Master these fundamental physics concepts to excel on the ACT Science section.
The ACT Science section tests your ability to analyze data and understand physics concepts through passages about forces, motion, electricity, and energy. The section presents scientific information in graphs, tables, and research summaries, and requires you to draw conclusions from the information provided.
Success on the 2025 ACT Science section requires understanding core physics concepts like acceleration, momentum, and energy conservation. The 300 topics below cover the most commonly tested physics concepts that appear across multiple ACT Science passages.
We recommend familiarizing yourself with these core concepts, but don't feel pressured to memorize every detail. Focus on understanding the big ideas and how they relate to each other, as the ACT Science section tests your ability to analyze and apply concepts rather than recall specific facts.
The rate at which an object’s velocity changes over time.
Example: Imagine you’re at a red light in a car. When the light turns green, you press the gas pedal, causing your velocity to increase from 0 mph to 60 mph over 10 seconds. Your average acceleration can be calculated by Δv/Δt. In SI units, that’s changing from 0 to about 27 m/s in 10 s, giving an acceleration of 2.7 m/s². The “push” you feel against the seat is a direct sensation of this acceleration.
The speed of an object in a specific direction.
Example: If you walk 3 meters per second eastward, your velocity is 3 m/s east. If you turn to walk north while keeping the same speed, your velocity changes direction even if the numerical value (speed) remains the same.
The distance traveled per unit of time, without regard to direction.
Example: Driving 60 miles in 2 hours yields an average speed of 30 mph. Whether you drive north or south doesn’t matter for speed—only distance over time.
The straight-line distance and direction from an object’s starting point to its ending point.
Example: If you start at home, walk 4 blocks east, then 3 blocks west, your final position is 1 block east of home. Your displacement is not 7 blocks total but rather 1 block in the eastward direction from your starting location.
A quantity that has both magnitude and direction.
Example: A plane flying 500 km/h due north has a velocity vector described by “500 km/h, north.” If it changes course to northeast, that vector has a northern component and an eastern component.
A quantity that has only magnitude (no direction).
Example: Your mass is a scalar—“70 kilograms” doesn’t include any direction. Similarly, the temperature of 25 °C is a scalar measure with no directional component.
The product of an object’s mass and velocity, representing how difficult it is to stop.
Example: A truck with a mass of 2,000 kg moving at 20 m/s has a momentum of 40,000 kg·m/s. Stopping this truck requires a significant force over a certain time interval—more so than stopping a lighter vehicle moving at the same speed.
An object’s resistance to changes in its state of motion.
Example: When a bus quickly stops, passengers lurch forward. Their bodies want to continue moving at the previous speed due to inertia, illustrating why seat belts are necessary.
A push or a pull that can cause an object to accelerate.
Example: Pushing a stalled car requires a large force to overcome friction and inertia. The more people pushing (greater total force), the faster the car can start moving.
The attractive force that acts between any two masses.
Example: Dropping a pen from your hand shows Earth’s gravitational pull. Astronauts on the International Space Station appear weightless because they’re in free fall around Earth, still influenced by gravity but moving fast enough laterally to orbit.
The amount of matter in an object; not dependent on gravity.
Example: A 2 kg bowling ball has the same mass on Earth and on the Moon, even though it weighs less on the Moon (because the Moon’s gravitational pull is weaker).
The force of gravity on an object (mass × gravitational acceleration).
Example: On Earth, where g ≈ 9.8 m/s², a 2 kg object weighs 2 × 9.8 = 19.6 N. On the Moon, g is about 1.6 m/s², so the same object weighs only 3.2 N.
The force that opposes motion when two surfaces are in contact.
Example: Rubbing your hands together rapidly makes them warm. The kinetic friction between your palms converts mechanical energy to thermal energy (heat).
The perpendicular force exerted by a surface on an object resting on it.
Example: Placing a book on a table: The table presses up on the book with a force equal to the book’s weight in order to keep it at rest (in the vertical direction).
The pulling force transmitted through a string, rope, or cable.
Example: When you swing a bucket of water in a circle, the rope exerts tension on the bucket, pulling it inward and preventing it from flying off in a straight line.
A measure of the force that can cause an object to rotate about an axis.
Example: Using a long wrench to loosen a lug nut on a car tire increases the lever arm and thus torque, making it easier to turn the nut compared to using a short wrench.
The product of the force on an object and the distance the object moves in the direction of that force.
Example: Lifting a 10 N book 2 meters off the floor requires 10 × 2 = 20 joules of work. If you simply hold the book (no movement), you do no physical work on it in the physics sense.
The ability to do work or cause change.
Example: A charged phone battery has chemical potential energy. When you use the phone, that energy becomes electrical energy powering the device and eventually converts to heat or light.
The energy of motion, calculated by (1/2)mv².
Example: A baseball of mass 0.15 kg moving at 40 m/s has KE = (1/2) × 0.15 × 40² = 120 J. When it hits a catcher’s mitt, that energy dissipates as heat and sound.
The stored energy of an object due to its position or state.
Example: A 1 kg book on a shelf 2 meters high has a gravitational potential energy of 1 × 9.8 × 2 = 19.6 J. Dropping it converts this potential energy into kinetic energy.
The sum of kinetic and potential energy in a system.
Example: In a pendulum, at its highest swing, the energy is mostly potential. At its lowest point, it’s mostly kinetic. The total mechanical energy (ignoring air resistance) remains nearly constant.
The rate at which work is done or energy is transferred.
Example: A 100-watt light bulb converts 100 joules of electrical energy into light and heat every second. If you climb stairs quickly, you exert more power than if you climb slowly (same work, less time).
A device that changes the direction or magnitude of a force (e.g., lever, pulley, wheel and axle).
Example: Using a crowbar to lift a heavy crate: You apply a small force over a larger distance, and the crowbar multiplies that force to lift the crate’s weight.
The ratio of useful work output to total work input, expressed as a percentage.
Example: If a pulley system requires you to pull 200 J of energy but only 160 J is transferred into lifting a load (the rest lost as heat/friction), its efficiency is (160/200) × 100% = 80%.
A disturbance that transfers energy through space or a medium.
Example: If you drop a pebble in a pond, circular ripples move outward. Water molecules mostly move up and down, but energy is carried horizontally through the water.
The number of wave cycles passing a point per unit time.
Example: If 10 wave crests pass a pier each second, the wave’s frequency is 10 Hz. High-frequency sound waves in a whistle create a high-pitched note.
The distance between two corresponding points on consecutive waves (e.g., crest to crest).
Example: In the visible light spectrum, red light has a longer wavelength (~700 nm) compared to blue light (~450 nm). This difference affects how we perceive color.
The height of a wave from its midpoint (rest position) to its crest or trough.
Example: For a sound wave, a larger amplitude translates to a louder sound. For a water wave, a bigger amplitude means taller waves.
The time it takes for one full wave cycle to pass a point.
Example: If a wave has a frequency of 5 Hz, each cycle takes 0.2 seconds (period = 1/frequency).
The bouncing back of a wave when it encounters a barrier.
Example: Shining a flashlight on a mirror: The angle at which light hits the mirror (angle of incidence) equals the angle at which it bounces off (angle of reflection).
The bending of a wave as it passes from one medium to another and changes speed.
Example: A straw in a glass of water appears bent at the surface because light traveling from water to air changes speed, causing the light rays to bend and create an optical illusion.
The spreading out of waves as they pass through an opening or around obstacles.
Example: When ocean waves squeeze through a narrow harbor entrance, they fan out into the open water on the other side. Similarly, you can hear sound around a corner due to diffraction.
When two waves meet, they superimpose and can form a resultant wave of greater or lower amplitude.
Example: Noise-canceling headphones generate sound waves 180° out of phase with incoming noise, destructively interfering to reduce or cancel the unwanted sound.
The change in frequency of a wave in relation to an observer moving relative to the source.
Example: An ambulance siren sounds higher in pitch as it drives toward you and lower as it moves away, because the sound waves are compressed (higher frequency) when approaching and stretched (lower frequency) when receding.
A longitudinal wave that travels through a medium via compression and rarefaction of particles.
Example: Plucking a guitar string causes vibrations that compress and expand the air molecules around the string. These pressure changes travel to your ear and you perceive them as sound.
A wave of electric and magnetic fields that does not require a medium to travel.
Example: Sunlight (visible light) travels 150 million km through the vacuum of space to reach Earth, carrying energy that warms the planet.
A quantum (particle) of electromagnetic radiation, carrying energy proportional to its frequency.
Example: Solar panels absorb photons from sunlight. Each photon’s energy can dislodge an electron in the panel’s material, creating an electric current.
A property of particles that causes them to experience a force in an electromagnetic field (positive or negative).
Example: Rubbing a balloon on your hair transfers electrons, giving the balloon a negative charge. The hair then becomes positively charged and is attracted to the balloon.
The flow of electric charge, typically through a conductor.
Example: Switching on a flashlight closes the circuit, and electrons begin to flow through the bulb’s filament, making it glow.
The energy per unit charge that drives electric current between two points.
Example: A 9V battery can push electrons through a small circuit (like an LED), giving each coulomb of charge 9 joules of electrical potential energy.
A measure of how much a material opposes the flow of electric current.
Example: A thin wire in a circuit has higher resistance than a thick wire of the same material, causing it to heat up more if current is high (e.g., in a light bulb filament).
A law stating that V = IR, where V is voltage, I is current, and R is resistance.
Example: If a 3 Ω resistor has a current of 2 A running through it, the voltage across it must be 3 × 2 = 6 V. If the voltage changes, current changes accordingly.
A closed path through which electric current flows.
Example: A simple flashlight circuit has a battery, switch, and a bulb. Only when the switch is “on” does the current complete the loop and light the bulb.
A circuit in which components are arranged in a single path; the same current flows through each component.
Example: In old-style string lights, if one bulb burns out (opening the circuit), the entire string of lights goes off because current can’t complete the single path.
A circuit in which components are arranged so current can flow through multiple branches independently.
Example: Household wiring is parallel: Turning off a kitchen light doesn’t affect the living room lights because they’re on separate branches.
The ability of a system to store electric charge, measured in farads (F).
Example: In a camera flash circuit, a capacitor charges up, then releases its stored energy quickly to power the flash, producing a bright burst of light.
The property of a conductor (coil) by which a change in current generates an induced voltage (EMF).
Example: Transformers on power lines rely on inductance to step voltage up or down. Changing current in the primary coil induces a voltage in the secondary coil.
A field around a magnet or moving electric charge where magnetic forces are exerted.
Example: Iron filings around a bar magnet align to show the invisible magnetic field lines from its north pole to its south pole.
A magnet created by electric current flowing through a coil of wire wrapped around a ferromagnetic core.
Example: Junkyard cranes use powerful electromagnets to pick up cars. Turning off the current causes the magnet to lose its magnetic power, releasing the metal.
A changing magnetic field in a coil induces an electromotive force (voltage) in that coil.
Example: In a generator, rotating a coil within a magnetic field induces an electric current. The faster the rotation or the stronger the magnetic field, the greater the voltage generated.
The direction of induced current opposes the change that created it.
Example: If you move a magnet toward a coil, the coil’s induced current creates a magnetic field that repels the approaching magnet, resisting the motion.
The electric force between two charged objects is proportional to the product of their charges and inversely proportional to the square of the distance between them.
Example: Two charged balloons repel more strongly if they are closer together. Doubling their separation distance reduces the force by a factor of four.
A region around a charged object in which electric forces act on other charged objects.
Example: A charged balloon can deflect a stream of running water because the water’s neutral molecules align with the balloon’s electric field and are attracted or repelled.
A material that allows electric charges (or heat) to flow freely.
Example: Copper wires in household circuits let electrons move easily. That’s why copper is used widely for electrical wiring.
A material that resists or prevents the flow of electric charge (or heat).
Example: Wires are coated with plastic insulation so you can safely handle the cord without allowing current to pass through your hand.
A material whose conductivity is between that of an insulator and a conductor, often manipulated by doping.
Example: Silicon wafers in computer chips are doped with small amounts of other elements to control how electricity flows, enabling transistors to switch current on and off.
The study of heat, work, and forms of energy transformation.
Example: In an internal combustion engine, fuel combustion (chemical energy) is converted into mechanical work (pistons moving), with some energy lost as heat.
The transfer of thermal energy from a hotter object to a cooler one.
Example: Placing a warm hand on a cold metal railing transfers heat to the metal until both reach a common temperature.
A measure of the average kinetic energy of particles in a substance.
Example: Particles in boiling water at 100 °C move faster on average than those in ice water at 0 °C, though both contain water molecules.
The transfer of heat by direct contact between molecules.
Example: Touching a hot stove burner transfers heat rapidly to your hand, which is why it feels hot and can cause a burn.
The transfer of heat by the movement of fluids (liquids or gases).
Example: In a heated room, the air near a radiator warms and rises while cooler air sinks, creating a circulation pattern that distributes heat.
The transfer of energy by electromagnetic waves, without requiring a medium.
Example: Standing in sunlight, you feel warmth on your skin even though the air might be cold; that warmth is due to infrared radiation from the Sun.
The amount of heat needed to raise the temperature of 1 gram of a substance by 1 °C.
Example: Water’s high specific heat (4.18 J/g·°C) means it warms up and cools down more slowly than metals, which is why coastal climates are milder.
A measure of the disorder or randomness in a system.
Example: A solid ice cube (ordered structure) melts into liquid water (less ordered) at room temperature. This naturally increases entropy, aligning with the second law of thermodynamics.
A theoretical heat engine operating on a reversible cycle (Carnot cycle), offering maximum possible efficiency.
Example: Real engines can never be perfectly reversible or reach Carnot’s ideal efficiency; friction and other irreversibilities always lower their actual efficiency.
Force per unit area applied on a surface.
Example: Wearing snowshoes spreads your weight over a larger area, reducing pressure on the snow so you don’t sink as deeply.
The upward force exerted by a fluid on a submerged or partially submerged object.
Example: When you get into a swimming pool, you feel lighter because the water pushes up on your body with a force equal to the weight of the water displaced.
As the speed of a fluid increases, its pressure decreases.
Example: Airplane wings are shaped so that air travels faster over the top surface, decreasing air pressure above the wing and creating lift that can support the plane’s weight.
The buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object.
Example: A cargo ship floats because it displaces a volume of water whose weight is equal to the ship’s total mass.
Explains the properties of gases in terms of the motion of their molecules.
Example: In a sealed container, gas molecules collide with the walls, creating pressure. Increasing temperature makes molecules move faster, raising pressure if the volume is constant.
PV = nRT, relating pressure (P), volume (V), amount of gas (n), temperature (T), and the gas constant (R).
Example: Heating a sealed container of air (constant n, constant V) increases T, which increases P. Pressure may build up to dangerous levels if the container isn’t vented.
A phase change from solid directly to gas, bypassing the liquid state.
Example: Dry ice (solid CO₂) in normal atmospheric conditions quickly changes to gaseous CO₂. The visible “smoke” you see is condensed water vapor around the cold gas.
The splitting of a heavy atomic nucleus into two smaller nuclei, releasing energy.
Example: In nuclear power plants, uranium-235 atoms split when struck by neutrons. This releases heat, which turns water into steam that drives turbines to generate electricity.
The joining of two light atomic nuclei to form a heavier nucleus, releasing energy.
Example: In the Sun’s core, hydrogen nuclei fuse to form helium, releasing massive amounts of energy that radiate outward and eventually reach Earth as sunlight.
The spontaneous emission of particles or radiation from unstable atomic nuclei.
Example: Radium slowly decays, emitting alpha particles. Over time, it transforms into different elements until a stable nucleus is formed.
The basic structural and functional unit of all living organisms.
Example: A single human cell (e.g., a skin cell) has organelles like mitochondria, ribosomes, and a nucleus. Trillions of these cells communicate and work together to keep your body functioning.
A single-celled organism without a nucleus or membrane-bound organelles (e.g., bacteria).
Example: Escherichia coli (E. coli) lives in the human gut. Despite having no nucleus, it carries out all vital processes such as metabolism, growth, and reproduction by binary fission.
An organism whose cells contain a nucleus and membrane-bound organelles.
Example: A plant cell has a nucleus containing DNA, mitochondria for energy, and chloroplasts for photosynthesis. All these organelles are enclosed by membranes.
A specialized structure within a cell that performs a specific function.
Example: The Golgi apparatus modifies and packages proteins. In secretory cells (like in the pancreas), the Golgi is often extensive because these cells produce large quantities of proteins for export.
A selectively permeable barrier surrounding the cell, regulating substance entry and exit.
Example: The phospholipid bilayer, studded with proteins, allows some molecules (like oxygen) to pass freely while transporting others (like glucose) through protein channels or pumps.
A rigid outer layer found in plants, fungi, and some prokaryotes, providing support and protection.
Example: Plant cell walls primarily contain cellulose. This rigid structure helps them maintain shape—why a plant stands upright without a skeleton.
The gel-like material inside the cell membrane that contains organelles.
Example: Metabolic pathways like glycolysis happen in the cytoplasm. The fluid portion is called the cytosol, through which molecules and organelles move.
The cell’s central organelle containing genetic material (DNA).
Example: In eukaryotic cells, the nucleus has a double membrane (the nuclear envelope) with pores controlling the movement of molecules like mRNA in and out.
The organelle that produces ATP (energy) through cellular respiration.
Example: Muscle cells have abundant mitochondria because they require large amounts of ATP for contraction. Each mitochondrion has its own DNA, suggesting an evolutionary origin from independent bacteria.
An organelle in plant and algal cells where photosynthesis occurs.
Example: The green pigment chlorophyll inside chloroplasts absorbs light energy, typically red and blue wavelengths, converting it into chemical energy stored in glucose.
A cellular structure that synthesizes proteins by translating messenger RNA.
Example: Ribosomes may float freely in the cytoplasm or attach to the rough endoplasmic reticulum, where proteins destined for membranes or secretion are produced.
A network of membranes involved in protein (rough ER) and lipid (smooth ER) synthesis.
Example: Liver cells have abundant smooth ER for lipid synthesis and detoxification. Pancreatic cells have lots of rough ER because they secrete digestive enzymes (proteins).
An organelle that modifies, sorts, and packages proteins and lipids for transport.
Example: Proteins from the rough ER travel to the Golgi, get modified (e.g., sugars added), then are packaged into vesicles for delivery to their destination inside or outside the cell.
A membrane-bound organelle containing enzymes that break down waste and foreign material.
Example: White blood cells engulf bacteria into vesicles that fuse with lysosomes, where enzymes degrade the bacteria. This is part of the immune response.
A storage sac within a cell, often large in plant cells.
Example: The central vacuole in plant cells can store water, nutrients, or toxins. When it’s full of water, it provides turgor pressure, helping the plant stand upright.
The molecule carrying genetic instructions for growth, development, and reproduction.
Example: In human cells, DNA is coiled into chromosomes. During replication, each DNA strand unwinds so it can be copied to ensure each new cell has a complete genome.
A molecule involved in protein synthesis; can also be genetic material in some viruses.
Example: In eukaryotes, mRNA carries the genetic “blueprint” from the nucleus to ribosomes in the cytoplasm. Some viruses like HIV use RNA (retroviruses) to store their genetic information.
A segment of DNA that codes for a specific protein or trait.
Example: The gene for insulin in humans directs cells in the pancreas to produce insulin protein, which regulates blood sugar levels.
A thread-like structure of nucleic acids and proteins carrying genetic information.
Example: Humans have 23 pairs of chromosomes in each somatic cell. During cell division, chromosomes condense and become visible under a microscope.
A type of cell division resulting in two identical daughter cells, important for growth and repair.
Example: When you scrape your knee, skin cells around the wound divide by mitosis, producing new cells to heal the cut.
A specialized cell division that produces gametes (sex cells) with half the usual number of chromosomes.
Example: In human ovaries and testes, meiosis reduces the chromosome number from 46 to 23 so that fertilization (egg + sperm) restores the full 46.
A sex cell (egg or sperm) containing half the number of chromosomes of a normal body cell.
Example: Human sperm cells have 23 chromosomes. When a sperm fertilizes an egg (also 23 chromosomes), the resulting zygote has 46.
The fusion of male and female gametes to form a zygote.
Example: In flowering plants, pollen (male gamete) lands on the stigma, grows a tube to the ovule, and fuses with the egg cell, creating a seed.
The cell formed by the union of two gametes; the earliest stage of a developing organism.
Example: A human zygote begins as a single cell but undergoes rapid mitotic divisions (cleavage), eventually forming an embryo.
The process by which cells build proteins from amino acids, directed by mRNA.
Example: Beta cells in the pancreas synthesize insulin protein, which is folded into a specific shape. The finished protein is then secreted into the bloodstream.
The process of copying DNA into messenger RNA (mRNA).
Example: If a cell needs hemoglobin, transcription factors bind to the hemoglobin gene region in DNA, and RNA polymerase synthesizes mRNA, which then exits the nucleus.
The process where ribosomes read mRNA to assemble amino acids into proteins.
Example: In your muscle cells, the mRNA for actin is read by ribosomes, linking amino acids in the correct sequence to create the actin protein used in muscle fibers.
A change in the DNA sequence that can result in altered traits.
Example: A single base substitution in the gene coding for hemoglobin can lead to sickle cell disease. This one genetic “misspelling” changes the shape of red blood cells.
The maintenance of a stable internal environment in an organism.
Example: Humans maintain a constant body temperature (~37 °C) using mechanisms like sweating (to cool down) or shivering (to generate heat).
All the chemical reactions in an organism necessary to maintain life.
Example: After eating, carbohydrates break down into glucose, which can be used (via cellular respiration) to produce ATP, the universal energy currency.
The process by which green plants and some other organisms convert light energy into chemical energy (glucose).
Example: In a leaf, chlorophyll absorbs sunlight. Energy from photons splits water molecules and ultimately helps fix carbon dioxide into glucose, releasing oxygen as a byproduct.
The process of converting biochemical energy from nutrients into ATP.
Example: Muscle cells break down glucose in the presence of oxygen to produce ATP. Excess CO₂ produced is carried to the lungs and exhaled.
Respiration that requires oxygen to generate ATP.
Example: Running a marathon primarily uses aerobic respiration in muscle cells, enabling sustained ATP production as long as the oxygen supply meets demand.
Respiration that does not use oxygen, often producing less ATP.
Example: During a sprint, muscle cells may switch to lactic acid fermentation (a type of anaerobic respiration) when oxygen is scarce, producing lactic acid that causes muscle fatigue.
A protein catalyst that speeds up biochemical reactions without being consumed.
Example: The enzyme amylase in saliva breaks down starch into simpler sugars, accelerating a reaction that would otherwise be very slow at body temperature.
The reactant that an enzyme acts upon.
Example: In the reaction catalyzed by lactase, lactose (milk sugar) is the substrate. Lactose intolerance occurs when there’s insufficient lactase to digest lactose properly.
The region on an enzyme where the substrate binds and the reaction occurs.
Example: The shape and chemical properties of an enzyme’s active site ensure only the correct substrate can fit—like a lock and key.
A structural change in a protein causing it to lose its function, often due to heat or pH changes.
Example: Cooking an egg white (mainly albumin protein) causes permanent denaturation. Its structure changes from clear and runny to solid and opaque.
The main energy currency of cells.
Example: ATP releases energy when its terminal phosphate bond is broken, forming ADP (adenosine diphosphate). This energy fuels cellular processes like muscle contraction.
The product formed when ATP loses a phosphate group; can be recycled back into ATP.
Example: During strenuous exercise, ADP can be quickly recharged into ATP by various pathways (e.g., glycolysis, oxidative phosphorylation), enabling continuous muscle activity.
An organism that makes its own food, typically through photosynthesis or chemosynthesis.
Example: Green plants and algae use sunlight to synthesize sugars from CO₂ and water. Some bacteria near deep-sea vents use chemical energy from sulfur compounds (chemosynthesis).
An organism that cannot make its own food and must consume other organisms.
Example: Animals, fungi, and many bacteria are heterotrophic—lions eat zebra muscle tissue, fungi absorb nutrients from decaying plant matter.
An organism (usually an autotroph) that serves as a food source for others in a food chain.
Example: Grass in a savanna ecosystem captures energy from the sun. Zebras graze on the grass, and lions feed on zebras, so grass is the producer.
An organism that obtains energy by feeding on other organisms.
Example: A cow (primary consumer) eats grass, while a human eating the cow’s meat is a secondary consumer. Each step up the food chain relies on producers at its base.
An organism that breaks down dead or decaying organisms, recycling nutrients back into the environment.
Example: Earthworms and bacteria in the soil decompose leaves and animal remains, releasing nutrients like nitrogen that plants can reuse.
A community of living organisms and their physical environment interacting as a system.
Example: A pond ecosystem includes fish, algae, aquatic plants, insects, and microorganisms, plus nonliving factors like water quality and sunlight.
A large community of plants and animals occupying a major habitat (e.g., rainforest, desert).
Example: The tundra biome is characterized by permafrost, low temperatures, short growing seasons, and specialized plants (mosses, lichens) adapted to cold.
The variety of life forms in a given area or ecosystem.
Example: The Amazon Rainforest boasts extremely high biodiversity, with countless insect species, birds, mammals, and plants coexisting.
The natural environment where an organism lives and grows.
Example: The habitat of a polar bear is the Arctic sea ice. Loss of sea ice due to climate change threatens its ability to hunt seals.
The role or function of an organism or species in an ecosystem.
Example: Bees’ niche includes pollinating flowers while gathering nectar. This activity aids plant reproduction and provides bees with food.
A position in a food chain, based on how an organism obtains energy.
Example: Producers (like grass) are at the first trophic level, herbivores (like grasshoppers) at the second, small carnivores (like frogs) at the third, and larger carnivores (like snakes or hawks) even higher.
A linear sequence of organisms through which nutrients and energy pass as one eats another.
Example: Sun → Grass → Grasshopper → Frog → Snake → Hawk. Each step transfers only a fraction of the energy to the next level.
A complex interconnection of food chains showing the various feeding relationships among organisms.
Example: In a forest, mice eat seeds, owls eat mice, hawks can also eat mice or small birds, and scavengers or decomposers consume the remains of all. These overlapping links form a *web*.
A close and long-term biological interaction between two different biological organisms.
Example: The fish called clownfish and sea anemone form a symbiotic relationship: clownfish clean the anemone while gaining protection from its stinging tentacles.
A symbiotic relationship in which both organisms benefit.
Example: A bee obtains nectar from a flower (food), while the flower benefits by having its pollen spread to other blooms.
A symbiotic relationship where one organism benefits and the other is neither harmed nor helped significantly.
Example: Orchids growing on tree branches gain better access to sunlight, while the tree isn’t noticeably affected by the orchid’s presence.
A symbiotic relationship in which one organism (the parasite) benefits and the other (the host) is harmed.
Example: Ticks feeding on a dog’s blood can weaken the dog and transmit diseases, while the tick gains nutrition.
The process by which populations of organisms change over generations due to genetic variation and natural selection.
Example: Over millions of years, whales evolved from land-dwelling, four-legged mammals into aquatic animals with flippers. Fossil records and DNA studies support this transformation.
The process by which organisms better adapted to their environment tend to survive and reproduce.
Example: After an environmental change darkened tree bark, moths with darker coloring were better camouflaged and survived at higher rates, passing on the dark-wing genes.
An inherited characteristic that enhances an organism’s ability to survive and reproduce.
Example: A cactus’s spines are modified leaves that reduce water loss, an adaptation to desert environments.
The formation of new and distinct species through evolution.
Example: A population of birds may become geographically isolated on an island. Over many generations, genetic differences accumulate until they can no longer interbreed with the original population.
Random changes in allele frequencies in a population, more noticeable in small populations.
Example: In a small group of ten rabbits, if only two breed successfully (by chance) and they both have a rare allele, that allele can quickly become common.
The total collection of genes in a population at any one time.
Example: A population of wolves has a certain range of coat colors, eye colors, etc. All the alleles for these traits combined make up the wolf pack’s gene pool.
A variant form of a gene.
Example: For blood type, alleles A, B, and O exist for the same gene, leading to possible genotypes (AA, AO, BB, BO, AB, or OO).
The observable characteristics or traits of an organism.
Example: A rose plant’s phenotype might be red flowers, tall stem, and thorny branches, all influenced by its genetic makeup and environment.
The genetic makeup of an organism (combination of alleles).
Example: A pea plant with genotype “Tt” (T = tall allele, t = short allele) might be tall (phenotype) because T is dominant, but it still carries the recessive t allele.
A diagram used to predict the genotypic outcomes of a particular cross or breeding experiment.
Example: Crossing Tt (tall) with Tt (tall) shows a 1:2:1 genotype ratio (TT : Tt : tt) and a 3:1 phenotype ratio (tall : short) in the offspring.
An allele that masks the expression of a recessive allele in a heterozygote.
Example: If “B” is a dominant allele for brown eyes, individuals with BB or Bb genotype will have brown eyes, overshadowing a recessive allele for blue eyes.
An allele whose effects are masked by a dominant allele in a heterozygote.
Example: If blue eyes require two recessive alleles (bb), then a person must have the genotype bb to show the blue-eyed phenotype.
Having two identical alleles for a trait (RR or rr).
Example: A pea plant could be homozygous dominant (RR) for round seeds or homozygous recessive (rr) for wrinkled seeds.
Having two different alleles for a trait (Rr).
Example: A pea plant with genotype Rr would display round seeds (dominant trait) but carry the allele for wrinkled seeds.
A chemical signal secreted by glands, regulating bodily functions and behaviors.
Example: Insulin produced by the pancreas lowers blood sugar levels by signaling cells (especially in the liver and muscle) to absorb glucose.
A nerve cell responsible for transmitting signals through electrical and chemical impulses.
Example: Motor neurons carry signals from the spinal cord to muscles, triggering contraction. Sensory neurons send signals from skin (touch) to the brain.
The body’s defense against infectious organisms and foreign invaders.
Example: When you get a cut, white blood cells attack bacteria that enter, while other immune responses help heal the wound.
A disease-causing microorganism (virus, bacterium, fungus, or parasite).
Example: Influenza virus infects respiratory cells, causing flu symptoms such as fever, cough, and fatigue.
The smallest unit of an element that retains the chemical properties of that element.
Example: A single helium atom in a balloon has a nucleus (2 protons, 2 neutrons) and 2 electrons orbiting it. Even one helium atom is fundamentally the same element as an entire balloonful of helium.
A substance composed of only one type of atom, defined by its number of protons.
Example: Hydrogen is the simplest element with just one proton. Gold (Au) has 79 protons in each atom, making it uniquely gold regardless of how many atoms or molecules you have.
A substance formed when two or more elements chemically bond in fixed proportions.
Example: Water (H₂O) always has 2 hydrogen atoms per oxygen atom. If the ratio changes, it’s no longer water (e.g., H₂O₂ is hydrogen peroxide).
The smallest unit of a compound (or element) formed by two or more atoms held together by covalent bonds.
Example: One water molecule (H₂O) has distinct properties, like polarity. When many water molecules gather, you get liquid water with characteristic cohesion and surface tension.
An atom or molecule with a net electric charge due to loss or gain of electrons.
Example: Table salt consists of Na⁺ (sodium ion, lost one electron) and Cl⁻ (chloride ion, gained one electron). These ions arrange in a crystal lattice.
A positively charged subatomic particle found in an atom’s nucleus.
Example: A hydrogen atom has one proton in its nucleus. Adding or removing protons changes the identity of the element itself.
A neutral (uncharged) subatomic particle found in the nucleus.
Example: Carbon typically has 6 neutrons, but carbon-14 has 8 neutrons. This difference makes carbon-14 radioactive.
A negatively charged subatomic particle that orbits the atom’s nucleus.
Example: In a neutral helium atom, 2 electrons orbit the nucleus. If one electron is removed, the resulting helium ion is He⁺.
The number of protons in an atom’s nucleus, defining the element.
Example: Nitrogen has an atomic number of 7, so any atom with exactly 7 protons is nitrogen, regardless of the number of neutrons.
The total number of protons and neutrons in an atom’s nucleus.
Example: Carbon-12 has 6 protons and 6 neutrons, giving a mass number of 12. Carbon-13 has 7 neutrons (mass number 13).
Atoms of the same element with different numbers of neutrons.
Example: Hydrogen has three common isotopes: protium (no neutrons), deuterium (1 neutron), and tritium (2 neutrons). Chemically similar, but different masses.
An electrostatic attraction between oppositely charged ions.
Example: In sodium chloride (NaCl), Na⁺ and Cl⁻ form a crystalline lattice. Each sodium ion is surrounded by chloride ions, giving salt its characteristic cubic crystals.
A bond where two atoms share one or more pairs of electrons.
Example: In a methane molecule (CH₄), carbon shares one pair of electrons with each hydrogen, forming four covalent bonds.
A bond found in metals where electrons are free to move through a lattice of positively charged ions.
Example: Copper’s “sea of electrons” allows it to conduct electricity and heat efficiently, and also gives it malleability and ductility.
A molecule with an uneven distribution of charge, resulting in partial positive and negative poles.
Example: Water (H₂O) has a partial negative charge on the oxygen and partial positives on the hydrogens. This leads to hydrogen bonding and many of water’s unique properties (like high boiling point).
A molecule with an even distribution of electrons, lacking distinct poles.
Example: CO₂ is linear, with dipoles that cancel out, making it a nonpolar molecule despite containing polar bonds.
A weak bond between a hydrogen atom (bound to F, O, or N) and another electronegative atom.
Example: Hydrogen bonds between water molecules cause ice to float (ice is less dense than liquid water) and allow water to remain liquid over a wide temperature range.
A measure of an atom’s ability to attract and hold electrons in a bond.
Example: Oxygen strongly pulls electrons toward itself in covalent bonds, leading to polar molecules like water.
A unit representing 6.022 × 10²³ particles (Avogadro’s number) of a substance.
Example: One mole of water (H₂O) is about 18 grams, containing 6.022 × 10²³ molecules. This is essential for relating mass to number of molecules.
6.022 × 10²³, the number of particles in exactly one mole of a substance.
Example: If you have one mole of carbon atoms, you have 6.022 × 10²³ carbon atoms, which weighs about 12 grams for carbon-12.
The mass of one mole of a substance, measured in grams per mole (g/mol).
Example: The molar mass of CO₂ is ~44 g/mol (12 for carbon + 2 × 16 for oxygen).
Specifies the actual number of each type of atom in a molecule.
Example: Benzene’s molecular formula is C₆H₆, indicating 6 carbons and 6 hydrogens in each molecule.
The simplest whole-number ratio of atoms of each element in a compound.
Example: Benzene’s empirical formula is CH (6:6 reduces to 1:1), but its molecular formula is C₆H₆.
The percentage by mass of each element in a compound.
Example: Water is about 11.2% hydrogen by mass and 88.8% oxygen. If you have 100 g of water, about 11.2 g is H and 88.8 g is O.
The calculation of reactants and products in chemical reactions based on the law of conservation of mass.
Example: Burning methane (CH₄ + 2O₂ → CO₂ + 2H₂O). If you start with 16 g of CH₄ (1 mole), you need 2 moles of O₂, producing 44 g of CO₂ and 36 g of H₂O.
A chemical equation with the same number of atoms of each element on both sides.
Example: 2H₂ + O₂ → 2H₂O ensures 4 hydrogen and 2 oxygen atoms on each side, satisfying conservation of mass.
A starting substance in a chemical reaction, undergoing change.
Example: In photosynthesis, CO₂ and H₂O are reactants that the plant converts into glucose (C₆H₁₂O₆) and O₂ with light energy.
A substance formed as a result of a chemical reaction.
Example: In the combustion of propane (C₃H₈), the products are CO₂ and H₂O.
The reactant that is completely consumed first, limiting the amount of product formed.
Example: If you have 10 eggs and 15 slices of bread to make egg sandwiches, eggs might be limiting. Once you run out of eggs, you can’t make more sandwiches.
The reactant that remains after a chemical reaction has reached completion.
Example: In the sandwich example, bread would be in excess once the eggs are used up.
A substance that speeds up a chemical reaction without being permanently changed itself.
Example: A platinum catalyst in a car’s catalytic converter helps convert toxic exhaust gases into less harmful products faster.
A measure of the total energy (heat content) in a thermodynamic system.
Example: Exothermic reactions release energy (negative ΔH). When natural gas burns, the products have lower enthalpy than the reactants.
A reaction that releases heat (ΔH < 0).
Example: Combustion of candle wax produces heat and light. Touching a metal spoon near the flame quickly shows the spoon warming.
A reaction that absorbs heat (ΔH > 0).
Example: Instant cold packs use ammonium nitrate dissolving in water, absorbing heat from the surroundings and making the pack feel cold.
A measure of the disorder in a system.
Example: Dissolving salt in water increases entropy because the ordered crystal structure becomes dispersed ions moving freely in solution.
A thermodynamic quantity representing the maximum usable energy from a reaction. ΔG = ΔH - TΔS.
Example: A negative ΔG indicates a spontaneous reaction under given conditions. Rust formation (oxidation of iron) has a negative ΔG at normal temperatures, so it proceeds spontaneously.
The minimum energy required to start a chemical reaction.
Example: Even though burning paper is exothermic, you need a spark or flame to break initial bonds and begin the reaction.
The state in which the forward and reverse reactions occur at the same rate, and concentrations remain constant.
Example: In N₂ + 3H₂ ⇌ 2NH₃, at equilibrium, ammonia is formed as quickly as it breaks down. Changing temperature or pressure can shift equilibrium.
If a dynamic equilibrium is disturbed, the system shifts to counteract the disturbance and restore equilibrium.
Example: Increasing pressure in ammonia synthesis drives the reaction toward the side with fewer gas moles (NH₃), thus producing more ammonia.
A substance that donates hydrogen ions (H⁺) in solution (pH < 7).
Example: Hydrochloric acid (HCl) in water fully dissociates into H⁺ and Cl⁻. Your stomach acid uses HCl to help break down food.
A substance that accepts hydrogen ions or produces hydroxide ions (OH⁻) in solution (pH > 7).
Example: Sodium hydroxide (NaOH) in water forms Na⁺ and OH⁻. Commonly used in drain cleaners due to its ability to dissolve grease and hair.
A scale (0–14) indicating the acidity or alkalinity of a solution, based on [H⁺].
Example: Lemon juice (pH ~2) is acidic, water (pH 7) is neutral, and ammonia (pH ~11) is basic. Each pH unit represents a tenfold change in [H⁺].
A scale indicating the hydroxide ion concentration in a solution; pOH + pH = 14 at 25°C.
Example: A solution with pH 3 has pOH 11. That solution is strongly acidic with a high [H⁺] and low [OH⁻].
A solution that resists changes in pH when small amounts of acid or base are added.
Example: Blood contains bicarbonate (HCO₃⁻) which neutralizes added H⁺ or OH⁻, keeping blood pH around 7.4.
A technique to determine the concentration of a solute by reacting it with a solution of known concentration.
Example: Adding a base of known molarity to an acid until the endpoint (color change) reveals the acid’s concentration.
A process in which an atom or ion loses electrons.
Example: When iron rusts, Fe atoms lose electrons to oxygen. Fe → Fe²⁺ + 2e⁻ is the oxidation half-reaction.
A process in which an atom or ion gains electrons.
Example: In the same rusting process, oxygen gains electrons to form oxide ions (O²⁻), combining with Fe²⁺ to make iron oxide (Fe₂O₃).
A reaction that involves the transfer of electrons between two species (oxidation and reduction).
Example: A battery’s operation relies on a redox reaction: Zinc is oxidized at the anode, and manganese oxide is reduced at the cathode, generating electric current.
The species that causes oxidation and is itself reduced.
Example: In Zn + Cu²⁺ → Zn²⁺ + Cu, Cu²⁺ is the oxidizing agent because it gains electrons (reduction).
The species that causes reduction and is itself oxidized.
Example: In the same reaction, Zn metal is the reducing agent, losing electrons to Cu²⁺ and forming Zn²⁺.
The substance dissolved in a solvent to form a solution.
Example: Salt is the solute in saltwater, separating into Na⁺ and Cl⁻ ions.
The substance (often a liquid) in which a solute dissolves.
Example: Water is the “universal solvent,” dissolving a wide variety of substances due to its polarity.
A homogeneous mixture of two or more substances.
Example: Sugar water is uniform throughout—spoonfuls from anywhere in the mixture taste equally sweet.
The maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature.
Example: At 20°C, ~36 g of NaCl dissolve in 100 g of water. Adding more salt leads to undissolved crystals.
The amount of solute per amount of solution or solvent.
Example: A 10% salt solution has 10 g of salt per 100 g of solution. It can also be expressed in molarity.
Moles of solute per liter of solution.
Example: A 1 M NaCl solution has 1 mole of NaCl (about 58.44 g) dissolved per liter of total solution volume.
Moles of solute per kilogram of solvent.
Example: A 1 m solution of glucose contains 1 mole of glucose (about 180 g) per 1 kg of water.
An insoluble solid that emerges from a liquid solution during a reaction.
Example: Mixing silver nitrate (AgNO₃) with sodium chloride (NaCl) produces silver chloride (AgCl), a white precipitate, and soluble sodium nitrate (NaNO₃).
The point at which no more solute can dissolve in the solvent at a given temperature.
Example: If you keep adding sugar to iced tea and stirring, eventually sugar crystals remain undissolved at the bottom, indicating saturation.
Properties of solutions that depend on the ratio of solute particles to solvent molecules, not on the identity of the solute.
Example: Adding salt to water raises its boiling point and lowers its freezing point; these changes depend on how many dissolved particles are present.
The increase in boiling point of a solvent when a solute is dissolved.
Example: Salting water for cooking pasta increases its boiling point slightly, allowing water to get hotter before boiling.
The decrease in freezing point of a solvent when a solute is dissolved.
Example: Roads are salted in winter because the salt solution on the road’s surface has a lower freezing point than pure water, reducing ice formation.
A conductor through which electricity enters or leaves a medium (electrolyte).
Example: In a battery, the anode (negative electrode) and cathode (positive electrode) are immersed in an electrolyte to facilitate electron flow.
A substance that conducts electricity when dissolved in water.
Example: Sports drinks contain electrolytes (like Na⁺, K⁺, Cl⁻) that help maintain electrical conductivity in your body’s fluids.
A mixture of two or more metals or a metal and another element.
Example: Steel is an alloy of iron and carbon, stronger and less prone to corrosion than pure iron.
The level of reactivity of a metal, typically increasing down a group in the periodic table.
Example: Alkali metals like sodium have high metallic character and react vigorously with water, forming NaOH and H₂ gas.
A tabular arrangement of elements based on atomic number, electron configuration, and recurring properties.
Example: Elements in the same group (column) share similar chemical behaviors, e.g., the halogens in Group 17 are all highly reactive nonmetals.
A vertical column in the periodic table; elements in a group often share properties.
Example: Group 1 (alkali metals) includes lithium, sodium, potassium—all have one valence electron.
A horizontal row in the periodic table; properties change progressively across a period.
Example: In period 2, lithium (left side) is a highly reactive metal, while neon (far right) is an inert noble gas.
Elements in Group 1, highly reactive with one valence electron.
Example: Potassium must be stored in oil to prevent it from reacting with moisture in air.
Elements in Group 2, reactive but less so than alkali metals, with two valence electrons.
Example: Magnesium burns with a bright white flame. It’s used in flares and fireworks.
Elements in the central block of the periodic table, often forming variable oxidation states.
Example: Iron can form Fe²⁺ or Fe³⁺. Copper can form Cu⁺ or Cu²⁺. This property leads to colorful compounds (e.g., Cu²⁺ is typically blue).
Elements in Group 17, highly reactive nonmetals with seven valence electrons.
Example: Fluorine (F₂) is the most reactive halogen, capable of forming bonds with almost all elements, including some noble gases under extreme conditions.
Elements in Group 18, mostly inert with full valence electron shells.
Example: Neon is used in glowing signs; helium is lighter than air and nonreactive, making it safe to use in balloons.
A large molecule composed of repeating structural units (monomers).
Example: Polyethylene (common plastic) is formed from ethylene (CH₂=CH₂) units. Natural polymers include cellulose in plant cell walls and proteins in living organisms.
A testable prediction or explanation based on limited evidence as a starting point for investigation.
Example: “If tomato plants are watered with a nutrient-rich solution, then they will produce more tomatoes.” You’d then design an experiment measuring the yield of tomatoes under different watering conditions.
A well-substantiated explanation of some aspect of the natural world, repeatedly confirmed through observation and experiment.
Example: The Cell Theory states that all living things are composed of cells and that all cells come from preexisting cells—supported by countless observations with microscopes and experiments on cell division.
A statement describing a consistently observed phenomenon, often expressed mathematically.
Example: Newton’s Law of Universal Gravitation quantifies the gravitational force between masses (F = Gm₁m₂ / r²). It describes *what* happens, not *why*.
A controlled procedure carried out to discover, test, or demonstrate a fact or theory.
Example: Testing the effect of pH on enzyme activity by preparing multiple test tubes with different pH levels, then measuring reaction rates. This controlled approach isolates the variable (pH).
The group in an experiment that does not receive the independent variable treatment, used as a baseline.
Example: If you’re testing a new fertilizer, the control group would be plants grown without that fertilizer. Comparing growth results isolates the effect of the fertilizer itself.
Any factor that can change in an experiment.
Example: In testing fertilizer impact, variables might include amount of fertilizer, sunlight, water, soil type, and plant species. Ideally, only fertilizer amount changes (independent variable), while others remain constant.
The factor deliberately changed or manipulated by the experimenter.
Example: Giving different amounts of fertilizer to different groups of tomato plants is the independent variable. You set these levels to see how they affect plant growth.
The factor measured or observed in response to changes in the independent variable.
Example: In the fertilizer experiment, the dependent variable might be plant height or the number of tomatoes produced after a set period.
A factor kept the same for all groups in an experiment to ensure fair testing.
Example: Using identical pots, soil, water schedules, and light conditions keeps all factors the same except the fertilizer amount.
Information collected during an experiment or study.
Example: Recorded plant heights every week, fruit counts, or nutrient content in leaves. Data can be qualitative (descriptive) or quantitative (numerical).
Data or observations described in words rather than numbers.
Example: “The plant leaves turned yellowish and had spots” or “the solution became cloudy” are qualitative observations.
Data expressed in numbers or measurable quantities.
Example: “Plant A grew 10 cm in two weeks” or “the reaction temperature was maintained at 37 °C” allow for statistical or mathematical analysis.
How close a measurement is to the true or accepted value.
Example: A thermometer reading 100.0 °C for boiling water at sea level is accurate (the accepted boiling point is ~100 °C under those conditions).
How consistently repeated measurements produce similar results.
Example: Weighing a 100 g mass multiple times and getting 99.98 g, 99.99 g, and 100.01 g shows high precision, even if it’s slightly off from exactly 100 g.
The size, length, or amount of something, determined by comparing to a standard unit.
Example: Using a meter stick to measure the length of a table in centimeters or a thermometer to measure temperature in °C.
The standard set of units used globally for scientific measurements.
Example: The base units include meters (m) for length, kilograms (kg) for mass, seconds (s) for time, kelvins (K) for temperature, and moles (mol) for amount of substance.
The SI base unit of length.
Example: One meter was historically tied to Earth’s measurements. It is now defined based on the distance light travels in a vacuum in 1/299,792,458 of a second.
A unit of volume in the metric system.
Example: Grocery stores often sell 2-liter soda bottles. This measures liquid volume in many countries.
A unit of mass in the metric system.
Example: A small paperclip typically has a mass of about 1 g. Everyday objects, like sugar packets, are also commonly measured in grams.
The SI base unit of time.
Example: Defined by the radiation frequency of cesium-133. Most clocks and watches track time in seconds.
The SI base unit of thermodynamic temperature.
Example: 0 K is absolute zero, the point of minimal thermal motion. Room temperature (~20 °C) is about 293 K.
The SI base unit measuring the amount of substance, equal to 6.022 × 10²³ particles.
Example: 1 mole of carbon-12 weighs exactly 12 grams. This standard helps chemists calculate quantities in reactions.
A problem-solving technique that uses conversion factors to move from one unit to another.
Example: Converting 3 days to seconds: 3 days × (24 hours / 1 day) × (3600 s / 1 hour) = 259,200 s.
A method of expressing very large or small numbers as a product of a number between 1 and 10 and a power of 10.
Example: 300,000,000 can be written as 3.0 × 10⁸. This format is especially useful in scientific calculations.
The digits in a measurement that are known with certainty plus one estimated digit.
Example: 0.0340 has three significant figures (3, 4, and the last 0), whereas 0.034 has two (3 and 4).
The difference between a measured value and the true value.
Example: If you measure 98.5 g for a 100 g standard mass, the *error* is -1.5 g.
The difference between the experimental value and the accepted value, divided by the accepted value, times 100%.
Example: (|98.5 - 100| / 100) × 100% = 1.5% error in your measurement.
The examination and interpretation of data to find patterns, relationships, or explanations.
Example: Graphing plant growth vs. fertilizer amount might show an optimal point—too little or too much fertilizer can reduce growth.
A summary of the experiment’s results and whether they support the hypothesis.
Example: “Plants with moderate fertilizer grew tallest, supporting the hypothesis that some fertilizer increases growth, but excessive fertilizer caused toxicity.”
The theory that Earth’s outer shell is divided into plates that move over the mantle.
Example: The movement of the Pacific Plate causes earthquakes along the San Andreas Fault. Over millions of years, these plates shift, forming mountains and rearranging continents.
The gradual movement of continents across Earth’s surface through geological time.
Example: Fossils of the same species found on the coastlines of South America and Africa suggest they were once connected (Pangaea).
The process by which new oceanic crust forms at mid-ocean ridges and slowly moves outward.
Example: At the Mid-Atlantic Ridge, magma emerges from below Earth’s crust, cools to form new basaltic rock, pushing older crust aside.
The preserved remains or traces of ancient organisms.
Example: Dinosaur bones in sedimentary rock date back tens of millions of years, giving insight into past life forms and ecosystems.
A system of chronological dating that relates geological strata to time.
Example: The Mesozoic Era (age of dinosaurs) and Cenozoic Era (age of mammals) help structure Earth’s 4.54-billion-year history.
The process by which wind, water, ice, or gravity transports soil and sediment from one location to another.
Example: The Grand Canyon was formed over millions of years by the Colorado River eroding rock layers.
The breakdown of rocks into smaller pieces by physical or chemical processes.
Example: Acid rain chemically weathers limestone, while freeze-thaw cycles physically crack rocks.
A model describing the transformations between igneous, sedimentary, and metamorphic rocks.
Example: Magma cools to form igneous rock; weathering and compaction form sedimentary rock; heat and pressure transform sedimentary rock into metamorphic rock, which can melt again.
A naturally occurring, inorganic solid with a definite chemical composition and crystal structure.
Example: Quartz (SiO₂) forms six-sided crystals and ranks 7 on the Mohs hardness scale, used in glassmaking and electronics.
Rock formed by the cooling and solidification of molten magma or lava.
Example: Granite forms slowly beneath Earth’s surface (coarse-grained), while basalt forms at the surface from lava (fine-grained).
Rock formed by the accumulation and compaction of sediment, often containing fossils.
Example: Sandstone forms from layers of compressed sand. Fossils can be preserved if dead organisms are quickly buried in sediment.
Rock formed when existing rock is altered by heat, pressure, or chemical processes.
Example: Limestone transforms into marble under high pressure and temperature within Earth’s crust.
The continuous movement of water on, above, and below Earth’s surface.
Example: Water evaporates from oceans, condenses into clouds, falls as precipitation, flows in rivers, and returns to oceans.
Water released from clouds in forms such as rain, snow, or hail.
Example: When moisture in clouds condenses enough to overcome updrafts and gravity, it falls to Earth as precipitation.
The process by which water changes from liquid to gas at the surface.
Example: After a rainfall, puddles evaporate on a sunny day as water molecules gain enough energy to escape into the air.
The release of water vapor from plants into the atmosphere.
Example: Leaves lose water through stomata, cooling the plant and contributing moisture to the air (especially in forests).
The transformation of water vapor into liquid droplets.
Example: Dew forms overnight on grass when the air cools enough for water vapor to condense on cooler surfaces.
The circulation and transformation of carbon between living organisms and the environment.
Example: Carbon in the atmosphere as CO₂ is taken up by plants for photosynthesis and returned by respiration or combustion.
The cycling of nitrogen among the atmosphere, soil, and organisms.
Example: Nitrogen-fixing bacteria convert N₂ into ammonia, plants use nitrates, animals eat the plants, and decomposers return nitrogen to the soil or atmosphere.
A layer in Earth’s stratosphere containing a concentration of ozone (O₃) that protects life by blocking most UV radiation.
Example: Thinning of the ozone layer over Antarctica allows more harmful UV rays to reach Earth’s surface, increasing skin cancer risks.
The trapping of the Sun’s heat in Earth’s lower atmosphere by greenhouse gases.
Example: CO₂, methane, and water vapor absorb and re-radiate infrared light, keeping Earth’s temperature warmer than it would be otherwise.
The recent and ongoing increase in Earth’s average surface temperature, largely due to greenhouse gas emissions.
Example: Ice core data show that current CO₂ levels are higher than at any point in the last 800,000 years, correlating with rising global temperatures.
Long-term changes in temperature, precipitation, wind patterns, and other climate factors.
Example: More frequent heatwaves, shifting wildlife habitats, melting polar ice, and sea-level rise are attributed to climate change.
Energy from sources that are replenished naturally, like sunlight or wind.
Example: Solar panels convert solar radiation into electricity. Wind turbines harness wind currents to spin generators.
Energy from sources that cannot be replenished within a human timescale, such as fossil fuels.
Example: Coal, oil, and natural gas take millions of years to form. Using them faster than they replenish leads to eventual depletion.
Organic material from plants or animals used as a renewable energy source.
Example: Wood, crop residues, and animal waste can be burned for heat or converted to biofuels like ethanol and biodiesel.
Carbon-based energy sources formed from the remains of ancient organisms, like coal, oil, and natural gas.
Example: Gasoline is refined from crude oil. Burning fossil fuels releases CO₂, a major greenhouse gas.
Energy harnessed from the Sun’s radiation using solar panels or other technology.
Example: Photovoltaic cells generate electricity directly from sunlight, powering homes, streetlights, and even spacecraft.
Energy generated from air flow using wind turbines.
Example: Wind farms in open plains or offshore can power thousands of homes. Reliability depends on consistent wind speeds.
Heat from Earth’s interior that can be harnessed for power.
Example: Iceland uses geothermal sources for heating buildings and generating electricity, tapping hot springs and steam vents.
All regions of Earth inhabited by living organisms.
Example: Includes deserts, rainforests, coral reefs, and all ecosystems where life exists, from deep-sea vents to mountaintops.
The layer of gases surrounding Earth.
Example: The troposphere is where weather occurs, while the stratosphere above contains the protective ozone layer.
The rigid outer layer of Earth, consisting of the crust and upper mantle.
Example: Tectonic plates are segments of the lithosphere that move on the more ductile asthenosphere beneath.
All the water on Earth’s surface, underground, and in the air.
Example: Oceans, lakes, rivers, groundwater, and atmospheric moisture (clouds) are parts of the hydrosphere.
The study of how organisms interact with each other and their environment.
Example: An ecologist might research predator-prey relationships in a savanna, such as how lion populations affect gazelle numbers and plant growth.
A group of organisms of the same species living in the same area at the same time.
Example: A forest with 200 oak trees forms an oak population. If these trees reproduce successfully, the population persists and evolves over time.
Different populations of various species living together in a defined area.
Example: A coral reef community includes corals, fish, crustaceans, algae, and many other species, all interacting in complex webs.
The gradual change in species composition in a community over time.
Example: After a forest fire, pioneer species like grasses colonize first. Over many years, shrubs and trees appear, eventually restoring a mature forest if conditions permit.
The maximum population size an environment can sustain over time given resource availability.
Example: Deer in a forest may exceed carrying capacity if predators are removed, leading to overgrazing and a crash in the deer population.
A living component that affects the population of another organism or the environment.
Example: Predators, competitors, parasites, and mutualists (like pollinators) are all biotic factors influencing survival and reproduction.
A nonliving component of the environment.
Example: Temperature, soil pH, sunlight, and water availability shape which organisms can thrive in an area.
Any inherited characteristic that increases an organism’s chance of survival and reproduction.
Example: Arctic foxes have thick fur and small ears to minimize heat loss in sub-zero environments—critical for survival in polar regions.
The complete set of genes or genetic material in an organism.
Example: The Human Genome Project mapped all human genes, enabling advances in genetic testing, personalized medicine, and evolutionary studies.
The application of computational methods to analyze biological data, such as genetic sequences.
Example: Comparing DNA sequences of viruses helps track disease outbreaks and identify mutations relevant for vaccine development.
A systematic process for experimentation to explore observations and answer questions—includes making observations, forming a hypothesis, testing, analyzing data, and drawing conclusions.
Example: Observing that plants near a window grow faster, you hypothesize that increased sunlight boosts growth. Design a controlled experiment, analyze results, then conclude whether the hypothesis holds.
The application of scientific knowledge for practical purposes, especially in industry.
Example: CRISPR gene editing emerged from basic research into bacterial immune systems, showcasing how science leads to technological advances in medicine and agriculture.
While these concepts form a strong foundation for the ACT Science section, remember that the test will also cover other areas of science including chemistry and biology. Practice applying these concepts to various scenarios to build your confidence and improve your score.