in it and swimming in the sea is easier.
Water is a universal solvent, a good coolant, and used for transport, electricity and cleaning.
Soil texture: sandy (large particles, drains fast, less fertile), silt (medium, rich in organic matter), clay (fine, holds most water, hard to plough).
When water freezes, its volume increases but mass stays the same. So the density of ice is less than that of water and it floats.
Q2. Explain the anomalous behaviour of water.
Most liquids become denser on cooling. Water becomes denser only up to 4°C. Below 4°C it expands and its density decreases. So water is densest at 4°C.
Q3. Why should a glass bottle filled completely with water never be kept in a freezer?
Water expands when it turns into ice. The increased volume pushes on the glass and the bottle can crack.
Q4. What will happen if only one crop is grown again and again in a field?
The same nutrients get used up every time, so soil fertility decreases. Crops should be rotated; leguminous crops like moong, peanut and soyabean restore fertility.
Q5. Why is the density of sea water more than that of rain water?
Sea water has many salts dissolved in it. The dissolved salts increase its mass per unit volume, so its density is higher.
Exam tip
Remember the numbers: 4°C (maximum density of water), 1,01,400 N/m² (air pressure at sea level), and pH ranges of soil.
Nutrition is the process of taking in food and using it for energy, growth and repair. Plants mostly make their own food (autotrophic), while animals depend on others (heterotrophic). The chapter covers photosynthesis, nitrogen fixation, special modes of nutrition in plants, and types of nutrition in animals.
Key points
Nutrients: macro-nutrients (carbohydrates, proteins, fats – needed in large amounts) and micro-nutrients (minerals, vitamins – small amounts).
Photosynthesis (प्रकाशसंश्लेषण): leaves use sunlight and chlorophyll to make glucose from carbon dioxide and water; oxygen is released. Light energy becomes chemical energy.
CO₂ enters through tiny pores called stomata; chlorophyll is in chloroplasts.
Xylem carries water and minerals up from roots; phloem carries food from leaves to other parts.
Plants cannot use gaseous nitrogen directly. Rhizobium in root nodules of legumes and azotobacter in soil fix nitrogen as nitrates. Lightning also fixes nitrogen (NO → NO₂ → HNO₃).
Symbiotic nutrition: two organisms help each other, e.g. lichen (alga gives food, fungus gives water, minerals and shelter).
Parasitic plants: Cuscuta – complete parasite (no chlorophyll); Loranthus – partial parasite (green leaves, takes water and minerals from host).
Insectivorous plants (Drosera burmanii, pitcher plant) grow in nitrogen-poor soil and trap insects.
Saprophytes (mushroom, yeast) feed on dead and decaying matter by releasing digestive enzymes.
By food: herbivores, carnivores, omnivores, scavengers (vulture, hyena), decomposers (microbes).
Important terms
Autotrophic nutrition – making one's own food.
Heterotrophic nutrition – depending on others for food.
Assimilation – use of absorbed food by cells for energy, growth and repair.
Pseudopodia – false feet of amoeba used to surround food.
Saprophyte – plant that feeds on dead organic matter.
Formulas
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in presence of sunlight and chlorophyll)
Nutrient
Deficiency effect in plants
Nitrogen
Stunted growth, yellow leaves
Phosphorus
Early leaf-fall, late flowering
Potassium
Weak stem, wilting
Magnesium / Iron
Yellowing of leaves
5 important questions
Q1. What is photosynthesis? Write its equation.
The process in which green plants make glucose from carbon dioxide and water using sunlight and chlorophyll, giving out oxygen. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Q2. How do plants get nitrogen?
Rhizobium in root nodules of leguminous plants and azotobacter in soil convert atmospheric nitrogen into nitrates. During lightning, nitrogen and oxygen form nitric acid, which reaches the soil with rain and forms nitrogen salts.
Q3. Why is loranthus called a partial parasite?
It has green leaves and makes its own food, but it takes water and minerals from the host tree.
Q4. We cook many dishes at home. Are we autotrophs?
No. We only cook food that comes from plants and animals. We cannot make food from CO₂ and water, so we are heterotrophs.
Q5. Distinguish between ectoparasites and endoparasites.
Ectoparasites live on the body surface and suck blood (louse, tick). Endoparasites live inside the host's body (tapeworm, roundworm).
Exam tip
Match-the-pair questions are common: Cuscuta–parasite, Drosera–insectivorous, Mushroom–saprophyte, Lichen–symbiosis.
Good food must be safe and clean. This chapter explains how food gets spoiled or wasted, how we can protect and preserve it, and what food adulteration is and how to detect it at home.
Key points
Food is spoiled when its colour, smell, texture, taste or quality changes and its nutrients are lost.
Causes of spoilage: factors inside the food (fruits turning black, peanuts becoming rancid), wrong handling, damp storage, poor transport, reaction with metals, and micro-organisms or insects from air, water or soil.
Quantitative wastage: hand sowing, poor threshing, bad storage, wrong distribution, serving too much at feasts.
Qualitative wastage: over-cooking, washing vegetables after cutting, too many preservatives, rough handling of fruits, delay in transport.
Food protection = preventing spoilage by microbes and pests. Food preservation = using preservatives to stop spoilage from internal factors for a long time.
Methods: freezing, drying (dehydration), air-tight packing, filling nitrogen gas in wafer packets, smoking, natural preservatives (salt, sugar, oil), chemical preservatives (vinegar/acetic acid, citric acid, sodium benzoate), insecticides on grain bags, irradiation, pasteurization.
Pasteurization (Louis Pasteur): heat milk to about 80°C for 15 minutes, then cool it quickly.
Irradiation plants in Maharashtra: Lasalgaon (onions, potatoes) and Navi Mumbai (spices).
Adulteration (भेसळ): removing a useful part, mixing cheaper or harmful material, or adding stones, sand, iron filings, urea etc.
FSSAI was set up under the Food Safety and Standardization Act, 2006. The Prevention of Food Adulteration Act was passed in 1954; the 1976 amendment allows life imprisonment for harmful adulteration.
World Food Day: 16th October.
Important terms
Food spoilage – loss of quality and nutrients in food.
Preservative – substance that keeps food from spoiling.
Irradiation – exposing food to X-rays, gamma rays or electron beams to kill microbes and increase shelf-life.
Adulterant – unwanted substance mixed in food.
Expiry date – last date up to which packed food is safe.
Formulas
Simple home tests for adulteration:
Food
Adulterant
Test and result
Milk
Water
Drop on a tilted glass slide does not leave a milky white mark
Chilli powder
Brick dust
Red layer settles at the bottom of water
Turmeric
Metanil yellow
Turns red with conc. HCl and stays red
Rava
Iron filings
Filings stick to a magnet
5 important questions
Q1. How is milk pasteurized?
Milk is heated to a fixed temperature (about 80°C for 15 minutes) and then cooled suddenly. This kills most microbes and the milk keeps longer.
Q2. Why should we not eat adulterated food?
Adulterants can cause stomach upset or poisoning, harm organs if eaten for a long time, and some can even cause cancer.
Q3. Why is nitrogen gas filled in packets of wafers?
Nitrogen is an inert gas. It stops the growth of fungus and insects and keeps the wafers fresh and crisp.
Q4. Give five ways to prevent food wastage.
Serve only what you can eat; avoid over-cooking; re-use leftovers properly; buy only what is needed; store food correctly and use packed food before the expiry date.
Q5. Differentiate between food protection and food preservation.
Food protection prevents spoilage caused by microbes and pests. Food preservation uses preservatives to prevent spoilage due to internal factors for a long period.
Exam tip
Learn the adulteration test table and the names of preservation methods with one example each.
Quantities like mass, weight, length, time, speed and temperature are physical quantities. Each is written as a value with a unit. This chapter explains scalar and vector quantities, mass and weight, standard units (MKS, CGS, SI), and why accurate measurement matters.
Key points
A physical quantity = value + unit. In "2 kilometres", 2 is the value and kilometre is the unit.
Scalar quantity: fully described by magnitude only – length, area, mass, temperature, density, time, work.
Vector quantity: needs magnitude and direction – displacement, velocity, weight.
Mass (वस्तुमान) is the amount of matter in a body. It is a measure of inertia. It is the same everywhere. Unit: kilogram, gram.
Weight (वजन) is the gravitational force with which the earth pulls a body towards its centre. It changes from place to place.
Weight is maximum at the poles, minimum at the equator, less at high altitudes, and about one-sixth on the moon. Mass does not change.
A two-pan balance compares masses, not weights.
Hand-span or cubit cannot be standard units because they differ from person to person.
Fundamental quantities: length, mass, time; their units are standard units.
Standard kilogram: platinum-iridium cylinder kept at the International Bureau of Weights and Measures, Paris. The standard metre was the distance between two lines on a platinum-iridium bar there.
Standards of metre, kilogram, second, kelvin, ampere and candela are kept at the National Physical Laboratory, New Delhi.
The Weights and Measures (Legal Metrology) department checks balances and weights so customers are not cheated.
Important terms
Inertia – tendency of matter to resist change in its state.
Standard unit – a fixed, accepted unit available to all and not variable.
Standardization – checking a measuring device against a standard measure.
TMC – thousand million cubic feet (used for water in dams).
Mass is the amount of matter; it is scalar, constant everywhere, measured in kg. Weight is the earth's gravitational pull on that mass; it is a vector, changes from place to place, and is less on the moon.
Q2. Why is weight more at the poles than at the equator?
The poles are closer to the centre of the earth than the equator. So the gravitational pull, and hence weight, is greater at the poles.
Q3. Why can a hand-span not be used as a standard unit?
The length of a hand-span differs from person to person, so the same length would be measured differently. A standard unit must be the same for everyone.
Q4. Give examples of scalar and vector quantities.
Scalar: mass, length, time, temperature, density. Vector: displacement (20 km towards north), velocity (500 km/h towards Mumbai), weight.
Q5. What are the main causes of errors in measurement?
Using an unsuitable device, using the device wrongly, a tampered or unstandardized balance, and reading the scale from a wrong angle.
Exam tip
"Mass vs weight" and "scalar vs vector" differences are asked almost every year – learn them as tables.
This chapter explains how motion is described using distance, displacement, speed, velocity and acceleration. It introduces force, Newton's first law of motion, and how work is done when a force moves a body.
Key points
Distance: actual length of the path travelled, without direction. It is a scalar.
Displacement (विस्थापन): shortest straight-line distance from start to end point in a given direction. It is a vector.
Speed = distance ÷ time (scalar). Velocity = displacement ÷ time (vector). Unit of both: m/s.
Speed and velocity are equal in magnitude only when the body moves in a straight line in one direction.
Average velocity is for the whole journey; instantaneous velocity is at a particular moment.
Acceleration = change in velocity ÷ time taken. Unit: m/s². It is a vector.
The interaction that causes acceleration is force. Force can change speed, direction or shape.
Friction slows moving bodies; less friction (talcum powder on carrom board) lets them move longer.
Newton's First Law: if no force acts, a body at rest stays at rest and a moving body keeps moving with the same velocity in the same direction.
1 newton (N) is the force that gives a 1 kg mass an acceleration of 1 m/s².
Work is done only when a force causes displacement. Work = force × displacement in the direction of force.
SI unit of work is the joule (J); CGS unit is the erg. Energy is the ability to do work.
Important terms
Scalar – quantity with magnitude only (distance, speed).
Vector – quantity with magnitude and direction (displacement, velocity, acceleration, force).
Friction – force that opposes motion between surfaces.
1 joule – work done when a force of 1 N moves a body by 1 m in its direction.
Formulas
Speed = Distance ÷ Time
Velocity = Displacement ÷ Time
Acceleration a = (final velocity − initial velocity) ÷ time
Work W = F × s (W in J, F in N, s in m)
5 important questions
Q1. Ranjit walks 500 m, 700 m and 300 m (total 1500 m) to school in 25 minutes. His displacement is 1000 m. Find his speed and velocity.
Time = 25 × 60 = 1500 s. Speed = 1500 ÷ 1500 = 1 m/s. Velocity = 1000 ÷ 1500 ≈ 0.66 m/s. They differ because he did not walk in a straight line.
Q2. A bird flies from a wire, circles around and returns to the same perch. What is its distance and displacement?
The distance is the full length of its flight path. Its displacement is zero because its starting and final positions are the same.
Q3. A ball's speed increases from 2 cm/s to 4 cm/s in 2 seconds. Find its acceleration.
a = (4 − 2) ÷ 2 = 1 cm/s².
Q4. State Newton's first law of motion with an example.
A body remains at rest or keeps moving with constant velocity in a straight line unless a force acts on it. Example: a carrom coin keeps sliding until friction stops it.
Q5. A force of 20 N moves a box 5 m in its direction. Find the work done.
W = F × s = 20 × 5 = 100 J.
Exam tip
Always convert minutes to seconds and km to m before using formulas, and write the unit in the final answer.
When some objects are rubbed together, they become electrically charged. Like charges repel and unlike charges attract. This chapter explains how charges are produced, charging by contact and induction, the gold leaf electroscope, lightning and the lightning conductor.
Key points
All matter contains positive and negative charges. When they are equal, the object is neutral; when unbalanced, it is charged.
Benjamin Franklin named the charges positive (+) and negative (−).
Each atom has a fixed positive charge and moving negative charges. On rubbing, negative particles move from one object to the other.
The object that gains negative particles becomes negatively charged; the one that loses them becomes positively charged.
Glass rubbed with silk and plastic rubbed with wool get opposite charges.
Like charges repel, unlike charges attract. A charged object also attracts uncharged objects. Repulsion is the sure test of a charge.
Charge produced by friction stays at the place of rubbing for a short time, so it is called static electricity (स्थितिक विद्युत). Moist air removes it – so do experiments in dry weather.
Charging by contact: touching a charged comb to another comb. Charging by induction: a charged comb bends a thin stream of water without touching it; the induced charge lasts only while the comb is near.
Gold leaf electroscope detects charge: leaves open when a charged object is near the disc; touching the disc with a hand discharges them.
Thales (about 2500 years ago) saw rubbed amber attract feathers; Thomas Browne named it electricity (1646).
In 1752 Franklin's kite experiment showed lightning is electric charge.
Lightning happens when a large negative charge at the base of a cloud flows to the ground very fast, producing heat, light and sound.
Lightning helps form nitrogen oxides (soil fertility) and ozone.
Lightning conductor: forked copper strip on top of a building connected to a cast-iron plate in a pit of coal and salt; it safely carries charge into the earth.
Important terms
Frictional electricity – charge produced by rubbing.
Induction – charging a body by bringing a charged body near it without contact.
Electroscope – device to detect electric charge.
Earthing – connecting to the ground so charge flows into the earth.
Formulas
No numerical formulas. Rules to remember:
Like charges → repulsion
Unlike charges → attraction
Charged + uncharged object → attraction
5 important questions
Q1. Why are static electricity experiments done in dry weather?
Moisture in the air absorbs and removes static charges quickly, so the effects are not seen properly in humid weather.
Q2. Explain how a charged comb attracts a stream of water.
The negatively charged comb pushes the negative particles of the water away from the near side. That side becomes positive by induction, so it is attracted to the comb.
Q3. Describe a gold leaf electroscope.
It has a copper rod with a metal disc on top and two thin gold leaves at the bottom, placed in a bottle. A charged object near the disc gives both leaves the same charge, so they repel and open. Touching the disc discharges them.
Q4. How does a lightning conductor protect a building?
Its forked copper tip at the top collects the charge from the cloud, and the copper strip carries it to an iron plate buried in coal and salt, so the charge spreads safely into the earth.
Q5. Why is repulsion a sure test of charge, not attraction?
A charged object can attract both oppositely charged and uncharged objects. Repulsion happens only between two charged objects with like charges.
Exam tip
Draw and label the gold leaf electroscope and the lightning conductor – both are common diagram questions.
Heat always flows from a hotter body to a colder one. This chapter explains the three modes of heat transfer – conduction, convection and radiation – good and bad conductors, expansion and contraction of solids, liquids and gases, and how a thermos flask works.
Key points
Conduction (वहन): heat passes from the hot part of a solid to the cold part, particle to particle. It needs a medium.
In the wax-and-pin experiment, pins fall first from the copper bar – copper conducts heat fastest; steel is slower.
Convection (अभिसरण): in liquids and gases, warm fluid becomes less dense and rises, cold fluid sinks – forming convection currents. It needs a medium.
Heating the top of a test tube boils the water there, but ice at the bottom does not melt – warm water does not sink, so convection does not happen downward.
Radiation (प्रारण): heat transfer without any medium. The sun's heat reaches the earth by radiation.
Dark/black surfaces absorb more heat; white and shiny surfaces reflect it. So we wear white in summer and dark clothes in winter.
Good conductors: copper, aluminium, steel. Bad conductors: wood, plastic, glass, cloth, air.
Solids, liquids and gases expand on heating and contract on cooling (ring-and-ball, flask-and-tube, balloon-on-bottle experiments).
Gaps are left at joints of rails and concrete bridges to allow expansion in summer.
Mercury or alcohol is used in thermometers because they expand evenly.
Thermos (Dewar) flask – made by Sir James Dewar in 1892: double-walled glass with vacuum between (stops conduction and convection) and silvered shiny walls (stop radiation).
Infrared cameras use heat radiation to see at night.
Important terms
Transfer of heat – flow of heat from one place to another.
Convection current – circulating stream of a liquid or gas carrying heat.
Good conductor – substance that allows heat to pass easily.
Expansion – increase in size or volume on heating.
Formulas
No formulas. Summary table:
Mode
Medium needed?
Happens in
Example
Conduction
Yes
Solids
Spoon in hot tea gets hot
Convection
Yes
Liquids, gases
Sea and land breezes
Radiation
No
Vacuum/space
Sun's heat reaching earth
5 important questions
Q1. Why does a halwai wrap cloth around the handle of his spoon?
The metal spoon conducts heat from the hot milk to the handle. Cloth is a bad conductor, so it protects his hand.
Q2. Why are heaters fitted near the floor and air conditioners near the ceiling?
Warm air from a heater rises and cold air sinks, so heat spreads through the room by convection. Cool air from an AC near the ceiling sinks and pushes warm air up to be cooled.
Q3. Explain how a thermos flask keeps liquids hot.
The vacuum between the double walls stops conduction and convection. The silvered walls reflect heat back, stopping radiation. So heat stays inside for a long time.
Q4. Why are houses in Rajasthan painted white?
White surfaces reflect most of the sun's heat and absorb little, so the houses stay cooler.
Q5. Why are gaps left between rails?
In summer, rails expand due to heat. The gaps allow this expansion; otherwise the rails would bend.
Exam tip
For "give reasons" questions, name the mode of heat transfer (conduction, convection or radiation) in your answer.
Disasters can be natural (famine, cloudburst, flood, lightning, volcano, tsunami, storm) or man-made. We cannot stop most natural disasters, but with planning, warnings and precautions we can reduce loss of life and property. This chapter explains these disasters and what to do.
Key points
Famine (दुष्काळ): long, severe shortage of food grains and water. Drought is its main cause; floods, pests, locusts, cold waves and earthquakes also cause it.
Man-made causes of famine: war, unrest, no transport routes, uncontrolled population growth, misuse and over-lifting of water, deforestation, too many chemicals.
Asia is the most famine-affected continent. Chhatrapati Shivaji Maharaj and Shahu Maharaj ran famine relief and water schemes; Malik Ambar built a drinking water canal in Aurangabad.
To reduce famine: planned use and reuse of water, rainwater harvesting, tree plantation, using weather forecasts.
Cloudburst: very heavy rain (100 mm per hour or more) over a small area. Examples: Mumbai, 26 July 2005 (about 950 mm in 8–10 hours); Leh, 6 August 2010.
Flood control: small dams in hills, percolation tanks, levelling river beds, new forests, linking rivers. National Flood Commission set up in 1976.
Lightning: only about 5% of flashes reach the ground; about 40 flashes happen every second; it is hotter than the sun's surface.
Volcano: hot molten material from inside the earth erupts through the crust; forms hills, pollutes air, buries forests. It cannot be stopped, only predicted.
Tsunami ("harbour wave" in Japanese): waves caused by an earthquake or volcano on the sea floor; travel at 800–900 km/h and can rise to about 30 m at the coast. Geostationary satellites help give warnings.
Storms: cut weak branches, take shelter, turn off gas and electricity, inform relatives.
UNDP (set up in 1965) sends aid, equipment and volunteers. International Day for Disaster Reduction: 13th October.
Important terms
Disaster – sudden event causing heavy loss of life and property.
Cloudburst – sudden heavy rainfall over a small area.
Tsunami – huge sea waves caused by an undersea earthquake or volcano.
Disaster management – planning, warning, rescue and relief to reduce damage.
Formulas
No formulas. Lightning safety – do's and don'ts:
Do not stand on open ground, under a tree, on a high place, or near poles and towers.
Do not touch wire fences; get off bicycles, tractors and boats.
Keep 15 feet between persons; do not use plugged-in appliances or phones.
Crouch low on dry wood, plastic or jute, feet together, hands on knees.
A pucca house is the safest place.
5 important questions
Q1. What are the causes of famine?
Natural: drought, floods, storms, cold waves, pests, locusts, crop diseases, earthquakes. Man-made: war, unrest, lack of transport, population growth, misuse of water, deforestation.
Q2. What is a cloudburst?
When very high temperature near the land turns falling rain back into vapour, clouds become overloaded. Then rapid condensation causes sudden rain of 100 mm per hour or more over a small area.
Q3. How is a tsunami formed? What are its effects?
An earthquake or volcano on the ocean floor pushes water up, forming fast waves that become very high near the coast. They destroy buildings, boats, harbours and trees and cause great loss of life.
Q4. What measures can control floods?
Small dams in mountain areas, percolation tanks, flattening river beds, planting forests and linking rivers.
Q5. Find the odd one out: famine, earthquake, cloudburst, railway accident.
Railway accident – it is a man-made disaster; the others are natural.
Exam tip
Learn the lightning safety rules point-wise; "what precautions will you take" is a favourite question.
The cell is the basic structural and functional unit of life. This chapter explains the discovery of cells, the compound microscope, the parts of plant and animal cells, and micro-organisms – where they live, how they grow, and how they are useful (and sometimes harmful) to us.
Key points
Levels of organization: cell → tissue → organ → organ system → organism.
Robert Hooke (1665) saw box-like compartments in cork and named them "cells" (Latin cella = small room).
Schleiden and Schwann (1838): all living things are made of cells. Rudolf Virchow (1885): all cells come from pre-existing cells.
Leeuwenhoek (1673) made a microscope and first saw live bacteria and protozoa.
Cells are measured in micrometres and nanometres and observed with a compound microscope; organelles are studied with an electron microscope.
Cells have many shapes (round, rod, spiral, columnar) related to their work – e.g. nerve cell, muscle cell, RBC.
Plant cell: cell wall, chloroplasts, one large vacuole. Animal cell: no cell wall, small vacuoles; plasma membrane is the outer covering.
Organelles: nucleus (controls the cell), endoplasmic reticulum (modifies proteins), Golgi bodies (distribute proteins), mitochondria (powerhouse – produce energy), chloroplasts (photosynthesis), vacuoles (throw out wastes), lysosomes, ribosomes.
Cells with membrane-bound organelles are eukaryotic. Bacteria have only plasma membrane, cytoplasm and a nucleoid – they are prokaryotic.
Micro-organisms (सूक्ष्मजीव) are smaller than 100 micrometres; found in air, water, soil, food and bodies. Groups: algae, fungi, protozoa, bacteria, viruses.
They grow best at about 25–37°C in moist, warm conditions; in bad conditions they form a thick protective covering.
Useful microbes: decompose garbage into manure, clean sewage, fix nitrogen in root nodules, make curd, cheese, bread and idli (fermentation), antibiotics, vaccines, and help in tanning and fibre making.
Important terms
Organelle – small part inside a cell that does a special job.
Prokaryotic cell – cell without a true nucleus and membrane-bound organelles.
Fermentation – change of one carbon compound into another by microbes, releasing heat and CO₂.
Antibiotic – medicine made with microbes that kills or stops disease germs (e.g. penicillin).
Vaccine – preparation made using microbes that gives immunity against a disease.
Formulas
1 cm = 10 mm; 1 mm = 1000 micrometres; 1 micrometre = 1000 nanometres
Q1. Distinguish between plant cell and animal cell.
Plant cells have a cell wall, chloroplasts and one large vacuole, and a definite shape. Animal cells have no cell wall or chloroplasts, have small vacuoles, and the plasma membrane is the outermost layer.
Q2. Why are mitochondria called the powerhouse of the cell?
Mitochondria produce the energy needed for all the activities of the cell.
Q3. Why does dough or idli batter rise?
Microbes in the batter cause fermentation. Carbon dioxide gas is released, which makes the batter swell and rise.
Q4. How do microbes help keep the surroundings clean?
Microbes in soil and dung decompose organic waste like peels and garden waste into good manure. They are also used to decompose sewage.
Q5. Why should a course of antibiotics be completed as the doctor says?
Antibiotics used wrongly or stopped midway can be harmful and may not fully kill the germs. So we must follow the doctor's prescription and not take medicines on our own.
Exam tip
Draw the plant and animal cell diagrams and label at least six organelles; remember the scientists with their years.
The Muscular System and Digestive System in Human Beings
Quick summary
Muscles help us move, give our body shape and keep vital organs working. The digestive system breaks down food into a soluble form that can be absorbed into the blood. This chapter covers types of muscles, how they work, and the organs and glands of the digestive system.
Key points
Muscles are bundles of fibres that contract and relax. They are joined to bones by tendons.
The body has more than 600 muscles, about 40% of an adult's body weight; about 30 muscles are in the face.
Voluntary muscles work by our will (arms, legs). Involuntary muscles work on their own (heart, stomach, intestine).
Types: skeletal muscles (attached to bones, voluntary), cardiac muscles (heart, involuntary, beat about 70 times per minute), smooth muscles (stomach, intestine, blood vessels – involuntary and slow).
Muscles work in groups: when the biceps (front of upper arm) contracts, the triceps (back) relaxes.
Largest muscle is in the thigh; smallest is attached to the stapes bone in the ear. Study of muscles = myology.
Proteins, carbohydrates, exercise and good posture keep muscles healthy.
Digestion (पचन): converting food into a soluble form and absorbing it into blood.
Teeth: incisors, canines, premolars, molars; covered with hard enamel (a calcium salt).
Saliva has the enzyme ptyalin (salivary amylase): starch → maltose.
Stomach: gastric juice = hydrochloric acid + pepsin + mucus; proteins are mainly digested here.
Liver – largest gland; stores glucose; makes bile (stored in gall bladder) which helps digest fats.
Pancreas secretes pancreatic juice with enzymes.
Small intestine (about 6 m): most digestion and absorption happen here.
Large intestine (about 1.5 m): only water is absorbed; the appendix is attached to its first part.
Important terms
Tendon – tough tissue joining muscle to bone.
Enzyme – protein that speeds up a specific chemical reaction in the body; works best at body temperature.
Enamel – hardest outer covering of a tooth.
Bile – digestive juice from the liver that helps digest fats.
Formulas
No formulas.
Organ / Gland
Juice or enzyme
Work
Salivary glands
Ptyalin
Starch → maltose
Stomach
HCl, pepsin, mucus
Protein digestion
Liver
Bile
Fat digestion
Pancreas
Pancreatic juice
Digests carbohydrates, proteins, fats
Small intestine
Intestinal juice
Completes digestion, absorption
5 important questions
Q1. Distinguish between voluntary and involuntary muscles.
Voluntary muscles move according to our will, like arm and leg muscles. Involuntary muscles work without our control, like the muscles of the heart, stomach and intestine.
Q2. Name the three types of muscles with examples.
Q3. How do the biceps and triceps help bend the arm?
When we bend the arm, the biceps contracts and the triceps relaxes. To straighten it, the triceps contracts and the biceps relaxes. Muscles always work in pairs or groups.
Q4. What is the role of the liver in digestion?
The liver is the largest gland. It produces bile, stored in the gall bladder. Bile goes to the small intestine and helps in the digestion of fats.
Q5. What would happen if the stomach muscles did not move when food entered?
The food would not be churned and mixed with gastric juice, so it would not be digested properly.
Exam tip
Practise the labelled diagram of the human digestive system and remember the lengths: alimentary canal 9 m, small intestine 6 m, large intestine 1.5 m.
Changes happen around us all the time. They can be classified in many ways – natural or man-made, useful or harmful, fast or slow, reversible or irreversible, periodic or non-periodic, and physical or chemical. The chapter also explains corrosion and how to prevent it.
Key points
Natural changes happen on their own (ripening of fruit, spoiling of milk). Man-made changes are done by people (cooking, making bread).
Useful changes: sharpening a pencil, cooking food. Harmful changes: rusting, a tree falling in a storm.
Fast changes: bursting of a balloon. Slow changes: ripening of fruit, growth of a tree.
Reversible changes can go forward and back (melting wax, bending heated glass). Irreversible changes cannot (ripe mango to raw mango, ash to wood).
Periodic changes repeat after a fixed time (day and night, tides, seasons, clock hands). Non-periodic changes have no fixed interval (floods, a meteor, a bird flying away).
Physical change (भौतिक बदल): no new substance forms; composition stays the same – melting, boiling, dissolving, evaporation, breaking glass, making a table from wood.
Chemical change (रासायनिक बदल): a new substance with new properties forms – burning wood, rusting, ripening tomato, heating sugar till it turns black.
Corrosion: a reddish-brown layer (rust) on iron or a greenish layer on copper, caused by oxygen, moisture and chemical vapours in air. It weakens articles.
Prevention: galvanization (thin zinc coat on iron), tinning (tin coat on copper and brass), powder coating, painting.
While classifying, use one criterion at a time; the same change can belong to many groups.
Important terms
Evaporation – formation of vapour from a liquid (drying clothes, salt from sea water).
Corrosion – slow damage of metals by air, moisture and chemicals.
Galvanization – coating iron with zinc to prevent rusting.
Tinning – coating copper or brass with tin.
Formulas
No formulas. Quick comparison:
Physical change
Chemical change
No new substance
New substance forms
Composition unchanged
Composition changes
Usually reversible
Usually irreversible
Melting ice, dissolving salt
Burning paper, rusting
5 important questions
Q1. Distinguish between physical and chemical change.
In a physical change no new substance is formed and the composition remains the same, e.g. melting of wax. In a chemical change a new substance with different properties is formed, e.g. burning of wood.
Q2. What are periodic and non-periodic changes? Give examples.
Changes that repeat after a fixed interval are periodic (day and night, high and low tides). Changes whose repetition time is not fixed are non-periodic (floods, earthquakes).
Q3. Classify: conversion of milk into curd.
Natural, slow, irreversible, useful and chemical change.
Q4. What is corrosion? How can it be prevented?
Corrosion is the damage of metals by oxygen, moisture and chemical vapours, e.g. rusting of iron. It is prevented by galvanization, tinning, painting and powder coating.
Q5. Why is heating sugar till it turns black a chemical change?
The sugar changes into a black substance with completely different properties, and we cannot get sugar back from it.
Exam tip
For "classify the change" questions, give every applicable type (natural/man-made, fast/slow, reversible/irreversible, physical/chemical).
Everything around us is made of matter, which has mass and occupies space. Matter can be a pure substance – an element or a compound – or a mixture. This chapter explains elements and their symbols, metals, non-metals and metalloids, compounds and molecular formulae, mixtures, and methods to separate them.
Key points
Matter is made of tiny particles; its properties stay the same even when divided (chalk, ink, perfume).
Mass and volume are two main properties of matter.
A substance has one constituent (gold, water). A mixture has two or more substances (sherbet, air, soil, steel).
Element (मूलद्रव्य): molecules made of only one type of atom; cannot be broken into different substances. 118 elements are known; 92 occur naturally.
Democritus called the smallest particle atomos (indivisible). Dalton (1803): atoms cannot be created, divided or destroyed. Berzelius started today's letter symbols.
Symbols: H, He, C, N, O, Na (sodium), Mg, Al, Si, P, S, Cl, K (potassium), Ca; W (tungsten, from Wolfram), Ag (silver, Argentum), Au (gold, Aurum).
Metals are malleable, ductile, lustrous, sonorous and conduct heat and electricity. Non-metals (phosphorus, sulphur, chlorine) do not. Metalloids (arsenic, silicon, selenium) show both kinds of properties.
Alloy: a mixture of a metal with other elements to improve properties – brass, steel, 22-carat gold.
Compound (संयुग): formed by chemical combination of two or more elements; its properties differ from its elements. Water (H₂O) puts out fire though hydrogen burns and oxygen helps burning.
Heating sugar leaves black carbon – sugar is a compound.
In a mixture, proportion is not fixed, no chemical change happens and components keep their properties.
Distillation: salt water is boiled, vapour cools in a condenser and pure water collects; salt stays behind.
Separating funnel: separates immiscible liquids (kerosene floats on water).
Centrifugation: fast spinning separates fine solids from liquids (milk, blood).
Chromatography: separates dissolved substances using solubility and stickiness to paper (ink colours rise to different heights).
Important terms
Molecule – smallest particle of a substance that can exist independently.
Atom – smallest particle of an element.
Molecular formula – short form of a compound using symbols and number of atoms (H₂O, CO₂).
Immiscible liquids – liquids that do not mix (kerosene and water).
Formulas
Water: H₂O (2 hydrogen atoms + 1 oxygen atom)
Carbon dioxide: CO₂ (1 carbon + 2 oxygen)
Oxygen gas: O₂ (two oxygen atoms in one molecule)
5 important questions
Q1. Distinguish between a compound and a mixture.
A compound is formed by chemical combination in a fixed proportion and has new properties (water). A mixture is formed without any chemical change, its proportion is not fixed and components keep their properties (sherbet).
Q2. Explain distillation of salt water.
Salt water is heated in a round-bottom flask. Water vapour passes through a condenser, cools and drips into a conical flask as pure water. Salt remains in the flask.
Q3. How will you separate a mixture of semolina, salt and iron filings?
Move a magnet through it to remove iron filings. Add water to dissolve the salt and filter out the semolina. Evaporate the water to get the salt back.
Q4. What are metalloids? Give examples.
Elements that show some properties of metals and some of non-metals, e.g. arsenic, silicon, selenium.
Q5. Why are symbols of some elements not from their English names?
They come from Latin or German names – Na from Natrium (sodium), K from Kalium (potassium), Ag from Argentum (silver), Au from Aurum (gold), W from Wolfram (tungsten).
Exam tip
Learn the symbols of the first 20 elements and the principle behind each separation method.
We use many man-made materials every day. This chapter explains three of them – toothpaste, detergents (soaps and synthetic detergents) and cement – what they are made of, how they work and how they are prepared.
Key points
Earlier, people cleaned teeth with acacia bark, neem twigs, coal powder, ash, salt and pomegranate rind.
The first commercial toothpaste was made in New York in 1873.
Main toothpaste ingredients: calcium carbonate and calcium hydrogen phosphate (remove dirt and polish teeth). Fluoride prevents tooth decay and strengthens enamel and bones.
"Detergent" comes from Latin detergere = to wipe away. Ritha, shikekai, soap, washing soda, washing powder and shampoo are all detergents.
A detergent molecule is long; one end holds water and the other holds oil. So oil and water mix and dirt is removed.
Spreading on a surface is surface activity; such substances are surfactants. Lather is one effect.
Natural detergents: soap nut (ritha) and soap pod (shikekai) contain saponin; safe for skin, silk and wool.
Soap is a sodium or potassium salt of fatty acids. Hard soap (sodium salt) – for clothes. Soft soap (potassium salt) – for bathing.
Soap does not lather in hard water (well, tube-well); it forms scum. Synthetic detergents (made from fats or kerosene) work in hard water too.
Soap is made by boiling oil (fat) with sodium hydroxide (alkali), then adding salt solution so the soap floats and separates.
Cement is a greenish-grey powder of silica, alumina, lime, iron oxide and magnesia.
Portland cement: about 60% lime, 25% silica, 5% alumina, the rest iron oxide and gypsum; named after stone from the Isle of Portland, England.
Romans made cement from volcanic ash and lime; John Smeaton (1756) developed water-setting cement.
Concrete = cement + water + sand + gravel.
Important terms
Detergent – substance that cleans away dirt.
Surfactant – substance that spreads easily on a surface.
Saponin – natural cleaning chemical in ritha and shikekai.
Hard water – water in which soap does not lather easily.
Concrete – mixture of cement, water, sand and gravel used in construction.
Natural resources are things we get from nature – minerals, fuels, forests and ocean wealth. This chapter explains how minerals and ores form and are used, how fossil fuels like coal, petroleum and natural gas formed, the uses of forests, and the resources of the oceans.
Key points
Resources in the earth's crust (lithosphere): minerals, ores, mineral oil, fuels, rocks, water.
Only a few metals (gold, silver, copper, platinum, bismuth) occur free; most occur as compounds.
Ore: a mineral with a high proportion of metal from which the metal can be extracted economically. Sand and soil impurities in an ore are called gangue.
Minerals form from cooling magma/lava, evaporation, changes in temperature and pressure, and from living things (shells, conches).
Types: metal minerals (iron, gold, bauxite, manganese), non-metal minerals (mica, sulphur, gypsum, graphite), energy minerals (coal, mineral oil, natural gas), gems (diamond, ruby, emerald).
Iron ores: magnetite, haematite, limonite, siderite. Bauxite is the main ore of aluminium (about 55% aluminium). Copper – electric wires. Mica – bad conductor, used in electrical equipment.
Coal formed from buried forests under heat and pressure – a fossil fuel. Types: peat, lignite, bituminous, anthracite (best grade).
Mineral oil (petroleum) formed from dead sea organisms; found 1000–3000 m deep; refined by fractional distillation into petrol, diesel, kerosene, naphtha, tar, lubricating oil.
Natural gas is mainly methane (CH₄); transported as CNG and LNG. CNG is clean – no solid waste, few pollutants.
LPG: propane and butane (30:70) liquefied to 1/240 of volume; ethyl mercaptan is added so leaks can be smelled.
Limited fossil fuels and rising demand cause an energy crisis; alternatives – hydrogen, biofuels, methanol, ethanol.
ONGC was set up on 14 August 1956. Bombay High oil well drilled by 'Sagar Samrat' in 1974.
Forests cover about 30% of world land; they stop soil erosion, control floods, help water percolate, protect wildlife, and give wood, rubber, gum, honey, essential oils and medicines.
Oceans give minerals (tin, copper, manganese, uranium), pearls, shells, oil, gas and tidal energy.
Important terms
Ore – mineral from which a metal is extracted profitably.
Gangue – unwanted impurities in an ore.
Fossil fuel – fuel formed from remains of ancient organisms (coal, petroleum, natural gas).
Energy crisis – danger of fuel reserves getting used up.
Marine resources – resources obtained from seas and oceans.
An ore is a mineral that has enough metal to extract it profitably. Many minerals have very little metal or none, so they are not ores.
Q2. How was coal formed?
Millions of years ago, forests were buried under layers of soil. High pressure and the earth's heat slowly changed the plant remains into coal. So it is a fossil fuel.
Q3. Why is ethyl mercaptan added to LPG?
LPG has no smell. Ethyl mercaptan has a strong smell, so any leakage can be detected immediately and accidents avoided.
Q4. Why is natural gas called an eco-friendly fuel?
It burns completely, leaves no solid waste, gives only small amounts of CO₂ and water, and produces few other pollutants.
Q5. Write any four protective functions of forests.
They prevent soil erosion, slow down rain water and help it percolate, control floods, and maintain the balance of gases in the air.
Exam tip
"Coal is black gold, petroleum is liquid gold" – be ready to explain why in two lines each.
Light shows some interesting effects in nature. This chapter explains scattering of light (why the sky is blue and the setting sun red), shadows made by point and extended sources (umbra and penumbra), and solar and lunar eclipses.
Key points
We see a beam of light only because tiny particles (dust, fog, milk drops) in its path scatter light into our eyes.
A laser beam is not seen in clear water but is seen in slightly milky (turbid) water – scattering of light (प्रकाशाचे विकिरण).
The sky is bright in the day because sunlight is scattered by particles and gas molecules in the air. Without an atmosphere, the sky would look dark even in the day.
Molecules of nitrogen and oxygen scatter blue light the most, so the sky appears blue.
At sunrise and sunset, sunlight travels a longer path through air; more blue is scattered away, so the red-yellow light reaches us and the sun looks red.
A light source like a pinhole is a point source; a candle or bulb without the pinhole is an extended source.
A point source gives only a dark shadow – the umbra. An extended source gives a dark umbra and a faint outer penumbra.
For an object smaller than the source, the umbra becomes smaller and disappears as the screen moves away.
An eclipse happens when the sun, moon and earth come in a straight line.
Solar eclipse: moon comes between sun and earth; occurs only on a new moon (amavasya) day; can be total or partial. Never watch it with naked eyes – harmful UV rays; use special goggles.
Lunar eclipse: earth comes between sun and moon; occurs only on a full moon (pournima) night; can be total or partial; safe to watch with naked eyes and lasts a few hours.
Occultation: a planet or star passing behind the moon (e.g. star Rohini in 2016).
Important terms
Scattering – spreading of light in all directions by tiny particles.
Umbra – the completely dark part of a shadow.
Penumbra – the partly lit, faint part of a shadow.
Point source – very small light source.
Extended source – large light source.
Formulas
No formulas.
Solar eclipse
Lunar eclipse
Order
Sun – Moon – Earth
Sun – Earth – Moon
Day
New moon
Full moon
Shadow
Moon's shadow on earth
Earth's shadow on moon
Viewing
Only with special goggles
Safe with naked eye
5 important questions
Q1. Why does the sky appear blue?
Sunlight is scattered by gas molecules in the atmosphere. Blue light is scattered the most, so blue light reaches our eyes from all parts of the sky.
Q2. Why does the sun look red at sunset?
At sunset, light travels a longer distance through the atmosphere. Most blue light is scattered away, and mainly red-yellow light reaches us directly.
Q3. What is the difference between umbra and penumbra?
Umbra is the completely dark central part of a shadow where no light reaches. Penumbra is the lighter outer part where some light reaches; it is formed only with an extended source.
Q4. Why should a solar eclipse not be seen with naked eyes?
During a solar eclipse harmful ultraviolet rays reach the earth and can damage the eyes. Special eclipse goggles must be used.
Q5. Why is a lunar eclipse seen only on a full moon night?
Only on a full moon night does the earth come exactly between the sun and the moon, so the earth's shadow can fall on the moon.
Exam tip
Draw neat diagrams of solar and lunar eclipses showing umbra and penumbra – they fetch easy marks.
Sound is produced when an object vibrates. This chapter explains oscillatory motion, amplitude, time period and frequency, pitch, intensity and sound level (decibel), and audible, infrasonic and ultrasonic sounds.
Key points
Sound is produced by the rhythmic vibration of an object; vibrations reach our ears through air like ripples in water. When vibration stops, sound stops.
A swing or a pendulum is an oscillator. Back-and-forth motion about a central position is oscillatory motion. Motion repeated at fixed intervals is periodic motion.
Amplitude: the maximum distance from the central position. Greater amplitude → louder sound.
Time period (T): time for one complete oscillation (B → A → C → A → B).
Frequency (n): number of oscillations in one second; unit hertz (Hz).
Time period of a pendulum increases with its length; it does not depend on amplitude.
Pitch depends on frequency. More tension in a string → higher frequency → higher (shriller) pitch. Musicians tune the tanpura and tabla by changing tension.
Sa, Re, Ga, Ma, Pa, Dha, Ni have increasing pitch. A mosquito's hum has higher pitch than a lion's roar.
Intensity ∝ (amplitude)². Doubling amplitude makes intensity four times.
Sound level is measured in decibels (dB), named after Alexander Graham Bell. 10 times intensity → +10 dB.
Normal conversation ≈ 60 dB; heavy traffic ≈ 70 dB; jet engine ≈ 130 dB. Above 100 dB can cause temporary deafness.
Humans hear 20 Hz to 20,000 Hz (audible sound).
Infrasonic (below 20 Hz): made by whales, elephants, rhinoceros. Ultrasonic (above 20,000 Hz): dogs can hear it; used to clean delicate ornaments and watch parts, in RADAR and in medical tests.
Important terms
Oscillation – one complete back-and-forth movement.
Amplitude – maximum displacement from central position.
Frequency – oscillations per second (Hz).
Pitch – how high or low (shrill or flat) a sound is.
Elasticity – property of returning to original shape after stretching.
Formulas
Frequency n = 1/T (n in Hz, T in seconds)
Time period T = time taken ÷ number of oscillations
Intensity ∝ (amplitude)²
5 important questions
Q1. A pendulum makes 20 oscillations in 40 seconds. Find its time period and frequency.
T = 40 ÷ 20 = 2 s. Frequency n = 1/T = 1/2 = 0.5 Hz.
Q2. What is the effect of tension on the pitch of a string?
When tension increases, frequency increases and the pitch becomes higher (shriller). When tension decreases, pitch becomes lower.
Q3. Distinguish between infrasonic and ultrasonic sound.
Infrasonic sound has frequency below 20 Hz (elephants, whales). Ultrasonic sound has frequency above 20,000 Hz (heard by dogs). Humans cannot hear either.
Q4. If the amplitude of vibration is doubled, what happens to the intensity?
Intensity is proportional to the square of amplitude, so it becomes 2² = 4 times.
Q5. Write any three uses of ultrasonic sound.
Cleaning delicate ornaments and tiny watch parts, RADAR systems, and medical imaging.
Exam tip
Remember the audible range (20 Hz – 20,000 Hz) and the formula n = 1/T; small numericals on these are common.
A magnet affects objects even from a distance through its magnetic field. This chapter explains how to make artificial magnets, why the earth behaves like a giant magnet, magnetic lines of force and their properties, how a magnetic field passes through materials, and metal detectors.
Key points
Magnets are made from alloys of iron, cobalt and nickel. Alnico (aluminium, nickel, cobalt) and Nipermag (iron, nickel, aluminium, titanium) are magnetic alloys.
Single touch method: rub one pole of a magnet repeatedly from one end of a steel bar to the other – weak, short-lasting magnetism.
Double touch method: place opposite poles of two magnets at the centre and drag them apart to the ends – stronger, longer-lasting magnetism.
William Gilbert showed that the earth is a gigantic magnet. Its magnetic south pole is near the geographic north pole, so the north pole of a freely suspended magnet points north.
The region around a magnet where its force acts is the magnetic field (चुंबकीय क्षेत्र).
Michael Faraday imagined invisible magnetic lines of force; iron filings around a bar magnet show their pattern.
Field is strong where lines are crowded (near the poles) and weak where they are spread out.
Properties of lines of force: imaginary; run from N pole to S pole outside the magnet; are in tension like a stretched spring; repel each other; never intersect; their number per unit area shows field strength.
Tension explains attraction of unlike poles; mutual repulsion of lines explains repulsion of like poles.
A magnetic field passes through cardboard, plastic and water, but its intensity decreases.
Repulsion is the real test of a magnet (attraction can happen with any iron object too).
Metal detectors work on electromagnets – used at airports, bus stations, in food processing and geology.
Important terms
Magnetic field – region around a magnet where its force can be felt.
Magnetic lines of force – imaginary lines showing the direction and strength of a magnetic field.
Magnetisation – process of making a magnet.
Electromagnet – a temporary magnet made by passing current through a coil around soft iron.
Formulas
No formulas. Remember: like poles repel, unlike poles attract; lines of force go from N to S outside a magnet.
5 important questions
Q1. Distinguish between the single touch and double touch methods of magnetisation.
In single touch, one pole of a magnet is rubbed repeatedly from one end of a steel bar to the other; magnetism is weak and short-lived. In double touch, opposite poles of two magnets are dragged from the centre to the two ends; magnetism is stronger and lasts longer.
Q2. Why does a freely suspended magnet always point north–south?
The earth is a huge magnet whose magnetic south pole is near the geographic north pole. The north pole of the suspended magnet is attracted towards it, so the magnet settles in the north–south direction.
Q3. Write any four properties of magnetic lines of force.
They run from north to south pole, they never intersect, they repel each other, and they are in tension like a stretched spring.
Q4. Why is repulsion the real test of a magnet?
A magnet attracts both a magnet's opposite pole and ordinary iron. But repulsion happens only between like poles of two magnets, so it proves that an object is a magnet.
Q5. Write the uses of metal detectors.
They check people at airports, bus stations and important buildings, find precious metal objects, detect iron pieces in food in factories, and locate metals in geology.
Exam tip
Draw the pattern of lines of force around a bar magnet with arrows from N to S.
Stars are born in clouds of gas and dust and go through a long lifecycle. This chapter explains the lifecycle of stars, the basic terms of sky watching (celestial sphere, zenith, nadir, ecliptic), zodiac signs and nakshatras, and how to identify constellations like Saptarshi, Sharmishtha and Mrug.
Key points
Stars are born from nebulae – clouds of mainly hydrogen gas and dust. Gravity pulls the particles together; the cloud contracts, becomes hot at the core and starts producing energy – a star is born.
Lifecycle: ordinary star → red giant → planetary nebula → white dwarf. A massive star → red super giant → supernova → neutron star or black hole.
Our solar system is a tiny part of the Milky Way galaxy, which has lakhs of stars.
The sky looks like a huge sphere around us – the celestial sphere (खगोल). The horizon divides it into two halves.
Zenith: point directly above our head. Nadir: point directly below our feet.
Celestial poles: where earth's axis meets the celestial sphere. Celestial equator: earth's equator extended to the sphere. Meridian: great circle through both celestial poles and our zenith and nadir.
Ecliptic: the circle along which the sun appears to move on the celestial sphere.
Stars rise and set 4 minutes earlier every day (a star rising at 8 pm today rises at 7:56 pm tomorrow).
The sun appears to move about 1° per day and the moon 12–13° per day eastward against the stars.
Constellation: a group of stars in a small part of the sky. Western astronomers divided the sky into 88 constellations.
12 zodiac signs (raashi): ecliptic divided into 12 parts of 30° each – Aries to Pisces.
27 nakshatras: the moon goes round the earth in about 27.3 days; the part it covers in one day (about 13°20') is a nakshatra. Its brightest star is the yogatara. One raashi = 2¼ nakshatras.
Saptarshi (Great Bear): seven stars like a kite; its side points to the Pole Star. Sharmishtha (Cassiopeia): five stars in an M shape, also used to find the Pole Star. Mrug (Orion): bright winter constellation; its three middle stars point to Vyadh (Sirius).
Sky watching: away from city lights, on a new-moon night, using the Pole Star as reference.
Important terms
Nebula – cloud of gas and dust where stars are born.
Celestial sphere – imaginary sphere on which stars appear to move.
Ecliptic – apparent path of the sun among the stars.
Constellation – group of stars forming a pattern.
Nakshatra – part of the sky the moon crosses in one day.
Formulas
One raashi = 360° ÷ 12 = 30°
One nakshatra = 360° ÷ 27 = 13°20'
One raashi = 27 ÷ 12 = 2¼ nakshatras
5 important questions
Q1. How is a star born?
In a nebula, gravity pulls hydrogen gas and dust together. The cloud contracts and becomes dense and round. Pressure and temperature at the core rise greatly and energy production begins – a star is formed.
Q2. A star rises at 8 pm tonight. When will it rise after a month? Why?
It rises 4 minutes earlier each day. In 30 days that is 120 minutes = 2 hours earlier, so about 6 pm. This happens because of the earth's revolution around the sun.
Q3. What are the zenith and nadir?
Zenith is the point on the celestial sphere exactly above our head; nadir is the point exactly below our feet.
Q4. How can we find the Pole Star?
Extend one side of the quadrangle of Saptarshi; it reaches the Pole Star. Also, the perpendicular bisector of the line joining the 3rd and 4th stars of Sharmishtha points to the Pole Star.
Q5. What is the difference between a zodiac sign and a nakshatra?
A zodiac sign is one of the 12 equal parts (30° each) of the ecliptic. A nakshatra is one of the 27 parts (13°20' each), the portion covered by the moon in one day.
Exam tip
Learn the star lifecycle flowchart and practise the "4 minutes earlier" calculation.