Science — Class 9

A simplified representation of a real system focusing only on what is most important. Ignores unnecessary details on purpose. E.g., treating Earth as a smooth sphere in earth science.

Describes a regular pattern observed in nature, often as a mathematical relationship. E.g., Newton's laws of motion. Does NOT explain why — just what happens.

Explains WHY patterns occur, based on evidence. NOT a guess. Always open to revision. E.g., Atomic theory explains molecule formation. More powerful than a law.

Indian physicist who treated star matter as a hot gas (ignoring complex details) to explain how star colour connects to temperature — a perfect example of scientific modelling.

International System of Units — agreed standards used worldwide. E.g., kg (mass), m (length), s (time). Avoids dangerous errors from unit confusion, like the aircraft fuel incident.

From Latin word "celeritas" meaning speed. Exactly 299,792,458 m/s. Scientific symbols come from history and international agreements, not convenience.

Finding an approximate answer using reasoning. More important in science than exact calculation. E.g., We breathe ~10,000 litres of air per day — estimated, not measured exactly.

Basic structural and functional unit of all living organisms. First observed by Robert Hooke in 1665 in cork using a self-designed microscope (200-300X magnification).

Cell with no membrane-bound nucleus — genetic material is in the nucleoid. Also lacks membrane-bound organelles. Smaller and simpler. E.g., Bacteria.

Larger, more complex cell with a well-defined nucleus and membrane-bound organelles. E.g., Plant cells, animal cells, fungi. Has chromosomes in nucleus.

Rigid outer covering outside plasma membrane. Made of cellulose in plants. Present in plants, fungi and bacteria. Absent in animal cells — which is why animals can change shape easily.

Powerhouse of the cell. Produces energy via cellular respiration. Has its own DNA. Number varies — cells needing more energy have more mitochondria (e.g., muscle cells).

Found only in plant cells. Contains chlorophyll. Site of photosynthesis — converts sunlight + CO₂ + water into glucose and oxygen. A type of plastid.

Cell division producing 2 genetically identical daughter cells. Each gets same DNA and chromosome number as parent. Used for growth, repair and asexual reproduction.

Two-step division producing 4 daughter cells, each with HALF the chromosomes. Occurs in reproductive organs (testes/ovaries). Creates genetic diversity. Restored at fertilisation.

Minimum distance between two points that allows them to be seen as separate. Human eye: 0.1 mm at 25 cm. Microscopes overcome this — light microscopes use lenses to magnify.

Group of similar cells working together for a specific function. Tissues form organs, organs form organ systems. Leads to division of labour in the body.

Plant tissue with continuously dividing cells. Found at growing tips (apical), sides (lateral) and nodes (intercalary). Responsible for plant growth in length and girth.

Transports water and dissolved minerals from roots to leaves. Made of dead cells for efficient conduction. Part of the plant's vascular system.

Transports food (glucose made by photosynthesis) from leaves to all parts of the plant. Made of living cells. Food moves in all directions — up and down.

Voluntary muscle attached to bones. Cylindrical cells with multiple nuclei and prominent striations. Under conscious control. E.g., Biceps, leg muscles.

Involuntary muscle in organs like stomach and intestines. Spindle-shaped cells with single nucleus and no striations. Controls slow, continuous movements like digestion.

Found only in the heart. Cylindrical branched cells with single nucleus, faint striations. Involuntary — works tirelessly and rhythmically throughout life without fatigue.

Nerve cell — basic unit of nervous tissue. Three parts: (1) Cell body (has nucleus), (2) Dendrites (receive signals), (3) Axon (carries signals to axon terminals → other cells).

Shortest distance between initial and final position, in a specific direction. Vector quantity (has magnitude + direction). Can be less than, equal to, or zero for the same distance.

v = final velocity, u = initial velocity, a = acceleration, t = time. Valid only for constant acceleration. Derived from the definition of acceleration.

s = displacement, u = initial velocity, a = acceleration, t = time. Gives position at any time during constant acceleration. Derived from velocity-time graph area.

Relates velocity, acceleration and displacement WITHOUT time. Derived by combining 1st and 2nd equations. Very useful when time is not given or not needed.

Constant speed in a circular path. Speed is constant but VELOCITY changes (direction changes continuously). Therefore acceleration exists, directed towards the centre (centripetal).

Homogeneous mixture with particle size <1 nm. Particles invisible, do not scatter light, cannot be filtered. E.g., Salt water, sugar water. Same composition throughout.

Particle size 1–1000 nm. Particles don't settle, show Tyndall Effect. Appears homogeneous but not truly so. E.g., Milk, butter, smoke, fog, blood.

Heterogeneous mixture, particle size >1000 nm. Visible particles that settle on standing. Can be separated by filtration. E.g., Muddy water, sand in water.

Scattering of light by colloid particles making the path of light visible. Named after John Tyndall. Distinguishes colloids from true solutions (solutions don't scatter light).

Spinning a mixture at high speed — denser components move outward and settle. Used to separate blood (RBCs from plasma), cream from milk, clarifying fruit juices.

Object at rest stays at rest; object in motion stays in motion with constant velocity — UNLESS a net force acts on it. Formulated by Newton based on Galileo's thought experiments.

F = ma. Net force = mass × acceleration. Direction of acceleration same as net force. 1 N = force giving 1 kg object an acceleration of 1 m/s².

When object A exerts force on B, B exerts equal and opposite force on A simultaneously. Forces act on DIFFERENT objects (so they don't cancel). Always in pairs.

Resistance to change in state of rest or motion. Coined by Newton. Greater mass = greater inertia. E.g., A heavy truck is harder to stop than a bicycle moving at the same speed.

Force opposing relative motion of surfaces in contact. Acts OPPOSITE to direction of motion. Always present when surfaces touch. Helps walking but hinders motion — can be reduced by lubricants.

g = 9.8 m/s² near Earth's surface (use 10 m/s² for quick estimates). Weight = gravitational force pulling object towards Earth's centre. Measured by spring balance.

W = F × s × cos θ. Work is done when force causes displacement. Zero work if: no displacement, OR force ⊥ displacement (θ=90°). E.g., Carrying bag horizontally = 0 work done on bag.

KE = ½mv². Energy due to motion. Depends on mass and SQUARE of velocity — doubling speed quadruples KE. All moving objects have KE (moving bicycle, rolling ball).

PE = mgh. Energy stored due to position. Valid only near Earth's surface where g is constant. At greater heights, g decreases so formula changes. E.g., Water in a dam, a raised hammer.

KE + PE = constant for a freely falling object (no friction). As object falls: PE decreases, KE increases by same amount. Total mechanical energy stays the same throughout the fall.

P = W/t = Work done ÷ Time. Rate of doing work. Unit: Watt (W). 1 W = 1 J/s. 1 HP = 746 W. A 100W bulb converts 100 J of electrical energy every second.

Ancient Indian philosopher who proposed that matter (dravya) if divided repeatedly reaches smallest indivisible particles called parmanus. Recorded in Sanskrit text Vaisesika Sutras — one of the earliest atomic theories.

Proposed electrons are embedded in a positively charged sphere — like plums in a pudding. Disproved by Rutherford's gold foil experiment which showed positive charge is concentrated, not spread out.

Atom = mostly empty space. Dense positively charged nucleus at centre, electrons revolve around it (planetary model). Nucleus diameter ≈ 10⁻¹⁵ m, atom diameter ≈ 10⁻¹⁰ m. Limitation: couldn't explain atomic stability.

Electrons move in fixed circular orbits (shells/energy levels) K, L, M, N. In each shell, energy is constant — electrons don't lose energy. Explained atomic stability. Shells named from X-ray experiments by Charles Barkla.

Discovered by James Chadwick in 1932. Neutral particle (no charge) with mass nearly equal to proton. Found in nucleus of all atoms except hydrogen. Explains why helium is 4× heavier than hydrogen despite having only 2× the protons.

Number of protons in the nucleus. Uniquely identifies an element. In a neutral atom, atomic number = number of electrons. E.g., Carbon Z=6 means 6 protons and 6 electrons.

Total number of nucleons (protons + neutrons) in the nucleus. A = Z + number of neutrons. E.g., ¹²C has 6 protons + 6 neutrons = mass number 12.

Atoms of the SAME element with same atomic number but DIFFERENT mass numbers (different neutrons). E.g., ³⁵Cl and ³⁷Cl — both have 17 protons but 18 and 20 neutrons respectively. Average atomic mass = weighted average.

Atoms of DIFFERENT elements with same mass number but different atomic numbers. E.g., Ca (Z=20), K (Z=19), Ar (Z=18) all have mass number 40. Same nucleons, different elements.

K shell (n=1): 2×1²=2. L shell (n=2): 2×2²=8. M shell (n=3): 2×3²=18. BUT outermost shell max = 8 electrons. Shells fill in order K→L→M→N.

Combining capacity of an atom. Equal to electrons that can be gained, lost or shared to achieve stable octet configuration. E.g., Sodium (2,8,1) has valency 1 — it loses 1 electron. Oxygen (2,6) has valency 2 — it gains 2 electrons.

Mass can neither be created nor destroyed in a chemical reaction. Total mass of reactants = total mass of products. E.g., 2g H₂ + 16g O₂ → 18g H₂O. Basis of Dalton's Atomic Theory.

A compound always contains the same elements in the same fixed ratio by mass, regardless of how it was made or where obtained. E.g., Water is always 1:8 (H:O) by mass wherever it comes from.

Formed by SHARING of electrons between atoms. Both atoms contribute electrons to a shared pair. E.g., H₂ (each H shares 1 electron), Cl₂, H₂O, CO₂. Low melting point, usually poor conductor of electricity.

Formed by TRANSFER of electrons. One atom loses electrons (becomes cation +), another gains (becomes anion −). E.g., NaCl — Na loses 1e⁻ → Na⁺, Cl gains 1e⁻ → Cl⁻. High melting point, conducts electricity when dissolved.

Criss-cross method: write symbols, write valencies below, swap valencies as subscripts. E.g., Ca²⁺ + Cl¹⁻ → CaCl₂. Al³⁺ + O²⁻ → Al₂O₃. Cancel equal subscripts to get simplest ratio.

Sum of atomic masses of all atoms in a molecule. E.g., H₂O = (1×2) + 16 = 18u. CO₂ = 12 + (16×2) = 44u. HNO₃ = 1+14+48 = 63u. Used for covalent compounds.

Mass of one formula unit of an ionic compound. E.g., NaCl = 23+35.5 = 58.5u. Ca(NO₃)₂ = 40+(14+48)×2 = 164u. Ionic compounds don't form molecules — they form crystal lattices.

Wave where particles vibrate PARALLEL to the direction of wave propagation. Sound is a longitudinal wave. Creates compressions (high density) and rarefactions (low density). Needs a medium — cannot travel through vacuum.

Distance between two consecutive crests (or troughs) of a wave. SI unit: metre (m). Symbol: λ (lambda). Shorter wavelength = higher frequency. Formula: v = λν (speed = wavelength × frequency).

Number of density oscillations per unit time at a fixed point. SI unit: Hertz (Hz) = per second. Symbol: ν (nu). Inversely related to time period: ν = 1/T. Higher frequency = higher pitch of sound.

Speed = Wavelength × Frequency. Speed of sound in dry air ≈ 331 m/s at 0°C, ≈ 344 m/s at 22°C. Sound travels faster in solids > liquids > gases. Speed increases with temperature and humidity.

Reflected sound heard distinctly after the original sound. To hear an echo, reflected sound must arrive at least 0.1s after original. Minimum distance from reflector = 17m (at 340 m/s). Used in SONAR to find underwater objects.

Sound with frequency above 20,000 Hz (20 kHz) — above human hearing. Used in: SONAR (sea depth), medical imaging (ultrasound scans), echolocation by bats and dolphins, cleaning delicate objects.

20 Hz to 20,000 Hz (20 kHz). Below 20 Hz = infrasound (elephants can detect). Above 20 kHz = ultrasound (bats, dogs, dolphins can detect). Range decreases with age. Loudness measured in decibels (dB).

Asexual reproduction in plants using vegetative parts (stem, root, leaf). Produces genetically identical offspring. E.g., Potato/ginger (underground stems), Bryophyllum (leaves), money plant (stem). Used in agriculture via cutting, grafting, layering.

Transfer of pollen grains from anther to stigma. Self-pollination = same flower/plant. Cross-pollination = different plant of same species. Agents: wind (wheat, maize), water (Vallisneria), insects (sunflower, marigold), birds (coral tree).

Pollen germinates on stigma → pollen tube grows down through style → male gamete reaches ovule → fuses with egg cell → Zygote forms. Zygote → embryo. Ovary → fruit. Ovule → seed.

External: gametes meet outside body — fish, frogs. Many eggs produced, low survival. Internal: fertilisation inside female body — reptiles, birds, mammals. Fewer eggs, higher survival rate.

Monthly cycle in human females. Ovulation ≈ 14th day. If egg not fertilised → menstruation. If fertilised → implantation in uterus → pregnancy (≈9 months). Stops during pregnancy. Controlled by hormones.

Asexual reproduction via spores — tiny, lightweight, single-celled structures produced in millions. Float in air, germinate when conditions are favourable. E.g., Rhizopus (bread mould), Aspergillus. All based on mitosis → genetically identical offspring (clones).

Systematic grouping of organisms based on similarities and differences. Makes study of vast diversity manageable. Modern classification is based on evolutionary relationships, cell structure, mode of nutrition.

Prokaryotes — no true nucleus. Unicellular. E.g., Bacteria, Blue-green algae (Cyanobacteria). Some autotrophs (photosynthesis), some heterotrophs. Oldest life forms on Earth.

Eukaryotes, mostly unicellular. E.g., Amoeba, Paramecium, Euglena, algae. Can be autotrophic (algae) or heterotrophic (amoeba). Often have cilia or flagella for movement.

Eukaryotes, heterotrophic (saprophytes — feed on dead matter). Cell wall made of chitin (not cellulose). E.g., Mushrooms, Penicillium, Rhizopus, yeast. Reproduce by spores.

Gymnosperms: seeds exposed on cones, no fruits, needle-like leaves. E.g., Pine, Cycas. Angiosperms: seeds enclosed in fruits, have flowers. E.g., Mango, wheat. Most diverse plant group on Earth.

Vertebrata: have vertebral column (backbone) — fish, amphibia, reptiles, birds, mammals. Invertebrata: no notochord/backbone — insects, worms, sponges, jellyfish. Vertebrates have more complex organ systems.

Layer of gases surrounding Earth. Mostly Nitrogen (78%) and Oxygen (21%). Protects life from harmful UV radiation, regulates temperature, enables weather. Without it, Earth would be like Mars — lifeless and extreme.

Continuous movement of water: Evaporation (ocean/land → atmosphere) → Condensation (clouds) → Precipitation (rain/snow) → Runoff/infiltration → back to ocean. Driven by solar energy and gravity.

Large-scale winds caused by uneven heating between equator and poles. Equator: warm air rises (low pressure). 30° N/S: cool air sinks (high pressure). Earth's rotation deflects winds — right in Northern Hemisphere (Coriolis effect).

Large continuous movements of ocean water. Driven by: planetary winds (surface currents), temperature differences, salinity differences, Earth's rotation, distribution of land. Warm currents (from equator) moderate climate of coastal regions.

Continuous transformation between three rock types: Igneous (solidified magma) → weathering/erosion → Sedimentary (compacted layers) → heat/pressure → Metamorphic → melting → Igneous again. Takes millions of years.

Carbon moves between atmosphere, living organisms, oceans and Earth's crust. Photosynthesis removes CO₂. Respiration/decomposition/combustion releases CO₂. Human activities (burning fossil fuels) disrupting the balance → climate change.