Every science stands on a few big ideas. Here are the foundations, one subject at a time — the concepts you need before the details start to make sense. Choose a subject and start exploring.
Everything is made of atoms. An element is defined by its number of protons — change the protons and you change the element. The periodic table arranges all ~118 elements so that columns (groups) share chemical behaviour: metals on the left, nonmetals on the right, and the famously unreactive noble gases in the last group.
Atoms bond to complete their outer electron shell. Ionic bonds: electrons are transferred from a metal to a nonmetal (table salt, NaCl). Covalent bonds: electron pairs are shared between nonmetals (water, CO₂). Metallic bonds: atoms share a "sea" of electrons — which is why metals conduct electricity and bend instead of shattering.
The particle model explains it all: in solids particles vibrate in fixed positions; in liquids they stay close but slide past each other; in gases they are far apart and move fast. Melting and boiling are physical changes — no new substance forms. Heating increases the particles' kinetic energy — try it in the diffusion simulator.
The pH scale runs 0–14: below 7 is acidic, 7 is neutral, above 7 is alkaline. Acids release hydrogen ions (H⁺) in water; alkalis release hydroxide ions (OH⁻). Mix them and they neutralise: acid + alkali → salt + water. Indicators such as universal indicator show pH by colour.
In a chemical reaction atoms are rearranged, never created or destroyed — so the total mass of products equals the total mass of reactants. Main types: synthesis, decomposition, combustion, and displacement. Reaction rates rise with temperature, concentration, surface area, and catalysts — enzymes are biology's catalysts.
Atoms are too small to count one by one, so chemists count in moles: 1 mole = 6.02 × 10²³ particles (Avogadro's constant). The molar mass of a substance (grams per mole) is read from the periodic table, and moles = mass ÷ molar mass. Example: water's molar mass is 18 g/mol, so 36 g of water is exactly 2 moles.
1. Objects keep their state of motion unless a force acts (inertia). 2. Force = mass × acceleration (F = ma) — heavier things need more force for the same acceleration. 3. Every action has an equal, opposite reaction. On Earth, gravity accelerates falling objects at about 9.8 m/s²; friction is the force that opposes sliding.
Energy only changes form. Kinetic energy: KE = ½mv² — double the speed, four times the energy. Gravitational potential energy: GPE = mgh. Power is how fast energy is transferred (watts = joules per second), and efficiency = useful energy out ÷ total energy in — no real machine is 100% efficient.
All waves obey wave speed = frequency × wavelength (v = fλ). Sound is a vibration that needs a material to travel through (~343 m/s in air) — it cannot cross a vacuum. Light needs no medium and travels at 3.00 × 10⁸ m/s in a vacuum. The electromagnetic spectrum runs from radio waves through visible light to gamma rays — same family, different wavelengths.
Current is the flow of electric charge (amperes). Voltage is the energy carried per unit of charge (volts). Resistance opposes the flow (ohms). They are linked by Ohm's law: V = I × R. In a series circuit the current is the same everywhere; in a parallel circuit each branch gets the full voltage — which is why one dead bulb doesn't darken your home.
Pressure = force ÷ area (P = F/A) — the same force spread over a smaller area presses harder, which is why sharp knives cut and wide tyres grip mud. Density = mass ÷ volume: whether something floats depends on its density compared with the fluid. In liquids, pressure grows with depth and pushes equally in all directions.
Temperature measures the average kinetic energy of a substance's particles. Heat energy transfers three ways: conduction (through touching solids), convection (rising warm fluids), and radiation (infrared waves, no medium needed — how the Sun warms the Earth). Most materials expand when heated. Water is unusual: its high specific heat capacity (4.18 J/g·°C) is why coastal climates stay mild.
Every living thing is made of cells. Animal and plant cells share a nucleus, cytoplasm, and membrane; plant cells add a cell wall, chloroplasts, and a large permanent vacuole. Cells specialise, group into tissues, then organs, then whole systems.
DNA is a double helix of two strands; its sequence of bases (A, T, C, G) spells genes, and genes carry the instructions for building proteins. Half your DNA came from each parent — which is why families resemble each other, and why no two people (except identical twins) share the same DNA.
Your body runs as cooperating systems — cardiovascular, respiratory, digestive, nervous, and more — each with its own organs and job, all depending on each other. Explore them with real photographs and textbook diagrams in the Human Body Explorer.
Two paired processes move the world's energy: photosynthesis stores light energy in glucose (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), and respiration releases that energy for cells to use — the same equation, reversed. Nearly all life on Earth runs on this exchange.