Comprehensive NCERT Science Notes for Class VIII

EDUCATIONAL PHILOSOPHY AND FRAMEWORK

• National Curriculum Framework (NCF) 2005:     * Recommends linking children’s school life to their life outside the school.     * Marks a departure from the legacy of bookish learning.     * Aims to discourage rote learning and sharp boundaries between subject areas.     * Follows the National Policy on Education (1986) toward a child-centred system.     * Recognizes that children generate new knowledge when given space, time, and freedom.     * Emphasizes treating children as participants in learning rather than receivers of fixed knowledge.
• Educational Methodology:     * Higher priority is given to contemplation, wondering, small group discussion, and hands-on activities.     * Interactive characters Boojho and Paheli are utilized to pose questions and build concepts from student experiences.     * Extended Learning: Non-evaluative, voluntary projects/activities designed for interaction with experts and society.

CROP PRODUCTION AND MANAGEMENT

• Agriculture Definition: When plants of the same kind are cultivated at one place on a large scale, it is called a crop (e.g., a wheat crop).
• Historical Context: Humans were nomadic until $10,000\,B.C.E.$, wandering for food and shelter before discovering land cultivation.
• Major Cropping Patterns in India:     1. Kharif Crops: Sown in the rainy season (June to September). Examples: Paddy, maize, soyabean, groundnut, and cotton.     2. Rabi Crops: Grown in the winter season (October to March). Examples: Wheat, gram, pea, mustard, and linseed.     3. Summer Crops: Pulses and vegetables grown during summer at many places.
• Basic Practices of Crop Production:     1. Preparation of Soil: Most important task involves turning and loosening soil (tilling/ploughing) to allow roots to breathe and earthworms/microbes to grow.     2. Sowing: Selection of clean, healthy, high-yield seeds; tested by floating in water (damaged seeds are hollow/lighter).     3. Adding Manure and Fertilisers: Nutrients for healthy growth.     4. Irrigation: Supply of water at regular intervals.     5. Protecting from Weeds: Removing undesirable plants.     6. Harvesting: Cutting mature crops.     7. Storage: Protecting produce from moisture, insects, and rats.
• Agricultural Implements:     * Plough: Made of wood/iron; contains a ploughshare (triangular iron strip) and a ploughshaft (long log of wood).     * Hoe: Used for removing weeds and loosening soil; has a long rod with a strong, broad bent plate of iron.     * Cultivator: Tractor-driven; saves labor and time.     * Seed Drill: Sows seeds uniformly at equal distance and depth; protects seeds from birds.
• Manure vs. Fertilisers:     * Fertiliser: Man-made inorganic salt produced in factories; rich in Nitrogen ($N$), Phosphorus ($P$), and Potassium ($K$); lacks humus.     * Manure: Natural organic substance from decomposition of cattle dung and plant residue; provides a lot of humus; improves soil texture and water-holding capacity.
• Irrigation Methods:     * Traditional: Moat (pulley system), chain pump, dhekli, and rahat (lever system). Cheaper but less efficient.     * Modern Sprinkler System: Vertical pipes with rotating nozzles; useful for uneven land.     * Modern Drip System: Water falls drop-by-drop at roots; best for fruit plants/gardens; zero water waste.
• Weeding and Harvesting:     * Weedicides: Chemicals like $2,4-D$ used to kill weeds without damaging crops.     * Threshing: Separating grain from chaff; done by a Combine machine (harvester + thresher).     * Silos and Granaries: Large-scale storage structures to protect grains from pests.

MICROORGANISMS: FRIEND AND FOE

• Classification: Four major groups: Bacteria, Fungi, Protozoa, and some Algae.
• Viruses: Microscopic but reproduce only inside host cells (bacteria, plant, or animal).
• Friendly Microbes:     * Curd and Bread: Lactobacillus bacterium promotes curd formation; Yeast converts sugar into alcohol (Fermentation, discovered by Louis Pasteur in $1857$).     * Medicinal Use: Antibiotics (e.g., Streptomycin, Tetracycline, Erythromycin) made from fungi/bacteria. Penicillin was discovered by Alexander Fleming in $1929$.     * Vaccines: Edward Jenner discovered the smallpox vaccine in $1798$. Vaccination introduces dead/weakened microbes to produce antibodies.     * Soil Fertility: Rhizobium and blue-green algae fix atmospheric nitrogen.
• Harmful Microbes (Pathogens):     * Communicable Diseases: Spread via air, water, food, or contact (e.g., cholera, common cold, chicken pox, TB).     * Carriers: Female Anopheles mosquito (malaria parasite Plasmodium); Female Aedes mosquito (dengue virus).     * Animal Diseases: Anthrax (bacterium Bacillus anthracis discovered by Robert Koch in $1876$); Foot and mouth disease in cattle (virus).
• Food Preservation Techniques:     * Chemical Method: Using preservatives like Sodium Benzoate or Sodium Metabisulphite.     * Salting/Sugar/Oil/Vinegar: Inhibits microbial growth.     * Pasteurisation: Heating milk to $70^\circ C$ for $15-30$ seconds, then chilling to kill microbes.
• Nitrogen Cycle: Atmosphere is $78\%\,N_2$. It is fixed into usable compounds by bacteria/blue-green algae, then cycled through plants, animals, and soil back to the atmosphere.

SYNTHETIC FIBRES AND PLASTICS

• Polymers: Large single units made of many repeating small chemical units (Greek: poly = many, mer = part/unit). Cellulose is a natural polymer made of glucose units.
• Synthetic Fibres:     * Rayon (Artificial Silk): Obtained by chemical treatment of wood pulp.     * Nylon: First fully synthetic fibre ($1931$); made from coal, water, and air. Stronger than steel wire.     * Polyester: Does not wrinkle; PET (polyethylene terephthalate) used for bottles/utensils; Terylene is a popular variety.     * Acrylic: Artificial wool; cheaper and more durable than natural wool.
• Plastics:     * Thermoplastics: Deform easily on heating (e.g., Polythene, PVC). Used for toys/combs.     * Thermosetting Plastics: Cannot be softened after molding (e.g., Bakelite for electrical switches; Melamine for floor tiles/fire-resistant fabrics).     * Environemntal Impact: Microbes cannot decompose plastics (Non-biodegradable). 5 R Principle: Reduce, Reuse, Recycle, Recover, Refuse.

MATERIALS: METALS AND NON-METALS

• Physical Properties:     * Metals: Malleable (beaten into sheets), Ductile (drawn into wires), Sonorous (ringing sound), High luster, Good conductors of heat/electricity. (Ex: Iron, Copper). Mercury is liquid at room temperature.     * Non-metals: Soft/Dull, Brittle (breaks into powder), Poor conductors. (Ex: Sulphur, Carbon).
• Chemical Properties:     * Reaction with Oxygen: Metals produce basic oxides (red litmus turns blue); Non-metals produce acidic oxides (blue litmus turns red).     * Reaction with Water: Sodium reacts vigorously (stored in kerosene); most non-metals do not react with water (Phosphorus stored in water to prevent reaction with air).     * Displacement Reactions: A more reactive metal replaces a less reactive metal in its salt solution (e.g., Zinc displaces Copper from Copper Sulphate).

COAL AND PETROLEUM

• Fossil Fuels: Formed from dead remains of living organisms millions of years ago.
• Coal:     * Coke: Tough, porous, black, almost pure carbon.     * Coal Tar: Thick black liquid; mixture of about $200$ substances.     * Coal Gas: By-product used as fuel in industries.
• Petroleum: Derived from organisms in the sea. Refining separates it into LPG, Petrol, Kerosene, Diesel, Paraffin wax, and Bitumen.
• Natural Gas: Stored as CNG (Compressed Natural Gas); cleaner fuel with less pollution.

COMBUSTION AND FLAME

• Combustion: Chemical process where a substance reacts with Oxygen to give off heat; requires fuel, oxygen, and reaching the Ignition Temperature (lowest temperature to catch fire).
• Inflammable Substances: Very low ignition temperature (e.g., Petrol, LPG).
• Flame Structure:     1. Innermost Zone: Unburnt wax vapors (black), coolest.     2. Middle Zone: Partial combustion (yellow), luminous, moderately hot.     3. Outermost Zone: Complete combustion (blue), non-luminous, hottest.
• Fuel Efficiency: Measured by Calorific Value (unit: $kJ/kg$). Hydrogen has highest calorific value ($150,000\,kJ/kg$).

CONSERVATION OF PLANTS AND ANIMALS

• Deforestation Consequences: Increased $CO_2$ (Global Warming), disturbed water cycle (Droughts), decreased soil fertility/infiltration, and Desertification.
• Classification of Areas:     * Biosphere Reserve: Large area for biodiversity conservation and tribal life (e.g., Pachmarhi).     * National Park: Reserved for wildlife to freely use habitats/natural resources (e.g., Satpura).     * Wildlife Sanctuary: Protection of animals from disturbance.
• Species Status:     * Endemic Species: Found exclusively in a particular area (e.g., Giant Squirrel in Pachmarhi).     * Endangered Species: Facing danger of extinction.     * Red Data Book: Record of all endangered animals and plants.

CELL: STRUCTURE AND FUNCTIONS

• Discovery: Robert Hooke ($1665$) using cork slices.
• Cell Concept: Basic structural and functional unit of life.
• Prokaryotic vs. Eukaryotic: Prokaryotic cells (e.g., Bacteria) lack a nuclear membrane; Eukaryotic cells (e.g., Onion, Human cheeks) have a well-organized nucleus with a membrane.
• Major Parts:     * Cell Membrane: Porous, allows movement of substances.     * Cytoplasm: Jelly-like substance containing organelles (Mitochondria, Ribosomes, etc.).     * Nucleus: Control center containing Chromosomes (thread-like, carry genes).     * Cell Wall: Additional outer layer in plant cells for protection/rigidity.     * Vacuoles: Large/central in plants; small in animals.     * Plastids: Found only in plants; Chloroplasts contain green chlorophyll for photosynthesis.

REPRODUCTION IN ANIMALS

• Sexual Reproduction: Fusion of male/female gametes forming a Zygote.     * Male Organs: Testes (produce sperms), sperm ducts, penis.     * Female Organs: Ovaries (produce ova/eggs), oviducts, uterus (development site).     * IVF (In Vitro Fertilisation): Fertilisation outside the body; "test-tube babies."     * Embryo and Foetus: Embryo is the developing ball of cells; Foetus is the stage where body parts are identifiable.     * Viviparous: Give birth to young (Humans); Oviparous: Lay eggs (Hens).
• Metamorphosis: Drastic changes from larva to adult (Tadpole to Frog).
• Asexual Reproduction: Single parent.     * Budding: Development from outgrowths (Hydra).     * Binary Fission: Division into two individuals (Amoeba).
• Cloning: Exact copy of a cell/organism. Dolly the sheep (born $5\,July\,1996$) was the first cloned mammal.

REACHING THE AGE OF ADOLESCENCE

• Adolescence: Period from age $11$ to $18/19$ characterized by reproductive maturity.
• Puberty Changes: Sudden height increase, voice change (Adam's Apple in boys), secondary sexual characters (facial hair, breast development).
• Endocrine Glands: Ductless glands; secrete hormones directly into the bloodstream.     * Pituitary: Growth hormone; master gland.     * Thyroid: Thyroxine (deficiency causes Goitre).     * Pancreas: Insulin (deficiency causes Diabetes).     * Adrenal: Adrenalin (stress management; salt balance).
• Sex Determination: Human cells have $23\,pairs$ of chromosomes. Females have $XX$; Males have $XY$. Father's sperm determines the baby's sex ($X$ sperm = girl, $Y$ sperm = boy).

FORCE AND PRESSURE

• Force: A push or pull resulting from interaction between objects. Can change speed, direction, or shape.
• Contact Forces: Muscular force, Friction.
• Non-contact Forces: Magnetic, Electrostatic, Gravitational (all objects exert gravitational force).
• Pressure: Force acting on a unit area ($P = \frac{F}{A}$). Smaller area = higher pressure.
• Atmospheric Pressure: Force of gravity on an air column. Equalized inside human bodies to prevent crushing.

FRICTION

• Mechanism: Caused by interlocking of irregularities in surfaces. Depends on force pressing surfaces together.
• Types:     * Static Friction: To start an object from rest.     * Sliding Friction: When object is moving; smaller than static friction.     * Rolling Friction: When body rolls; smallest friction. Replaces sliding via Ball Bearings.
• Fluid Friction (Drag): Friction exerted by gases and liquids; reduced by streamlined shapes.

SOUND

• Production: By vibrating bodies. In humans, via Larynx (vocal cords).
• Propagation: Needs a medium (Solid, Liquid, or Gas); cannot travel in vacuum.
• Characteristics:     * Frequency: Oscillations per second ($Hertz, Hz$). Determines Pitch.     * Amplitude: Maximum displacement. Determines Loudness (measured in decibels, dB).
• Hearing: Range for humans is $20\,Hz$ to $20,000\,Hz$.

LIGHT

• Reflection Laws:     1. Angle of incidence ($\angle i$) = Angle of reflection ($\angle r$).     2. Incident ray, normal, and reflected ray lie in the same plane.
• Periscope: Uses two parallel mirrors to see objects out of sight.
• Kaleidoscope: Multiple reflections creating beautiful patterns.
• Dispersion: Splitting of white light into seven colors (Rainbow).
• Eye Anatomy: Cornea, Iris (controls light), Pupil, Lens, Retina (contains cones/rods), Optic nerve.
• Braille System: $63$ dot patterns developed by Louis Braille for visually impaired readers.

STARS AND THE SOLAR SYSTEM

• Celestial Objects: Stars (emit own light), Planets (reflect light), Moon (phases cycle approx. $29\,days$).
• Units: Light Year = distance light travels in one year at $300,000\,km/s$.
• Constellations: Recognizable patterns like Ursa Major (Great Bear), Orion (Hunter), Cassiopeia ($W/M$ shape).
• Solar System Members:     * Sun: Nearest star.     * Inner Planets: Mercury, Venus, Earth, Mars (Few moons, close to Sun).     * Outer Planets: Jupiter, Saturn, Uranus, Neptune (Large number of moons, ring systems).     * Other: Asteroids (between Mars/Jupiter), Comets (long tails away from Sun), Meteors.

POLLUTION OF AIR AND WATER

• Air Pollution: Contamination by substances like $CO$, $CO_2$, $SO_2$, $NO_2$, and CFCs (damage ozone layer).     * Acid Rain: $SO_2$ and $NO_2$ form sulphuric/nitric acids; causes "marble cancer" on Taj Mahal.     * Greenhouse Effect: Trapping of heat by gases like $CO_2$, methane, and water vapor; lead to Global Warming.
• Water Pollution: Contamination by sewage, toxic industrial chemicals (arsenic, lead), and pesticides.     * Potable Water: Suitable for drinking; purified via filtration, boiling, or chlorination.     * Case Study: Ganga River is heavily endangered; National Mission for Clean Ganga was launched in $2016$.