🌿 Class 8 NCERT 2025–26

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🔬 Chapter 1

Exploring the Investigative World of Science

Science progresses through systematic inquiry. The scientific method — Observation → Question → Hypothesis → Experiment → Analysis → Conclusion — is a self-correcting cycle. Variables: Independent (what you change), Dependent (what you measure), Controlled (what stays constant).

Fig. 1.1

The Scientific Method — A Cycle of Discovery

Six steps in a continuous cycle: Observe → Question → Hypothesize (testable prediction) → Experiment (controlled test) → Analyse (find patterns) → Conclude. Science is self-correcting — results are always verified and repeated by others.

Fig. 1.2

Variables — Independent, Dependent, Controlled

Independent variable: what you deliberately change. Dependent variable: what you measure as a result. Controlled variables: everything else kept the same. Example — plant growth: sunlight (IV), plant height (DV), soil/water/temperature (CV).

Fig. 1.3

Observation vs Inference

Observation: what you directly detect with your senses. Inference: a conclusion drawn from observations using prior knowledge. "The grass is wet" (observation) vs "It rained last night" (inference — could also be dew or sprinkler).

🦠 Chapter 2

The Invisible Living World: Beyond Our Naked Eye

Microorganisms include bacteria, fungi, protozoa, algae, and viruses. First observed by Antonie van Leeuwenhoek. They decompose matter, fix nitrogen, make antibiotics — but some cause diseases. Cells are the basic unit of life; tissues → organs → organ systems → organism.

Fig. 2.1

Types of Microorganisms — Classification

Bacteria (prokaryote, no nucleus — rod/sphere/spiral), Fungi (cell wall of chitin, spores — yeast, Penicillium), Protozoa (animal-like, flagella — Amoeba, Paramecium), Algae (photosynthetic, aquatic — Spirogyra), Viruses (non-cellular, needs host — Influenza, COVID-19).

Fig. 2.5

Animal Cell vs Plant Cell — Structure

Both: nucleus (DNA), cell membrane, cytoplasm, mitochondria. Plant ONLY: cell wall (cellulose), chloroplasts, large central vacuole. Animal ONLY: centrioles, lysosomes. Cells are the basic unit of life (Robert Hooke, 1665).

Fig. 2.3

Compound Microscope — Parts & Function

Eyepiece (×10) → Body tube → Objective lens (×4,×10,×40,×100) → Stage → Condenser → Diaphragm → Mirror/lamp. Total magnification = eyepiece × objective. Leeuwenhoek's microscope: ×200–300.

Fig. 2.6

Microorganisms — Beneficial vs Harmful

Beneficial: Lactobacillus (curd), Yeast (bread/idli), Rhizobium (N₂ fixation), Penicillium (antibiotic), decomposers. Harmful: Mycobacterium tuberculosis (TB), Plasmodium (malaria), Vibrio cholerae (cholera), COVID-19 virus, food spoilage moulds.

Fig. 2.7

Levels of Organisation — Cell → Organism

Cell (basic unit, e.g. muscle cell) → Tissue (similar cells, e.g. muscle tissue) → Organ (different tissues, e.g. heart) → Organ System (e.g. circulatory system) → Organism (all systems = a living being). Each level is more complex than the last.

Fig. 2.11

Yeast Fermentation — Dough Rises

Yeast + warm water + sugar → CO₂ + ethanol. CO₂ bubbles trapped in dough → dough rises and becomes soft/porous. Used in bread, idli, dosa. Optimal temperature: 35–40°C. If overheated, yeast dies.

Fig. 2.13

Bacterial Cell — Prokaryote Structure

No true membrane-bound nucleus. Parts: Cell wall (peptidoglycan), Cell membrane, Cytoplasm, Nucleoid (circular DNA), Ribosomes (protein synthesis), Flagella (movement), Pili (attachment). Much smaller than eukaryotic cells (1–10 µm).

🏥 Chapter 3

Health: The Ultimate Treasure

WHO: "Health is a state of complete physical, mental, and social well-being." Diseases are communicable (spread person-to-person) or non-communicable (lifestyle/genetic). Vaccines build immunity. Antibiotics kill bacteria but NOT viruses. Antibiotic resistance is a global crisis.

Fig. 3.1

Aspects of Health — The Health Wheel

Physical health: fit body, nutrition, sleep. Mental health: emotional well-being, stress management. Social health: relationships, community participation. All three are interconnected — neglecting one affects the others.

Fig. 3.4

Disease Transmission Routes

Airborne: droplets (TB, Influenza, COVID-19). Waterborne: contaminated water (Cholera, Typhoid). Vector-borne: mosquito (Malaria — Plasmodium, Dengue). Direct contact: (Ringworm, Chickenpox). Foodborne: (Salmonella). Sexually transmitted: HIV.

Fig. 3.3

How Vaccination Builds Immunity

Vaccine contains weakened pathogens → immune system produces antibodies → memory B-cells formed. If real pathogen enters later → memory cells rapidly produce antibodies → fight infection before symptoms. Herd immunity protects even unvaccinated people.

Fig. 3.6

Communicable vs Non-Communicable Diseases

Communicable (infectious): spread via pathogens — TB, Malaria, COVID-19, Cholera, Dengue. Non-communicable (lifestyle/genetic): cannot spread — Diabetes (type 2), Hypertension, Heart disease, Cancer. Prevention: balanced diet, exercise, hygiene, vaccination.

Activity 3.5

Antibiotic Resistance — Why Complete the Course?

Sensitive bacteria die; resistant ones survive (natural selection). Stopping antibiotics early → remaining resistant bacteria multiply → antibiotic-resistant population. Superbugs (MRSA, XDR-TB) don't respond to common antibiotics. Always complete prescribed courses!

⚡ Chapter 4

Electricity: Magnetic and Heating Effects

Electric current in a wire creates a magnetic field (Oersted, 1820). An electromagnet: coiled wire around iron, carrying current. Heating effect (Joule heating): H = I²Rt — used in bulbs, heaters, fuses. A fuse melts when excess current flows, protecting circuits.

Fig. 4.1

Oersted's Discovery — Current Creates Magnetism

Hans Christian Oersted (1820): a compass needle deflects near a current-carrying wire — electricity and magnetism are linked! Reverse current → needle deflects opposite way. This discovery led to electromagnets and electric motors.

Fig. 4.3

Electromagnet — How Current Creates Magnetism

Coil of insulated wire wound around an iron core + battery. Current → magnetic field (right-hand rule). Iron core amplifies field ×1000. Stronger with: more coils, more current, soft iron core. Switch off → magnetism disappears instantly. Used in cranes, doorbells, MRI machines.

Fig. 4.3b

Magnetic Field Lines — Wire, Coil & Bar Magnet

Concentric circles around a straight wire. In a coil/solenoid: field lines combine to create uniform field inside — like a bar magnet with N and S poles. Right-hand thumb rule: thumb in current direction, curled fingers show field direction.

Fig. 4.6

Electric Bell — Using Electromagnet

Switch pressed → current → electromagnet attracts iron strip → hammer hits bell → circuit breaks → electromagnet off → spring pulls armature back → circuit completes again → repeats rapidly → continuous ringing! This make-and-break mechanism causes the bell to ring.

Fig. 4.8

Heating Effect — Resistance & Fuse

Joule's law: H = I²Rt. More current or resistance → more heat. Filament bulb: tungsten wire (melting point 3422°C) glows white-hot. Fuse: thin wire of low melting point — melts when excess current flows, breaking circuit and preventing fire.

💪 Chapter 5

Exploring Forces

A force is a push or pull that changes an object's speed, direction, shape, or size. Contact forces (muscular, friction, normal) require physical contact. Non-contact forces (gravity, magnetic, electrostatic) act at a distance. Pressure = Force ÷ Area (Pascals).

Fig. 5.1

Types of Forces — Contact vs Non-Contact

Contact: Muscular (push/pull with muscles), Friction (opposes motion), Normal reaction (perpendicular to surface). Non-Contact: Gravitational (F = Gm₁m₂/r²), Magnetic (attracts iron/nickel/cobalt), Electrostatic (between charged bodies). All forces have magnitude and direction (vectors).

Fig. 5.3

Friction — Types and Comparison

Static friction (object at rest — highest), Kinetic/sliding friction (object sliding), Rolling friction (object rolling — lowest). Order: Static > Kinetic > Rolling. Friction depends on: surface roughness, normal force. Lubrication, ball bearings reduce friction.

Fig. 5.6

Friction — Surface Irregularities (Microscopic)

Even "smooth" surfaces have tiny bumps at microscopic level. Their irregularities interlock → friction. Polishing reduces irregularities → less friction. Lubricants fill gaps → surfaces don't directly touch → much less friction. Ball bearings replace sliding with rolling.

Fig. 5.8

Gravity — Newton's Universal Law

F = Gm₁m₂/r². Every mass attracts every other mass. Earth pulls apple AND apple pulls Earth (equally!). Moon's gravity causes ocean tides. Moon stays in orbit because gravity provides centripetal force. Weight = mass × g (g = 9.8 m/s² on Earth).

Fig. 5.5

Pressure = Force ÷ Area

Same force on smaller area = higher pressure (needle, knife edge). Same force on larger area = lower pressure (snowshoes, camel's broad feet). Pressure in Pa = N/m². Liquid pressure increases with depth: P = ρgh.

🌪️ Chapter 6

Pressure, Winds, Storms, and Cyclones

Air exerts pressure in all directions. Wind blows from high to low pressure. Sea breeze (day) and land breeze (night) caused by differential heating. Thunderstorms develop from hot, humid air. Cyclones: intense low-pressure systems over warm seas — destructive but satellite-tracked.

Fig. 6.1–6.3

Pressure Applications — Broad Strap, Dam, Knife

Broad bag strap: large area → low pressure on shoulder. Narrow strap: small area → HIGH pressure → pain. Dam wall wider at base: water pressure increases with depth → needs larger area. Knife sharpened to thin edge: tiny area → huge pressure → cuts easily.

Fig. 6.2

Sea Breeze & Land Breeze — Daily Wind Cycle

Day (Sea Breeze): Land heats faster → warm air rises over land → low pressure over land → cool sea air blows in. Night (Land Breeze): Sea stays warm longer → warm air rises over sea → cool land air blows out. This daily cycle drives coastal weather patterns.

Fig. 6.5

Cyclone — Structure & Formation

Forms over warm sea (>27°C): humid air rises rapidly → strong low pressure → air spirals inward and upward → clouds and rain. Eye: calm, clear, 10–30 km wide. Eyewall: fiercest winds. Called Hurricanes (Atlantic) and Typhoons (Pacific).

Fig. 6.8

Thunderstorm & Lightning Formation

Hot humid air rises → cumulonimbus cloud forms. Ice crystals and water droplets collide → charge separation (+ at top, − at bottom). When charge difference is large enough → lightning discharge. Thunder = rapid air expansion from heat of bolt (~30,000 K).

Fig. 6.3

Atmospheric Pressure — Altitude & Experiments

Air pressure decreases with altitude (less air column above). At sea level: ~1013 hPa. At 5 km: ~540 hPa. At 10 km: ~265 hPa. Crushed can experiment: steam condenses inside → pressure drops → atmospheric pressure crushes can from outside.

⚗️ Chapter 7

Particulate Nature of Matter

All matter is made of tiny particles (atoms/molecules). Particles have spaces between them, are in constant motion, and attract each other. The three states differ in particle arrangement and energy. Diffusion is evidence for particle motion. Evaporation is a surface phenomenon; boiling is bulk.

Fig. 7.1

Three States of Matter — Particle Model

Solid: tightly packed in fixed positions, strong forces, vibrate only — definite shape and volume. Liquid: close but can slide, moderate forces — definite volume, no fixed shape. Gas: far apart, moving randomly, weak forces — no fixed shape or volume. State changes with temperature.

Activity 7.1

Matter is Made of Particles — Chalk Breakdown

Break chalk → smaller pieces → grind to powder → dissolve in water → particles invisible but still there. Shows matter = tiny particles. Sugar in tea: particles disperse but never disappear — conservation of matter! Eventually reach constituent particles that can't be divided further.

Fig. 7.3

Diffusion — Particles Mixing Spontaneously

Diffusion: spontaneous mixing from high → low concentration due to random particle motion. Gases fastest (large spaces), liquids slower, solids very slowly. Classic demo: NH₃ and HCl gases produce white smoke (ammonium chloride) when they diffuse and meet.

Fig. 7.5

Changes of State — Heating Curve

Solid → Liquid: melting (latent heat of fusion). Liquid → Gas: vaporisation (latent heat of vaporisation). Reverse: condensation, freezing. Solid → Gas: sublimation (dry ice, camphor, naphthalene). During phase change: temperature stays constant — all heat changes state.

Fig. 7.6

Evaporation vs Boiling — Key Differences

Evaporation: surface phenomenon — only fast surface particles escape. Occurs at ALL temperatures. Cooling effect. Faster with: higher temp, larger area, lower humidity, wind. Boiling: bulk phenomenon — ALL particles throughout liquid vaporise. Only at fixed boiling point (100°C). Much faster.

🧪 Chapter 8

Nature of Matter: Elements, Compounds, and Mixtures

Element: pure substance, one type of atom, cannot be broken down chemically. Compound: two or more elements chemically combined in fixed ratio. Mixture: physically combined. Periodic Table: 118 elements. Separation: filtration, evaporation, distillation, chromatography.

Fig. 8.2

Elements vs Compounds vs Mixtures

Element (Au, Fe, O): single type of atom, fixed properties. Compound (H₂O, NaCl, CO₂): atoms of ≥2 elements in fixed ratio, completely different properties, chemical bond. Mixture (air, soil, saltwater): components keep their properties, variable composition, separated by physical means.

Activity 8.1

Iron + Sulphur: Mixture vs Compound

Mixture: iron filings + sulphur — magnet separates iron, each retains properties, any ratio. Compound (FeS): heat mixture → iron sulphide forms — magnet CANNOT attract it, new grey solid with different properties, fixed ratio (Fe:S = 56:32 by mass). Classic demonstration of mixture vs compound!

Fig. 8.5

Separation Techniques

Filtration: separates insoluble solid from liquid (tea leaves). Evaporation: separates dissolved solid from solution (salt from seawater). Simple distillation: separates liquids by boiling point. Chromatography: separates mixtures by movement speed. Magnetic separation: iron filings from sand.

Fig. 8.6

Paper Chromatography — Separating Ink Colours

Ink spot at bottom of paper strip. Dip in water (solvent). Water rises by capillary action, carrying ink components. Different dyes travel at different speeds depending on solubility → separate into distinct colour bands. Black ink = mixture of many colours! Rf = distance by component ÷ distance by solvent.

Fig. 8.1

Periodic Table — Organisation of Elements

Dmitri Mendeleev (1869): 63 elements by atomic mass + properties. Modern table: 118 elements by atomic number. Periods (rows): 7. Groups (columns): 18. Metals (left) — Metalloids (middle) — Non-metals (right). Noble gases (Group 18) are unreactive.

💧 Chapter 9

The Amazing World of Solutes, Solvents, and Solutions

A solution is a homogeneous mixture of solute dissolved in solvent. Concentration = amount of solute per unit volume. pH scale (0–14) measures acidity/alkalinity. Indicators show pH. Acids + Bases → Salt + Water (neutralisation). Tyndall effect distinguishes colloids from true solutions.

Fig. 9.2

Solution Types — Saturated, Unsaturated, Supersaturated

Unsaturated: can dissolve more solute. Saturated: maximum solute dissolved at that temperature. Supersaturated: more than maximum dissolved (unstable — seed crystal causes rapid crystallisation). Solubility usually increases with temperature for solids. Gases dissolve more at lower temperature.

Fig. 9.4

pH Scale — Acids, Neutral, Bases

pH 0–6: Acids (H⁺ ions). pH 7: Neutral (pure water). pH 8–14: Bases (OH⁻ ions). Each step = 10× change. Stomach acid: pH 2. Lemon juice: pH 2–3. Blood: pH 7.4. Bleach: pH 13. Litmus: red in acid, blue in base.

Activity 9.3

Tyndall Effect — Colloid vs True Solution

True solution (salt water): particles 1 nm — light NOT scattered, beam invisible. Colloid (milk, fog, starch): particles 1–100 nm — scatter light → beam visible (Tyndall effect). Suspension: particles >100 nm — settles on standing. Milk in dark room with torch beam shows Tyndall effect clearly!

Fig. 9.6

Solubility Curve — Temperature vs Amount Dissolved

Solubility (g per 100g water) generally increases with temperature for solids. KNO₃: 13g (20°C) → 246g (100°C) — steep curve. NaCl: 35.7g (20°C) → 39.2g (100°C) — nearly flat. Used in fractional crystallisation to separate salts.

Fig. 9.8

Neutralisation — Acid + Base → Salt + Water

H⁺ + OH⁻ → H₂O. NaOH + HCl → NaCl + H₂O. Indicator changes colour as pH moves toward 7. Applications: antacids neutralise stomach acid; baking soda neutralises wasp stings (acidic); vinegar neutralises bee stings (alkaline).

💡 Chapter 10

Light: Mirrors and Lenses

Light travels in straight lines at 3×10⁸ m/s. Reflection: angle of incidence = angle of reflection. Concave mirror: converging — used in torches, dentists. Convex mirror: diverging — rear-view mirror. Convex lens: magnifying glass, eye. Concave lens: corrects myopia. Prism disperses white light to VIBGYOR.

Fig. 10.3

Concave & Convex Mirror — Image at Key Positions

Concave mirror: beyond C → real, inverted, diminished (camera). At C → same size. At F → image at infinity. Between F and P → virtual, erect, magnified (shaving mirror). Convex mirror: always virtual, erect, diminished — wide field of view (rear-view mirror).

Fig. 10.6–10.7

Uses of Concave & Convex Mirrors

Concave: torch/headlight (bulb at focus → parallel beam), doctor's head mirror (magnified image), solar cooker. Convex: car rear-view mirror (wide field of view), road safety mirror at blind corners, shop security mirror.

Fig. 10.7

Convex & Concave Lens — Image Formation

Convex (converging) lens: beyond 2F → real, inverted, diminished (camera). Between F and lens → virtual, erect, magnified (magnifying glass). Concave (diverging) lens: always virtual, erect, diminished (corrects myopia). Lens formula: 1/v − 1/u = 1/f.

Fig. 10.9

Dispersion — White Light to VIBGYOR

White light = mixture of 7 colours. Prism disperses because each colour travels at different speed in glass. Violet bends most, red bends least. VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red. Rainbow: raindrops act as tiny prisms. Newton first split white light (1666).

Fig. 10.11

Defects of Vision — Myopia & Hypermetropia

Myopia (short-sightedness): image in front of retina — far objects blurry. Eyeball too long. Corrected with concave lens. Hypermetropia (long-sightedness): image behind retina — near objects blurry. Eyeball too short. Corrected with convex lens (reading glasses).

🌙 Chapter 11

Keeping Time with the Skies

Earth rotates (24 hrs — day/night) and revolves around Sun (365.25 days — year). Moon orbits Earth in 29.5 days — causes phases. Waxing (Shukla Paksha): bright portion grows. Waning (Krishna Paksha): shrinks. Ancient timekeeping: sundials, water clocks, hourglasses. Solar, lunar, and lunisolar calendars.

Fig. 11.2

Earth's Motion — Rotation, Revolution & Seasons

Rotation (west→east, 24h): causes day and night. Revolution (365.25 days): causes seasons due to Earth's 23.5° axial tilt. Summer (Jun 21): Northern Hemisphere tilts toward Sun. Winter (Dec 21): tilts away. Equinox (Mar 21, Sep 23): equal day and night worldwide.

Fig. 11.2 · Phases

Waxing & Waning Moon — Shukla & Krishna Paksha

After New Moon (Amavasya): bright portion grows — Waxing (Shukla Paksha). After Full Moon (Purnima): bright portion shrinks — Waning (Krishna Paksha). Moon rises ~50 min later each day. Waxing Moon visible in western sky after sunset; waning Moon visible in eastern sky before sunrise.

Fig. 11.5

Moon Phases — 29.5 Day Cycle

Moon takes 29.5 days to orbit Earth. Phases depend on how much lit side faces Earth. New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → New Moon. We always see the same face (synchronous rotation).

Fig. 11.7

Sundial & Ancient Timekeeping

Sundial uses shadow of gnomon — works because Earth rotates 15° per hour. Water clock (clepsydra): water drips at constant rate. Hourglass: sand falls at constant rate. Modern atomic clocks: accurate to 1 second in 300 million years. GPS satellites use atomic clocks.

Fig. 11.9

Calendar Types — Solar, Lunar & Lunisolar

Solar (Gregorian): 365 days, based on Earth's revolution. Lunar (Islamic Hijri): 354 days, based on Moon — drifts 11 days/year. Lunisolar (Hindu Panchang): tracks both, adds leap month (Adhika Masa) every ~3 years. Indian National Calendar (Saka era): 365 days, starts Chaitra 1.

🌿 Chapter 12

How Nature Works in Harmony

Ecosystems: biotic (living) + abiotic (non-living) components. Energy flows through food chains and webs. Nutrients cycle continuously. Only 10% energy transfers up each trophic level. Carbon and nitrogen cycles sustain life. Biodiversity is crucial — every species has a role.

Fig. 12.2

Forest Food Web — Energy Flow & Trophic Levels

Producers (plants): convert sunlight → food. Primary consumers (herbivores). Secondary consumers (carnivores). Tertiary consumers (top predators). Decomposers (bacteria, fungi): break dead matter → recycle minerals. 10% rule: only 10% energy transfers up each trophic level — rest lost as heat.

Fig. 12.4

Energy Pyramid — 10% Rule

Producers: 10,000 units. Primary consumers: 1,000. Secondary: 100. Tertiary: 10. Top predators: 1 unit. 90% lost as heat at each level. This is why large predators are rare — need huge amounts of prey. Why vegetarian diet is more energy-efficient than meat-based diet.

Fig. 12.5

Biotic & Abiotic Interactions

Biotic: producers, consumers, decomposers — interact via predation, competition, symbiosis, mutualism, parasitism. Abiotic: sunlight, water, soil, temperature, air. Biotic ↔ Abiotic: plants absorb sunlight + CO₂ + water; animals affect soil; decomposers return nutrients.

Fig. 12.5

Carbon Cycle

CO₂ absorbed by plants (photosynthesis) → organic carbon in food → released by respiration. Decomposers return C to soil. Burning fossil fuels adds extra CO₂ → greenhouse effect. Ocean absorbs CO₂. Limestone stores carbon for millions of years. Human disruption → global warming.

Fig. 12.7

Nitrogen Cycle

N₂ (78% of air) → Nitrogen fixation: Rhizobium in legume root nodules converts N₂ → NH₃. Lightning also fixes N. Nitrification: bacteria convert NH₃ → NO₂⁻ → NO₃⁻ (plant-available). Plants absorb nitrates → animals eat plants → death/excretion → ammonification → denitrification → N₂ back to air.

🌍 Chapter 13

Our Home: Earth, a Unique Life-Sustaining Planet

Earth is the only known planet with life. Right distance from Sun (Goldilocks zone), liquid water, oxygen-nitrogen atmosphere, magnetic field (magnetosphere). Structure: Crust → Mantle → Outer Core (liquid iron) → Inner Core (solid iron). The water cycle and atmosphere sustain all life.

Fig. 13.2

Earth's Internal Layers — Crust to Core

Crust: 5–70 km thick (oceanic: basalt; continental: granite). Mantle: 2900 km thick, semi-solid silicate rock, convection currents move tectonic plates. Outer Core: 2200 km, liquid iron-nickel, generates Earth's magnetic field. Inner Core: 1200 km radius, solid iron-nickel (~5000°C).

Fig. 13.2

Greenhouse Effect — Natural vs Enhanced

Natural: Sun's shortwave radiation passes through → Earth emits infrared → GHGs (CO₂, CH₄, H₂O, N₂O) trap some heat → Earth 33°C warmer. Without it: −18°C. Enhanced: human CO₂ from fossil fuels → traps more heat → global warming → climate change.

Fig. 13.4

Goldilocks Zone — Why Earth Supports Life

Habitable zone: range from star where liquid water can exist. Mercury/Venus: too hot. Mars: too cold. Earth: just right! Earth also has: right atmosphere (O₂, N₂), magnetic field (blocks solar radiation), large Moon (stabilises axial tilt), right mass (holds atmosphere).

Fig. 13.5

Atmospheric Layers — Troposphere to Exosphere

Troposphere (0–12 km): weather occurs, temperature falls with height. Stratosphere (12–50 km): ozone layer absorbs UV. Mesosphere (50–85 km): meteors burn, coldest layer. Thermosphere (85–600 km): aurora borealis, temperature rises to 1500°C (sparse). Exosphere: fades into space.

Fig. 13.7

Water Cycle — Evaporation to Precipitation

Evaporation (Sun's heat): water → vapour. Transpiration: plants release vapour. Both → Condensation (cooling at altitude) → clouds form. Precipitation: rain, hail, snow. Runoff → rivers → sea. Infiltration → groundwater. ~9 days for a water molecule to complete cycle. 97% of Earth's water is salty.