Class 8 Science — Complete Study Notes

Class 8 Science is divided into Biology, Chemistry, and Physics sections. This guide covers all NCERT chapters with diagrams, key points, and exam-focused questions.

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Biology Section

Chapter 1: Crop Production and Management

Agricultural Practices:

1. Preparation of soil (tilling/ploughing)
2. Sowing — using seeds at proper depth and spacing
3. Adding manure and fertilizers
4. Irrigation — supply of water
5. Protection from weeds — weeding
6. Harvesting — cutting the mature crop
7. Storage of grains

Types of Crops by Season:

| Season | Name | Period | Examples | |--------|------|--------|---------| | Kharif | Summer/Monsoon | June–September | Rice, Maize, Cotton, Soybean | | Rabi | Winter | October–March | Wheat, Gram, Pea, Mustard | | Zaid | Between kharif and rabi | March–June | Watermelon, Muskmelon, Cucumber |

Irrigation Methods:

• Moat (Pulley system): Traditional, bullock-powered
• Chain pump: Continuous chain with buckets
• Drip system: Water drips near roots — most efficient, saves 40-60% water
• Sprinkler system: Like rainfall, good for uneven ground

Fertilizer vs Manure:

| Fertilizer | Manure | |-----------|--------| | Inorganic, chemically made | Organic, from decomposed material | | Rich in specific nutrients | Contains all nutrients in small amounts | | Adds no humus | Adds humus, improves soil texture | | Example: Urea, DAP | Example: Compost, vermicompost |

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Chapter 2: Microorganisms

Types of Microorganisms:

| Type | Examples | Size | Visibility | |------|---------|------|-----------| | Bacteria | Lactobacillus, E. coli | 1–10 μm | Microscope | | Fungi | Yeast, Aspergillus, Penicillium | Variable | Some visible | | Protozoa | Amoeba, Paramecium | 10–100 μm | Microscope | | Algae | Spirogyra, Chlamydomonas | Variable | Some visible | | Virus | Influenza, HIV, SARS-CoV | 20–400 nm | Electron microscope only |

Useful Microorganisms:

1. Food production:

   • Yeast → bread, beer (fermentation)
   • Lactobacillus → curd (converts milk to curd)
   • Fungi → cheese making

2. Medicine:

   • Penicillin (antibiotic) — from Penicillium fungus (discovered by Alexander Fleming, 1928)
   • Vaccines made from weakened/killed microbes

3. Agriculture:

   • Rhizobium bacteria in root nodules → fix atmospheric nitrogen
   • Used as biofertilizers

4. Cleaning environment:

   • Decomposers break down dead matter → recycling nutrients

Harmful Microorganisms (Disease Agents):

| Disease | Causative Agent | Transmission | |---------|----------------|-------------| | Tuberculosis | Bacteria (Mycobacterium tuberculosis) | Air droplets | | Typhoid | Bacteria (Salmonella typhi) | Contaminated food/water | | COVID-19 | Virus (SARS-CoV-2) | Air droplets, contact | | Malaria | Protozoa (Plasmodium) | Anopheles mosquito | | Cholera | Bacteria (Vibrio cholerae) | Contaminated water | | Ringworm | Fungi | Direct contact |

Food Preservation Methods:

• Salt and sugar (osmosis removes water)
• Vinegar / Pickling (acidic — kills bacteria)
• Oil (prevents air contact)
• Refrigeration (slows microbial growth)
• Pasteurization: heating to 70°C for 15 seconds
• Dehydration/Sun drying
• Chemical preservatives: sodium benzoate, sodium meta-bisulphite

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Chapter 3: Cell — Structure and Functions

Cell Theory (3 main points):

1. All living beings are made of cells
2. Cell is the basic structural and functional unit of life
3. All cells arise from pre-existing cells

Types of Organisms:

• Unicellular: Single cell performs all functions (Amoeba, Paramecium, bacteria, yeast)
• Multicellular: Many cells, each specialized (humans, plants, animals)

Comparison: Plant Cell vs Animal Cell

| Feature | Plant Cell | Animal Cell | |---------|-----------|------------| | Cell wall | Present (cellulose) | Absent | | Chloroplast | Present | Absent | | Vacuole | Large central vacuole | Small, scattered | | Centriole | Absent | Present | | Shape | Fixed rectangular | Irregular |

Cell Organelles and Functions:

| Organelle | Function | |-----------|---------| | Nucleus | Controls all cell activities; contains DNA | | Mitochondria | Powerhouse — produces energy (ATP) via respiration | | Chloroplast | Photosynthesis in plant cells | | Ribosome | Protein synthesis | | Endoplasmic Reticulum | Transport network; Rough ER has ribosomes | | Golgi Apparatus | Packages and sends proteins | | Vacuole | Storage of water, food, waste | | Lysosome | Digestion of waste, "suicidal bags" |

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Chapter 4: Reproduction in Animals

Types of Reproduction:

Asexual Reproduction (one parent):

• Binary fission: Amoeba, bacteria (splits into two)
• Budding: Hydra, yeast (new organism grows as bud)

Sexual Reproduction (two parents):

• Requires male gamete (sperm) + female gamete (egg/ovum)
• Fertilization: fusion of sperm and egg

Viviparous vs Oviparous:

• Viviparous: Give birth to young ones (humans, cows, dogs)
• Oviparous: Lay eggs (birds, reptiles, fish, frogs)

Human Reproduction:

• Male gamete: Sperm (produced in testes)
• Female gamete: Ovum/Egg (produced in ovaries)
• Fertilization: in fallopian tube
• Zygote develops into embryo → foetus → baby (after 9 months)

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Chemistry Section

Chapter 5: Coal and Petroleum

Fossil Fuels: Formed from dead organisms buried under earth over millions of years.

• Coal: from dead plants, found in seams/layers
• Petroleum: from dead marine organisms, found in rock formations
• Natural Gas: trapped above petroleum deposits

Coal Products (Destructive Distillation of Coal):

• Coke: nearly pure carbon, used in steelmaking
• Coal tar: black thick liquid, makes dyes, drugs, explosives
• Coal gas: used as fuel, lighting

Petroleum Refining (Fractional Distillation):

| Product | Boiling Point | Uses | |---------|--------------|------| | Petroleum gas (LPG) | Below 40°C | Cooking fuel | | Petrol | 40–120°C | Vehicle fuel | | Kerosene | 120–240°C | Aviation fuel, stoves | | Diesel | 240–340°C | Heavy vehicles, generators | | Fuel oil | 340–400°C | Ships, industrial furnaces | | Lubricating oil | >400°C | Lubricants, wax | | Bitumen/Asphalt | Residue | Road surfacing |

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Chapter 6: Combustion and Flame

Conditions for Combustion:

1. Presence of fuel (combustible material)
2. Supply of oxygen/air
3. Heat above ignition temperature

Types of Combustion:

| Type | Description | Example | |------|-------------|---------| | Rapid | Burns quickly with flame | Burning LPG | | Spontaneous | Self-ignition, no external heat | Phosphorus in air | | Explosive | Very rapid, creates blast | Firecrackers |

Ignition Temperature: Minimum temperature at which a substance burns.

Inflammable substances: Very low ignition temperature — petrol, alcohol, LPG. Handle with care!

Parts of a Candle Flame:

Outer luminous zone → Complete combustion, blue/white, hottest
Middle yellow zone → Partial combustion, yellow glow
Dark inner zone → Unburned wax vapour, cool

Fire Extinguisher — How it Works:

• Removes heat OR removes oxygen OR removes fuel
• CO₂ extinguisher: smothers fire by displacing oxygen
• Water: cools below ignition temperature (NOT for electrical/oil fires)

Calorific Value: Amount of heat produced by complete combustion of 1 kg of fuel (unit: kJ/kg)

| Fuel | Calorific Value (kJ/kg) | |------|------------------------| | Wood | 17,000 | | Coal | 25,000–33,000 | | Petrol | 45,000 | | Natural Gas/LPG | 55,000 | | Hydrogen | 1,50,000 |

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Physics Section

Chapter 7: Force and Pressure

Force: A push or pull on an object.

• Can change: speed, direction, shape, size

Types of Forces:

• Contact forces: Friction, normal force, tension
• Non-contact forces: Gravity, magnetic force, electrostatic force

Pressure: Force per unit area

Formula: Pressure (P) = Force (F) / Area (A)

• Unit: Pascal (Pa) = N/m²

Atmospheric Pressure: Pressure exerted by air — 1,01,325 Pa at sea level

Liquid Pressure:

• Increases with depth: P = ρgh
• Acts equally in all directions (Pascal's Law)
• Applications: Hydraulic brakes, hydraulic lift

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Chapter 8: Friction

Friction: Opposing force acting between two surfaces in contact.

Types of Friction:

| Type | Description | Example | |------|-------------|---------| | Static | Before object moves | Pushing a heavy box | | Sliding/Kinetic | Object in sliding motion | Sliding on floor | | Rolling | Object rolling on surface | Wheels, ball bearings | | Fluid | In liquids/gases | Ship in water, bird in air |

Order: Rolling friction < Sliding friction < Static friction

Factors affecting friction:

• Nature of surfaces (rough/smooth)
• Weight of object (heavier = more friction)
• NOT dependent on area of contact (for most cases)

Advantages of Friction:

• Walking, gripping, braking, writing

Disadvantages of Friction:

• Wear and tear, heat generation, energy loss

Reducing Friction:

• Lubricants (oil, grease)
• Ball bearings
• Streamlining shapes
• Polishing surfaces

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Chapter 9: Sound

Production: Sound is produced by vibrating objects.

• Vibration → compression and rarefaction → longitudinal waves

Key Terms:

• Frequency (f): Number of vibrations per second — unit: Hertz (Hz)
• Amplitude (A): Maximum displacement — determines loudness
• Time Period (T): Time for one complete vibration — T = 1/f
• Wavelength (λ): Distance between two compressions

Human Hearing Range: 20 Hz to 20,000 Hz (20 kHz)

• Below 20 Hz = Infrasound (used by elephants, whales)
• Above 20,000 Hz = Ultrasound (used by bats, dolphins)

Ultrasound Applications:

• Sonar (detecting underwater objects)
• Medical imaging (echocardiography, pregnancy scan)
• Cleaning delicate instruments

Speed of Sound:

• In air (20°C): 344 m/s
• In water: ~1,500 m/s
• In steel: ~5,000 m/s
• Sound travels fastest in solids, slowest in gases

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Chapter 10: Light

Laws of Reflection:

1. Angle of incidence = Angle of reflection
2. Incident ray, reflected ray, and normal are in the same plane

Regular vs Diffuse Reflection:

• Regular reflection: Smooth surface, clear image (mirrors)
• Diffuse reflection: Rough surface, no clear image (walls)

Dispersion of Light:

• White light splits into 7 colors through prism: VIBGYOR
• Violet, Indigo, Blue, Green, Yellow, Orange, Red
• Violet bends most, Red bends least

Human Eye Parts:

| Part | Function | |------|---------| | Cornea | Transparent protective layer, bends light | | Iris | Colored part, controls pupil size | | Pupil | Black opening, adjusts light entry | | Lens | Focuses light on retina | | Retina | Detects light, sends signal to brain | | Optic nerve | Carries signals to brain |

Braille: System for visually impaired — raised dots pattern read by touch. Developed by Louis Braille.

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Quick Revision Checklist — Class 8 Science

Biology:

• [ ] Kharif crops (June–Sep) vs Rabi crops (Oct–Mar)
• [ ] Drip irrigation is most water-efficient
• [ ] Cell organelles and their functions
• [ ] Plant cell has cell wall, large vacuole, chloroplast
• [ ] Viviparous (live birth) vs Oviparous (egg laying)

Chemistry:

• [ ] Fossil fuels — coal, petroleum, natural gas
• [ ] Products of petroleum distillation (LPG to bitumen)
• [ ] Conditions for combustion (fuel + O₂ + heat)
• [ ] Types: Rapid, Spontaneous, Explosive combustion
• [ ] Calorific value: Hydrogen highest, Wood lowest

Physics:

• [ ] Pressure = Force/Area (Pascal)
• [ ] Rolling < Sliding < Static friction
• [ ] Sound: Frequency(Hz), Amplitude(loudness)
• [ ] Human hearing: 20 Hz to 20,000 Hz
• [ ] Laws of reflection
• [ ] VIBGYOR — dispersion order