Independent variable The factor a researcher deliberately changes to test its effect; plotted on the x-axis as the suspected cause.
Dependent variable The outcome that is measured; it depends on (responds to) the independent variable and is plotted on the y-axis.
Controlled variable (constant) A condition kept the same across all groups so it cannot influence the results.
Control group The group that does not receive the experimental treatment; it provides a baseline for comparison.
Hypothesis A testable, falsifiable, proposed explanation, often stated as an 'if/then' prediction about how variables relate.
Falsifiable A claim is falsifiable if there is a possible observation or experiment that could prove it wrong; a requirement for a scientific hypothesis.
Scientific theory A broad, well-substantiated explanation supported by a large body of evidence (e.g., evolution, plate tectonics) — far stronger than a guess.
Scientific law A statement (often mathematical) describing a consistent natural pattern, such as Newton's laws; it describes what happens but not why.
Accuracy vs. precision Accuracy = closeness to the true value; precision = closeness of repeated measurements to each other. Measurements can be precise but inaccurate.
Systematic error A consistent error that shifts every measurement in the same direction (e.g., a balance reading high), reducing accuracy but not precision.
Random error Unpredictable scatter in measurements from trial to trial; reduced by averaging many trials.
Independent vs. dependent on a graph Independent (manipulated) variable goes on the x-axis; dependent (measured/responding) variable goes on the y-axis.
Replication Repeating an experiment to confirm results are reliable and not due to chance; central to the self-correcting nature of science.
Peer review Evaluation of research by independent experts before publication; a key check on the quality and validity of scientific claims.
Observation vs. inference An observation is information gathered directly with the senses or instruments; an inference is a conclusion drawn from observations.
Qualitative vs. quantitative data Qualitative = descriptive (color, texture); quantitative = numerical/measured (mass, length, time).
SI base units Meter (length), kilogram (mass), second (time), kelvin (temperature), ampere (current), mole (amount), candela (luminous intensity).
Independent variable, one at a time A valid controlled experiment changes only one independent variable so any change in the outcome can be attributed to it.
Scientific method (iterative) Question → hypothesis → experiment → analyze data → conclusion; results often loop back to new questions, so it is iterative, not one-time.
Correlation vs. causation Two variables changing together (correlation) does not prove one causes the other; only a controlled experiment can establish causation.
Model (in science) A simplified representation (physical, mathematical, or conceptual) used to explain or predict phenomena, e.g., the atomic model.
Lab safety: chemical splash Always wear goggles; flush eyes/skin with water for 15 minutes and notify the instructor if a chemical contacts them.
Engineering design process Define the problem and constraints → brainstorm → design → build a prototype → test → evaluate → redesign (iterate).
Science vs. engineering Science seeks to understand the natural world; engineering applies that understanding to design solutions under real-world constraints.
Technology and society Scientific and engineering advances shape society (medicine, energy, communication) and raise ethical, economic, and environmental questions.
Significant figures The digits in a measurement that carry meaning about its precision; the result of a calculation should reflect the least precise measurement.
Mean, median, mode Mean = average; median = middle value of ordered data; mode = most frequent value. The median resists outliers.
Constraint (engineering) A limit on a design solution, such as cost, time, materials, safety, or available space.
Empirical evidence Information acquired through observation or experiment; the basis for testing scientific claims.
Pseudoscience Claims presented as scientific but lacking testability, falsifiability, or supporting evidence (e.g., astrology).
Proton, neutron, electron Protons (+, in nucleus), neutrons (neutral, in nucleus), electrons (−, orbit nucleus). Nearly all the atom's mass is in the nucleus.
Atomic number The number of protons in an atom; it defines the element's identity.
Mass number The sum of protons and neutrons in an atom's nucleus.
Isotopes Atoms of the same element (same protons) with different numbers of neutrons, and thus different mass numbers.
Ion An atom that has gained or lost electrons, giving it a net negative (anion) or positive (cation) charge.
Periodic table groups Columns of elements with similar chemical properties because they have the same number of valence electrons.
Periodic table periods Horizontal rows; properties change gradually from left (metals) to right (nonmetals) across a period.
Atomic radius trend Generally decreases left-to-right across a period (more nuclear charge pulls electrons in) and increases down a group.
Electronegativity trend Increases up and to the right; fluorine is the most electronegative element.
Ionization energy The energy required to remove an electron from a neutral atom; increases left-to-right across a period.
Valence electrons Electrons in the outermost shell; they determine an element's bonding behavior and reactivity.
Ionic bond A bond formed by the transfer of electrons from a metal to a nonmetal, producing oppositely charged ions that attract.
Covalent bond A bond formed when two nonmetal atoms share electrons.
Metallic bond Bonding in metals where positive ions sit in a 'sea' of delocalized electrons, explaining conductivity and malleability.
Law of conservation of mass Mass is neither created nor destroyed in a chemical reaction; the mass of reactants equals the mass of products.
Balanced equation A chemical equation with equal numbers of each type of atom on both sides, satisfying conservation of mass. Combustion reaction A fuel reacts with oxygen to release energy, e.g. C H X 4 + 2 O X 2 → C O X 2 + 2 H X 2 O \ce{CH4 + 2O2 -> CO2 + 2H2O} CH X 4 + 2 O X 2 CO X 2 + 2 H X 2 O Synthesis reaction Two or more reactants combine to form one product: A + B → A B \ce{A + B -> AB} A + B AB Decomposition reaction One reactant breaks into two or more products: A B → A + B \ce{AB -> A + B} AB A + B
Acid A substance that donates H⁺ ions (or accepts electrons); has a pH below 7 and turns litmus red.
Base A substance that accepts H⁺ (or donates OH⁻); has a pH above 7 and turns litmus blue.
pH scale Measures acidity from 0–14; 7 is neutral, below 7 acidic, above 7 basic. Each unit is a tenfold change in H⁺ concentration.
States of matter Solid (fixed shape/volume), liquid (fixed volume, takes shape of container), gas (fills container), plasma (ionized gas).
Phase changes Melting, freezing, vaporization (boiling/evaporation), condensation, sublimation (solid→gas), deposition (gas→solid).
Endothermic vs. exothermic Endothermic reactions absorb energy (surroundings cool); exothermic reactions release energy (surroundings warm).
Physical vs. chemical change Physical change alters form but not identity (melting, cutting); chemical change forms new substances (burning, rusting).
Element, compound, mixture Element = one kind of atom; compound = atoms chemically bonded in fixed ratio; mixture = physically combined, separable substances.
Solution A homogeneous mixture of a solute dissolved in a solvent (e.g., salt water). Density Mass per unit volume: ρ = m V \rho = \dfrac{m}{V} ρ = V m
Newton's first law (inertia) An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net force. Newton's second law Net force equals mass times acceleration: F = m a F = ma F = ma
Newton's third law For every action there is an equal and opposite reaction; forces come in pairs on different objects.
Speed vs. velocity Speed is distance per time (scalar); velocity is speed with direction (vector). Acceleration The rate of change of velocity: a = Δ v Δ t a = \dfrac{\Delta v}{\Delta t} a = Δ t Δ v Kinetic energy Energy of motion: K E = 1 2 m v 2 KE = \tfrac{1}{2}mv^2 K E = 2 1 m v 2 Potential energy (gravitational) Stored energy due to position: P E = m g h PE = mgh P E = m g h
Law of conservation of energy Energy cannot be created or destroyed, only transformed from one form to another; total energy stays constant. Work Force applied over a distance in the direction of motion: W = F d W = Fd W = F d Power The rate of doing work: P = W t P = \dfrac{W}{t} P = t W Momentum The product of mass and velocity: p = m v p = mv p = m v
Friction A force that opposes motion between surfaces in contact; converts kinetic energy to heat. Gravity An attractive force between masses; near Earth's surface it gives free-falling objects an acceleration of about 9.8 m/s 2 9.8\ \text{m/s}^2 9.8 m/s 2 Mass vs. weight Mass is the amount of matter (constant, kg); weight is the force of gravity on that mass (W = m g W = mg W = m g
Simple machines Lever, pulley, wheel and axle, inclined plane, wedge, and screw; they change the size or direction of a force.
Mechanical advantage How much a machine multiplies input force; it does not multiply work or energy (those are conserved).
Electromagnetic spectrum (order) Radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays — increasing frequency and energy, decreasing wavelength. Wave properties Wavelength (crest-to-crest distance), frequency (waves per second), amplitude (height), and speed (v = f λ v = f\lambda v = f λ
Transverse vs. longitudinal waves Transverse: medium moves perpendicular to wave travel (light); longitudinal: medium moves parallel (sound).
Reflection, refraction, diffraction Reflection = bouncing off a surface; refraction = bending when entering a new medium; diffraction = spreading around obstacles.
Sound A longitudinal mechanical wave that requires a medium; pitch depends on frequency and loudness on amplitude. Ohm's law Voltage equals current times resistance: V = I R V = IR V = I R
Series vs. parallel circuits Series: one path, current is the same everywhere; parallel: multiple paths, voltage is the same across branches.
Conductor vs. insulator Conductors (metals) let charge flow easily; insulators (rubber, glass) resist charge flow.
Electromagnet A magnet created by current flowing through a coil of wire; its strength depends on current and number of coils.
Heat transfer methods Conduction (direct contact), convection (fluid movement), and radiation (electromagnetic waves, no medium needed).
Temperature vs. heat Temperature measures the average kinetic energy of particles; heat is the energy transferred between objects at different temperatures.
Nuclear fission vs. fusion Fission splits a heavy nucleus releasing energy (nuclear plants); fusion joins light nuclei (powers the Sun), releasing even more energy.
Radioactive decay Unstable nuclei emit alpha, beta, or gamma radiation, transforming into more stable elements over time.
Half-life The time for half of a radioactive sample to decay; constant for a given isotope.
Energy forms Mechanical, thermal, chemical, electrical, nuclear, electromagnetic, and sound — all interconvertible.
Net force The vector sum of all forces on an object; zero net force means no acceleration (equilibrium).
Static electricity Buildup of electric charge on a surface, often from friction; like charges repel and opposite charges attract.
Magnet poles Every magnet has a north and south pole; like poles repel, opposite poles attract, and poles cannot be isolated.
Concentration (solution) The amount of solute per amount of solution; higher concentration means more dissolved solute.
Catalyst A substance that speeds a reaction by lowering activation energy without being consumed.
Activation energy The minimum energy needed to start a chemical reaction.
Molarity Moles of solute per liter of solution; a common measure of concentration. Boyle's law At constant temperature, the pressure and volume of a gas are inversely related: P 1 V 1 = P 2 V 2 P_1 V_1 = P_2 V_2 P 1 V 1 = P 2 V 2
Charles's law At constant pressure, a gas's volume is directly proportional to its absolute temperature. Avogadro's number 6.022 × 10 23 6.022 \times 10^{23} 6.022 × 1 0 23 Photosynthesis Plants convert carbon dioxide and water into glucose and oxygen using light energy: 6 C O X 2 + 6 H X 2 O → C X 6 H X 12 O X 6 + 6 O X 2 \ce{6CO2 + 6H2O -> C6H12O6 + 6O2} 6 CO X 2 + 6 H X 2 O C X 6 H X 12 O X 6 + 6 O X 2
Where photosynthesis occurs In the chloroplasts of plant cells, using the green pigment chlorophyll to capture light. Cellular respiration Cells break down glucose with oxygen to release energy (ATP): C X 6 H X 12 O X 6 + 6 O X 2 → 6 C O X 2 + 6 H X 2 O \ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O} C X 6 H X 12 O X 6 + 6 O X 2 6 CO X 2 + 6 H X 2 O
Where respiration occurs Mostly in the mitochondria of eukaryotic cells.
Photosynthesis vs. respiration They are roughly reverse processes: photosynthesis stores energy in glucose; respiration releases it. Products of one are reactants of the other.
ATP Adenosine triphosphate — the cell's main energy-carrying molecule, produced by respiration. Anaerobic respiration (fermentation) Energy release without oxygen; in muscle it produces lactic acid, in yeast it produces alcohol and C O X 2 \ce{CO2} CO X 2
Cell theory All living things are made of cells, the cell is the basic unit of life, and all cells come from pre-existing cells.
Prokaryotic vs. eukaryotic Prokaryotes (bacteria) lack a nucleus and membrane-bound organelles; eukaryotes (plants, animals) have both.
Nucleus The organelle that contains the cell's DNA and controls cell activities.
Mitochondria The 'powerhouse' organelle where most cellular respiration and ATP production occur.
Chloroplast The organelle in plant cells where photosynthesis takes place; contains chlorophyll.
Cell membrane The selectively permeable boundary controlling what enters and leaves the cell.
Cell wall A rigid outer layer in plant, fungal, and bacterial cells that provides support and protection; absent in animal cells.
Ribosome The site of protein synthesis (translation) in the cell.
Diffusion Movement of particles from high to low concentration; passive, requiring no energy.
Osmosis Diffusion of water across a selectively permeable membrane from low to high solute concentration.
Active transport Movement of substances against a concentration gradient, requiring energy (ATP).
DNA Deoxyribonucleic acid — a double helix of nucleotides (A-T, G-C base pairs) that stores genetic information.
Gene A segment of DNA that codes for a protein or trait.
Chromosome A condensed structure of DNA and protein; humans have 23 pairs (46 total).
Mitosis Cell division producing two genetically identical diploid cells, used for growth and repair.
Meiosis Cell division producing four genetically varied haploid gametes (sex cells) with half the chromosome number.
Dominant vs. recessive allele A dominant allele is expressed when present; a recessive allele is masked unless two copies are inherited.
Genotype vs. phenotype Genotype is the genetic makeup (e.g., Bb); phenotype is the observable trait (e.g., brown eyes).
Homozygous vs. heterozygous Homozygous = two identical alleles (BB or bb); heterozygous = two different alleles (Bb).
Punnett square A diagram predicting offspring genotype and phenotype ratios from the parents' alleles.
DNA replication The semiconservative copying of DNA before cell division; each new molecule keeps one original strand.
Transcription and translation Transcription copies DNA into mRNA; translation uses mRNA at the ribosome to build a protein.
Mutation A change in DNA sequence; it can be harmful, beneficial, or neutral and is the ultimate source of genetic variation.
Natural selection Individuals with traits better suited to the environment survive and reproduce more, passing those traits on.
Evolution Change in the inherited traits of a population over generations; descent with modification.
Adaptation An inherited trait that improves an organism's survival or reproduction in its environment.
Evidence for evolution Fossils, homologous structures, embryology, DNA/molecular similarities, and observed natural selection.
Species A group of organisms that can interbreed and produce fertile offspring.
Taxonomy (Linnaean order) Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species — broadest to most specific.
Producer An autotroph that makes its own food via photosynthesis (or chemosynthesis); the base of a food chain.
Consumer A heterotroph that eats other organisms; classified as herbivore, carnivore, or omnivore.
Decomposer An organism (bacteria, fungi) that breaks down dead matter and recycles nutrients into the ecosystem.
Food web Interconnected food chains showing energy flow through an ecosystem from producers up to top consumers.
10% rule (energy transfer) Only about 10% of energy passes from one trophic level to the next; the rest is lost as heat.
Carbon cycle Carbon moves among the atmosphere, organisms, oceans, and rocks via photosynthesis, respiration, decay, and combustion.
Nitrogen cycle Bacteria convert atmospheric nitrogen into usable forms (fixation) that organisms use to build proteins and DNA.
Water cycle Evaporation, condensation, precipitation, and collection circulate water through the environment.
Carrying capacity The maximum population size an environment can sustain given its resources.
Limiting factor A resource or condition (food, water, space, predators) that limits a population's growth.
Biome A large region defined by its climate and characteristic plants and animals (e.g., desert, tundra, rainforest).
Ecosystem A community of living organisms interacting with their nonliving (abiotic) environment.
Symbiosis types Mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, host harmed).
Predator-prey relationship Predator and prey populations cycle: prey rises, predators rise, prey falls, predators fall.
Homeostasis The maintenance of stable internal conditions (temperature, pH, water) despite a changing environment.
Enzyme A protein catalyst that speeds biological reactions; specific to its substrate and sensitive to temperature and pH.
Circulatory system The heart, blood, and vessels that transport oxygen, nutrients, and wastes throughout the body.
Respiratory system Lungs and airways that exchange oxygen and carbon dioxide between the body and the air.
Digestive system Organs that break down food into nutrients the body can absorb and use.
Nervous system The brain, spinal cord, and nerves that sense stimuli and coordinate responses.
Macromolecules of life Carbohydrates (energy), lipids (storage/membranes), proteins (structure/enzymes), and nucleic acids (genetic info).
Asexual vs. sexual reproduction Asexual produces genetically identical offspring from one parent; sexual combines two parents' genes, increasing variation.
Biodiversity The variety of life in an ecosystem; greater biodiversity generally makes an ecosystem more stable and resilient.
Ecological succession The gradual change in a community over time — primary (bare rock) or secondary (after a disturbance like fire).
Earth's layers (outer to inner) Crust, mantle, outer core (liquid), inner core (solid).
Mantle The thick layer beneath the crust that behaves as a slowly flowing solid; convection currents here drive plate movement.
Outer vs. inner core The outer core is liquid (molten iron and nickel) and generates Earth's magnetic field; the inner core is solid due to immense pressure.
Plate tectonics The theory that Earth's rigid outer shell is broken into plates that slowly move over the softer mantle below.
Divergent boundary Plates pull apart; magma rises to form new crust, creating mid-ocean ridges and rift valleys.
Convergent boundary Plates collide; the denser plate subducts, forming volcanoes, mountains, and deep ocean trenches.
Transform boundary Plates slide past each other horizontally, producing earthquakes (e.g., the San Andreas Fault).
Driver of plate motion Convection currents in the mantle, powered by Earth's internal heat, move the plates.
Continental drift (Pangaea) Wegener's hypothesis that continents were once joined in a single landmass (Pangaea) and have since drifted apart.
Earthquakes and volcanoes location Concentrated along plate boundaries, especially the Pacific 'Ring of Fire.'
Igneous rock Forms from the cooling and solidifying of molten magma or lava (e.g., granite, basalt).
Sedimentary rock Forms when sediments are compacted and cemented over time (e.g., sandstone, limestone); may contain fossils.
Metamorphic rock Forms when existing rock is changed by heat and pressure (e.g., marble from limestone).
Rock cycle The continuous transformation among igneous, sedimentary, and metamorphic rock through melting, weathering, and pressure.
Weathering vs. erosion Weathering breaks rock down in place (physical or chemical); erosion transports the pieces away by wind, water, or ice.
Mineral A naturally occurring, inorganic solid with a definite chemical composition and crystal structure.
Fossil Preserved remains or traces of past life, most often found in sedimentary rock.
Law of superposition In undisturbed rock layers, the oldest layers are at the bottom and the youngest at the top.
Relative vs. absolute dating Relative dating orders events (older/younger); absolute dating gives a numeric age, often via radioactive decay.
Layers of the atmosphere Troposphere, stratosphere (ozone), mesosphere, thermosphere, exosphere — from the surface upward.
Ozone layer A region in the stratosphere that absorbs most of the Sun's harmful ultraviolet radiation.
Greenhouse effect Atmospheric gases (CO₂, water vapor, methane) trap heat, warming Earth's surface; excess gas intensifies warming.
Water cycle (Earth) Evaporation, transpiration, condensation, precipitation, and runoff continually move water through Earth's systems.
Weather vs. climate Weather is short-term atmospheric conditions; climate is the long-term average weather of a region.
Air masses and fronts A front is the boundary between air masses; cold fronts often bring storms, warm fronts bring gradual precipitation.
Ocean currents Large-scale movements of seawater driven by wind, temperature, and salinity that redistribute heat around the globe.
Tides The regular rise and fall of sea level caused mainly by the Moon's gravitational pull, with the Sun contributing.
Solar system order Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune — inner rocky planets, then outer gas/ice giants.
Earth's rotation vs. revolution Rotation (spin on its axis, ~24 h) causes day and night; revolution (orbit around the Sun, ~365 days) defines a year.
Cause of seasons The tilt of Earth's axis (about 23.5°), not its distance from the Sun, causes the seasons.
Moon phases New, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent — driven by Sun-Earth-Moon geometry.
Solar vs. lunar eclipse Solar: Moon passes between Sun and Earth, blocking the Sun. Lunar: Earth passes between Sun and Moon, casting a shadow on the Moon.
Star life cycle Stars form from gas/dust nebulae, fuse hydrogen as main-sequence stars, then expand and end as white dwarfs, neutron stars, or black holes.
The Sun A medium-sized star that produces energy by nuclear fusion of hydrogen into helium; the source of Earth's energy.
Galaxy A vast system of stars, gas, and dust bound by gravity; our solar system lies in the Milky Way galaxy.
Big Bang theory The leading explanation that the universe began ~13.8 billion years ago from an extremely hot, dense state and has been expanding since.
Gravity in space Gravity holds planets in orbit around the Sun and moons around planets; orbital paths are ellipses.
Renewable vs. nonrenewable resources Renewable resources (solar, wind, hydro) replenish naturally; nonrenewable (coal, oil, gas) form over millions of years.
Topographic map A map using contour lines to show elevation; closely spaced lines indicate steep terrain.
Comet, asteroid, meteor Comets are icy bodies with tails; asteroids are rocky bodies (mostly between Mars and Jupiter); a meteor is debris burning up in the atmosphere. 
