Junkao Science Knowledge cheat sheet

Independent variable: the factor the experimenter deliberately changes; dependent variable: the factor measured in response; controlled variables are held constant to prevent confounding.

Control group receives no treatment or a placebo, providing the baseline against which the experimental group's results are compared.

A valid hypothesis must be testable and falsifiable: it predicts a specific, observable outcome that could prove it wrong.

Random assignment of subjects to groups reduces selection bias; random sampling of subjects from a population supports generalization.

Blinding prevents bias: single-blind hides treatment from subjects, double-blind hides it from both subjects and data collectors.

Correlation does not establish causation; a third variable may drive both observed factors.

Repeat trials and adequate sample size reduce the influence of random error on conclusions.

A scientific law describes a consistent observed relationship; a theory is a well-supported explanatory framework that can incorporate laws.

SI base units: meter (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), candela (luminous intensity).

Derived units: newton = kg·m/s², joule = N·m, watt = J/s, pascal = N/m², volt = J/C, ohm = V/A.

Precision refers to agreement among repeated measurements; accuracy refers to closeness to the true value.

Percent error = |measured − accepted| / accepted × 100%.

Density = mass / volume, commonly in g/cm³ or kg/m³; 1 g/cm³ = 1000 kg/m³.

Significant figures: nonzero digits and captured zeros count; leading zeros do not; trailing zeros count only with a decimal point.

When multiplying or dividing, round to the least number of significant figures; when adding or subtracting, round to the least precise decimal place.

Unit conversions use dimensional analysis: multiply by conversion factors arranged so unwanted units cancel.

Atomic number = protons; mass number = protons + neutrons; isotopes differ in neutron count only.

Law of conservation of mass: total mass of reactants equals total mass of products in a closed system.

Balancing equations changes coefficients only, never subscripts; each element's atom count must match on both sides.

Types of reactions: synthesis, decomposition, single replacement, double replacement, combustion, and neutralization.

pH below 7 is acidic, 7 is neutral, above 7 is basic; each unit represents a tenfold change in hydrogen ion concentration.

Exothermic reactions release energy (products lower in energy); endothermic reactions absorb energy.

Molar mass in g/mol equals the sum of atomic masses; moles = mass / molar mass.

States of matter: solid (fixed shape and volume), liquid (fixed volume, variable shape), gas (variable shape and volume), plasma (ionized gas).

Newton's first law: an object at rest or in uniform motion stays that way unless a net external force acts on it.

Newton's second law: F_net = m·a; acceleration is directly proportional to net force and inversely proportional to mass.

Newton's third law: for every action force there is an equal and opposite reaction force on a different object.

Weight = m·g, where g ≈ 9.8 m/s² near Earth's surface; mass is constant, weight varies with gravity.

Kinetic energy = ½mv²; gravitational potential energy = mgh; total mechanical energy is conserved absent friction.

Work = force × distance moved in the direction of the force; power = work / time.

Speed = distance / time; velocity includes direction; acceleration = change in velocity / time.

Friction opposes relative motion and converts mechanical energy into thermal energy.

Wave speed = frequency × wavelength; frequency is measured in hertz (cycles per second).

Transverse waves oscillate perpendicular to propagation; longitudinal waves oscillate parallel to propagation.

Reflection bounces waves; refraction bends waves as speed changes between media; diffraction spreads waves around obstacles.

Sound requires a medium; light travels through vacuum at about 3 × 10⁸ m/s.

Ohm's law: V = I·R; power = V·I = I²R = V²/R.

Series circuits share one current path and sum resistances; parallel circuits share voltage and divide current.

Electric current is the flow of charge; conventional current runs from positive to negative terminal.

Magnetic field lines run from north to south outside a magnet; a changing magnetic field can induce a current (electromagnetic induction).

Cell theory: all living things are made of cells, cells arise from existing cells, and the cell is the basic unit of life.

Prokaryotes lack a membrane-bound nucleus; eukaryotes have a nucleus and membrane-bound organelles.

Mitochondria carry out cellular respiration releasing ATP; chloroplasts perform photosynthesis; ribosomes synthesize proteins.

DNA bases pair A–T and C–G; transcription makes mRNA from DNA, translation builds protein at the ribosome.

Mitosis produces two genetically identical diploid cells; meiosis produces four haploid gametes with genetic variation.

Genotype is the allele combination; phenotype is the observable trait; dominant alleles mask recessive alleles in heterozygotes.

Natural selection requires heritable variation, differential reproductive success, and change in allele frequencies over generations.

Punnett squares predict offspring genotype ratios from parental alleles.

Trophic levels: producers (autotrophs), primary consumers, secondary consumers, decomposers; energy flows one way and matter cycles.

About 10% of energy transfers between trophic levels; the rest is lost as heat, limiting food chain length.

Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂; respiration reverses this to release energy.

Carbon, nitrogen, water, and phosphorus cycle through biotic and abiotic reservoirs.

Carrying capacity is the maximum population an ecosystem sustains; limiting factors include food, water, space, and disease.

Biodiversity increases ecosystem resilience; habitat loss is a leading cause of species decline.

Predator–prey cycles show coupled population oscillations; competition occurs when organisms need the same limited resource.

Invasive species disrupt food webs by outcompeting natives and lacking natural predators.

Earth's layers: crust, mantle, outer core (liquid), inner core (solid); plate tectonics drives earthquakes, volcanoes, and mountain building.

Rock types: igneous (cooled magma), sedimentary (compacted deposits), metamorphic (heat and pressure altered).

The rock cycle and water cycle move matter among reservoirs; evaporation, condensation, precipitation, and runoff drive weather.

Weather occurs in the troposphere; air pressure decreases with altitude; wind flows from high to low pressure.

Seasons result from Earth's axial tilt (about 23.5°), not distance from the Sun.

Gravity follows an inverse-square law: doubling distance reduces force to one quarter.

Moon phases arise from the changing angle between Earth, Moon, and Sun; a solar eclipse occurs at new moon, a lunar eclipse at full moon.

Kepler's laws: orbits are ellipses, planets sweep equal areas in equal times, and orbital period squared scales with semi-major axis cubed.

Qualitative data are descriptive observations; quantitative data are numerical measurements subject to statistical analysis.

A line graph shows continuous change over time; a bar graph compares discrete categories; a scatter plot tests correlation between two variables.

Measurement uncertainty should be reported with results; the smallest scale division limits reading precision.

Peer review subjects scientific claims to independent scrutiny before publication.

A model is a simplified representation and its predictions are valid only within stated assumptions and conditions.

Sample size and replication strengthen the reliability of conclusions drawn from experimental data.

Metric prefixes: kilo (10³), centi (10⁻²), milli (10⁻³), micro (10⁻⁶), nano (10⁻⁹).

Mean is the arithmetic average; median is the middle value; mode is the most frequent value.

Slope of a distance–time graph gives speed; slope of a velocity–time graph gives acceleration; area under a velocity–time graph gives displacement.

A conversion factor equals one because its numerator and denominator represent the same quantity in different units.

Temperature conversions: K = °C + 273; °F = (°C × 9/5) + 32.

Outliers can distort the mean but affect the median less, so median is preferred for skewed data.

Elements are pure substances of one atom type; compounds are chemically combined elements in fixed ratios; mixtures retain component properties.

Physical changes alter form without changing composition; chemical changes produce new substances with different properties.

Periodic table groups share valence electron counts and similar chemical properties; periods indicate energy levels.

Ionic bonds transfer electrons between metals and nonmetals; covalent bonds share electrons between nonmetals.

The mole contains Avogadro's number (6.02 × 10²³) of particles.

Solubility, conductivity, and reactivity are properties used to identify substances and classify matter.

Momentum = mass × velocity; in a closed system total momentum is conserved in collisions.

Simple machines (lever, pulley, inclined plane) trade force for distance while conserving work in an ideal system.

Energy forms include kinetic, potential, thermal, chemical, electrical, and nuclear; energy transforms but total energy is conserved.

Thermal energy transfers by conduction (contact), convection (fluid currents), and radiation (electromagnetic waves).

Gravitational field strength determines weight; on the Moon g is about one sixth of Earth's value.

Free-fall acceleration near Earth is about 9.8 m/s² downward, independent of mass when air resistance is negligible.

Amplitude relates to wave energy; wavelength relates to pitch (sound) or color (light); frequency determines pitch and color.

The electromagnetic spectrum, from longest to shortest wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

The law of reflection: angle of incidence equals angle of reflection, measured from the normal.

A convex lens converges light; a concave lens diverges light; the focal point is where parallel rays converge.

Static electricity arises from transfer of electrons; like charges repel and opposite charges attract.

Electric power in a circuit is the rate of energy transfer; a fuse or breaker interrupts excessive current.

Cell membrane is selectively permeable, controlling movement of substances into and out of the cell.

Osmosis is diffusion of water across a membrane from higher to lower water concentration; diffusion moves particles from high to low concentration.

Enzymes are proteins that catalyze reactions and are affected by temperature and pH.

A mutation is a change in DNA sequence; it may be neutral, harmful, or beneficial depending on environment.

Fossils, homologous structures, and molecular similarities provide evidence for common ancestry.

Adaptation is a heritable trait increasing fitness; acquired traits are not inherited.

Biotic factors are living components; abiotic factors include sunlight, water, temperature, and soil.

A food web shows interconnected food chains; removing a keystone species can collapse the web.

Symbiosis types: mutualism (both benefit), commensalism (one benefits, other unaffected), parasitism (one benefits, other harmed).

Ecological succession is the gradual replacement of species; primary succession starts on bare rock, secondary after disturbance.

Renewable resources replenish naturally (solar, wind); nonrenewable resources (fossil fuels) are finite.

Human impacts include pollution, deforestation, and greenhouse gas emissions driving climate change.

The Sun is a main-sequence star powered by nuclear fusion of hydrogen into helium in its core.

The solar system order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

Earth rotates on its axis once per day causing day and night; it revolves around the Sun once per year.

Weather fronts: cold fronts bring rapid storms; warm fronts bring gradual precipitation.

The greenhouse effect traps outgoing infrared radiation, warming the surface; excess greenhouse gases amplify it.

Seismic waves (P and S) reveal Earth's interior structure; S waves cannot travel through liquids.

Absolute magnitude measures intrinsic brightness; apparent magnitude measures brightness as seen from Earth.

A controlled experiment changes one independent variable while holding all other conditions constant.

Scientific explanations must be based on empirical evidence and be open to revision when new evidence appears.

Data tables should include units and labeled columns; graphs should label axes with units and include a title.

A hypothesis is a proposed explanation; a prediction is a specific expected outcome derived from it.

Replication by independent researchers strengthens confidence in a finding.

Bias can enter through sampling, measurement, or interpretation, so procedures should be documented and reproducible.

Volume of a rectangular solid = length × width × height; volume of a cylinder = πr²h.

Mass is measured with a balance; weight is measured with a spring scale.

Dimensional analysis cancels units: multiply by ratios equal to one to convert between units.

Scientific notation expresses numbers as a coefficient between 1 and 10 multiplied by a power of ten.

Percent error and percent yield compare experimental results to theoretical or accepted values.

Graph interpretation requires identifying independent (x-axis) and dependent (y-axis) variables.

Density can identify substances because it is an intensive property independent of sample size.

Boiling and melting points are characteristic physical properties useful for identification.

A catalyst speeds a reaction by lowering activation energy without being consumed.

Reaction rate increases with temperature, concentration, and surface area, and with presence of a catalyst.

Acids release hydrogen ions in solution; bases accept them; neutralization produces salt and water.

The periodic table arranges elements by increasing atomic number; metals lie left, nonmetals right.

Net force is the vector sum of all forces; balanced forces produce zero acceleration.

Inertia is an object's resistance to change in motion and depends on mass.

Projectile motion combines constant horizontal velocity with vertical acceleration due to gravity.

Centripetal force keeps an object in circular motion and points toward the center.

Power is the rate of doing work; efficiency = useful output energy / total input energy × 100%.

Pressure = force / area; pressure increases when force increases or area decreases.

Resistance depends on material, length, and cross-sectional area: longer and thinner wires have greater resistance.

Voltage is electric potential difference; current flows only in a closed circuit.

Insulators resist charge flow; conductors allow it; semiconductors fall between.

Electromagnets produce magnetic fields from current and are used in motors and generators.

A generator converts mechanical energy to electrical energy by induction; a motor does the reverse.

Sound intensity decreases with distance from the source; loudness is measured in decibels.

Photosynthesis occurs in chloroplasts using chlorophyll to capture light energy.

Cellular respiration in mitochondria converts glucose and oxygen into carbon dioxide, water, and ATP.

Asexual reproduction produces genetically identical offspring; sexual reproduction increases genetic diversity.

A gene is a segment of DNA coding for a protein; chromosomes carry genes.

Punnett square ratios for a monohybrid cross of heterozygotes are 3:1 phenotype and 1:2:1 genotype.

Evolution occurs in populations, not individuals, through changes in allele frequencies.

An ecosystem includes all living organisms and their physical environment interacting together.

Population density = number of individuals / area or volume.

Energy pyramids are always upright because energy decreases at each higher trophic level.

Decomposers recycle nutrients back into the soil, making them available to producers.

Conservation strategies include protected areas, habitat restoration, and sustainable resource use.

Bioaccumulation is the buildup of pollutants in organisms along a food chain.

Earth's atmosphere layers from surface upward: troposphere, stratosphere, mesosphere, thermosphere, exosphere.

The ozone layer in the stratosphere absorbs harmful ultraviolet radiation.

Tides result primarily from the Moon's gravitational pull on Earth's oceans.

The water cycle is driven by solar energy and gravity.

Stars form from collapsing clouds of gas and dust called nebulae.

The Big Bang theory is supported by cosmic microwave background radiation and the observed expansion of the universe.

Operational definitions specify exactly how a variable is measured so others can replicate the procedure.

Anecdotal evidence is weak because it lacks controls and systematic sampling.

Statistical significance indicates a result is unlikely due to chance, not that it is important.

Ethical research requires informed consent, safety review, and honest reporting of results.

Scientific knowledge advances through hypothesis testing, replication, and revision of models.

Graphing continuous data requires connecting points only when intermediate values are meaningful.

The SI system uses decimal prefixes and base units consistently across all scientific fields.

Derived quantities combine base units: speed (m/s), acceleration (m/s²), density (kg/m³), force (N).

Measurement error includes random error (scatter) and systematic error (consistent offset).

Averages of repeated measurements reduce random error but not systematic error.

Linear relationships graph as straight lines; inverse relationships graph as hyperbolas; direct square relationships graph as parabolas.

Significant figures communicate the precision of a measurement, not its accuracy.

Atoms are the smallest unit of an element retaining its properties; molecules are bonded atoms.

The law of definite proportions: a compound always contains the same elements in the same mass ratio.

Valence electrons determine bonding; noble gases are stable with full outer shells.

A chemical equation is balanced when atoms of each element are equal on both sides.

Endothermic reactions feel cold because they absorb thermal energy from surroundings.

Mixtures can be separated by physical means (filtration, distillation); compounds require chemical reactions.

Speed is a scalar; velocity is a vector with magnitude and direction.

Acceleration can be positive (speeding up) or negative (slowing down) relative to the direction of motion.

Work requires displacement in the direction of the applied force.

Energy cannot be created or destroyed, only transformed between forms.

Friction and air resistance dissipate mechanical energy as heat.

Mass is a measure of inertia and remains constant regardless of location.

Wavelength and frequency are inversely related when wave speed is constant.

Sound travels faster in solids than in gases because particles are closer together.

Light can be reflected, refracted, absorbed, or transmitted when it meets a surface.

Electrical energy is measured in kilowatt-hours; 1 kWh = 3.6 × 10⁶ J.

Magnetic poles always occur in pairs; cutting a magnet produces two smaller magnets.

The direction of induced current depends on the direction of magnetic field change.

The cell cycle includes interphase (growth and DNA replication) and mitosis (nuclear division).

DNA replication produces two identical DNA molecules before cell division.

Alleles are alternative forms of a gene; homozygous means identical alleles, heterozygous means different.

Sex-linked traits appear more often in males because they carry one X chromosome.

Genetic engineering modifies DNA directly; selective breeding selects desirable traits over generations.

Extinction occurs when a species cannot adapt to environmental change.

Producers convert solar energy into chemical energy through photosynthesis.

Consumers obtain energy by eating other organisms; decomposers break down dead matter.

Population growth is limited by carrying capacity and density-dependent factors like disease and competition.

The nitrogen cycle relies on bacteria to fix atmospheric nitrogen into usable forms.

Pollution can acidify water and soil, harming organisms and reducing biodiversity.

Sustainable practices balance resource use with ecosystem health for future generations.

Earth's magnetic field is generated by motion of molten iron in the outer core.

Earthquakes occur at plate boundaries where stress releases as seismic waves.

Weathering and erosion break down and transport rock, shaping landforms.

The Sun's energy drives atmospheric circulation and the water cycle.

The Moon's gravity causes tides, and its orbit produces phases and eclipses.

The solar system formed about 4.6 billion years ago from a rotating cloud of gas and dust.