Tamil Nadu Class 8 Science Key Terms Bank — Samacheer Kalvi

Physics Terms

Measurements

• SI unit: The International System of Units — the globally accepted measurement system (metre, kilogram, second).
• Least count: The smallest value that an instrument can measure accurately.
• Significant figures: Digits that carry meaning contributing to a measurement's precision.
• Systematic error: Errors that consistently shift measurements in one direction (e.g., faulty zero on a scale).
• Random error: Errors that vary unpredictably between measurements.

Force and Pressure

• Force: A push or pull that can change the state of motion or shape of an object. SI unit: Newton (N).
• Pressure: Force applied per unit area. P = F/A. SI unit: Pascal (Pa).
• Atmospheric pressure: The pressure exerted by the weight of the atmosphere on Earth's surface (~101,325 Pa).
• Pascal's Law: Pressure applied at any point in an enclosed fluid is transmitted equally in all directions.
• Hydraulic machine: A machine that uses Pascal's Law — small force on a small area creates large force on a large area.

Heat

• Heat: A form of energy that transfers from a body at higher temperature to one at lower temperature.
• Temperature: A measure of the average kinetic energy of particles in a substance. SI unit: Kelvin (K).
• Specific heat capacity: The amount of heat required to raise the temperature of 1 kg of a substance by 1°C. SI unit: J kg⁻¹ K⁻¹.
• Conduction: Transfer of heat through a solid without the movement of the material itself.
• Convection: Transfer of heat through fluids (liquids/gases) by movement of the fluid itself.
• Radiation: Transfer of heat through electromagnetic waves without any medium.

Light

• Reflection: The bouncing back of light from a surface.
• Refraction: Bending of light as it passes from one medium to another due to change in speed.
• Angle of incidence: Angle between the incident ray and the normal at the point of incidence.
• Angle of reflection: Angle between the reflected ray and the normal.
• Focal length: Distance between the pole and the principal focus of a mirror or lens.
• Real image: An image formed by actual convergence of rays; can be projected on a screen.
• Virtual image: An image formed by apparent divergence of rays; cannot be projected on a screen.
• Concave mirror: A mirror that is curved inward (like the inside of a spoon); can form real or virtual images.
• Convex mirror: A mirror that is curved outward; always forms virtual, diminished, erect images.

Electricity

• Electric current: Rate of flow of electric charge. I = Q/t. SI unit: Ampere (A).
• Potential difference (voltage): Work done per unit charge. V = W/Q. SI unit: Volt (V).
• Resistance: Opposition to the flow of current. R = V/I. SI unit: Ohm (Ω).
• Ohm's Law: At constant temperature, V = IR (voltage = current × resistance).
• Conductor: Material that allows electric current to flow easily (e.g., copper, silver).
• Insulator: Material that does not allow current to flow (e.g., rubber, wood).
• Heating effect: Conversion of electrical energy into heat when current flows through a resistance.
• Fuse: A safety device — thin wire that melts and breaks the circuit when excess current flows.

Chemistry Terms

Matter Around Us

• Matter: Anything that has mass and occupies space.
• Solid: State of matter with definite shape and volume; particles closely packed.
• Liquid: State of matter with definite volume but no definite shape.
• Gas: State of matter with neither definite shape nor definite volume.
• Sublimation: Direct change from solid to gas without passing through liquid state (e.g., camphor, dry ice).
• Evaporation: Conversion of liquid to gas at the surface, below boiling point.

Atomic Structure

• Atom: The smallest unit of an element that retains its chemical properties.
• Proton: Positively charged particle in the nucleus; mass ≈ 1 amu.
• Neutron: Neutral particle in the nucleus; mass ≈ 1 amu.
• Electron: Negatively charged particle orbiting the nucleus; mass negligible.
• Atomic number (Z): Number of protons in an atom's nucleus.
• Mass number (A): Total number of protons + neutrons.
• Isotopes: Atoms of the same element with the same atomic number but different mass numbers.
• Isobars: Atoms of different elements with the same mass number but different atomic numbers.

Periodic Classification

• Periodic table: A tabular arrangement of elements in order of increasing atomic number.
• Period: A horizontal row in the periodic table.
• Group: A vertical column in the periodic table.
• Periodicity: The regular variation in properties of elements in the periodic table.
• Alkali metals: Group 1 elements — reactive metals (Li, Na, K).
• Halogens: Group 17 elements — reactive non-metals (F, Cl, Br).

Acids, Bases and Salts

• Acid: A substance that releases H⁺ ions in water. pH < 7. Turns litmus red.
• Base: A substance that releases OH⁻ ions in water. pH > 7. Turns litmus blue.
• Salt: Ionic compound formed by neutralisation of acid with base.
• Neutralisation: Reaction of acid + base → salt + water.
• pH: A measure of hydrogen ion concentration. 0–6 = acidic, 7 = neutral, 8–14 = basic/alkaline.
• Indicator: A substance that changes colour in acid/base — used to test pH (e.g., litmus, phenolphthalein).

Carbon and Its Compounds

• Allotrope: Different physical forms of the same element (e.g., diamond and graphite are allotropes of carbon).
• Organic compound: Carbon-containing compound, usually with C-H bonds.
• Hydrocarbon: Compound containing only carbon and hydrogen atoms.
• Saturated hydrocarbon: Hydrocarbon with only single C-C bonds (alkanes) — e.g., methane (CH₄).
• Unsaturated hydrocarbon: Hydrocarbon with double or triple C-C bonds (alkenes, alkynes).

Biology Terms

Life Processes of Plants

• Photosynthesis: Process by which plants make food using CO₂ + H₂O + sunlight → glucose + O₂.
• Transpiration: Loss of water vapour from leaves through stomata.
• Stomata: Tiny pores on leaf surface; regulate gas exchange and transpiration.
• Chlorophyll: Green pigment in plants that absorbs sunlight for photosynthesis.
• Xylem: Vascular tissue that transports water and minerals from roots to leaves.
• Phloem: Vascular tissue that transports food (sugars) from leaves to other parts.

Life Processes of Animals

• Digestion: Process of breaking down food into simpler molecules the body can absorb.
• Respiration: Process of releasing energy from food molecules (usually glucose + O₂ → CO₂ + H₂O + energy).
• Excretion: Removal of metabolic waste products from the body.
• Blood: Fluid connective tissue containing RBC (oxygen transport), WBC (immunity), platelets (clotting), plasma.
• Peristalsis: Wave-like muscular contractions that move food through the digestive tract.

Microorganisms

• Microorganism: A living organism too small to be seen with the naked eye.
• Bacteria: Single-celled prokaryotic microorganisms; some cause disease, some are beneficial.
• Virus: Non-cellular entity that can only replicate inside a host cell; causes diseases (flu, COVID).
• Fungi: Eukaryotic organisms that decompose organic matter (e.g., yeast, mould).
• Antibiotic: Medicine produced by microorganisms (usually fungi) that kills bacteria.
• Vaccination: Introduction of a weakened pathogen to stimulate the immune system to produce antibodies.

Reproduction

• Reproduction: The biological process by which organisms produce offspring.
• Asexual reproduction: Reproduction involving only one parent; offspring are genetically identical.
• Sexual reproduction: Reproduction involving two parents and fusion of gametes.
• Binary fission: Type of asexual reproduction — one cell splits into two equal cells (e.g., Amoeba, bacteria).
• Pollination: Transfer of pollen from anther to stigma — self-pollination (same flower) or cross-pollination (different flower).
• Fertilisation: Fusion of male gamete (pollen) and female gamete (ovule) to form a zygote.