NGSA Science key terms

Anything that has mass and takes up space. It exists as solids, liquids, gases, and plasmas, and is made of atoms or molecules that can be rearranged but not created or destroyed in ordinary changes.

The smallest unit of an element that keeps that element's properties. It has a dense nucleus of protons and neutrons surrounded by electrons, and atoms combine to form molecules and compounds.

Two or more atoms bonded together, either of the same element or of different elements. Water, oxygen gas, and carbon dioxide are all molecules, and their arrangement determines their properties.

A pure substance made of only one kind of atom, identified by its atomic number. Elements cannot be broken into simpler substances by chemical means and are organized on the periodic table.

A substance formed when atoms of two or more different elements bond in fixed proportions. Its properties differ from those of its component elements, and it can be separated only by chemical means.

A combination of two or more substances that are not chemically bonded and keep their own properties. Components can be separated by physical means such as filtering, evaporation, or distillation.

A characteristic used to describe or identify a substance. Physical properties include density, melting point, and color; chemical properties describe how a substance reacts with other substances.

The mass of a substance per unit volume, often written as mass divided by volume. It explains floating and sinking and is a useful way to identify unknown materials.

A change in a substance's form or appearance without forming a new substance. Melting, dissolving, and cutting are examples; the chemical identity of the material stays the same.

A change in which substances react to form one or more new substances with different properties. Evidence can include color change, gas production, heat release, or a new solid forming.

One of the forms matter takes depending on particle energy and arrangement: solid, liquid, gas, or plasma. Changes between states are physical changes driven by adding or removing heat.

The principle that the total mass of matter stays the same in a closed system during a physical or chemical change. Atoms are rearranged, not created or destroyed.

The energy an object has because of its motion. For particles, higher kinetic energy means faster motion and usually a higher temperature; it is transferred in collisions.

Stored energy that depends on an object's position or arrangement, such as height above the ground or the stretch of a spring. It can convert into kinetic energy when released.

The movement of energy from one object, place, or form to another. Energy moves through waves, moving objects, heat flow, electrical circuits, and sound, and is never lost overall.

A push or pull on an object that can change its speed, direction, or shape. Forces have strength and direction, and the net force determines how an object's motion changes.

A change in an object's position over time relative to a reference point. Motion is described by distance, speed, direction, and acceleration, and is affected by forces.

A force that resists sliding or rolling between surfaces in contact, converting motion into heat. It acts opposite the direction of motion and can be reduced by lubrication or smooth surfaces.

A disturbance that carries energy from place to place without permanently moving matter. Waves have wavelength, frequency, and amplitude, and can be mechanical or electromagnetic.

The distance between two matching points on a repeating wave, such as from crest to crest. Longer wavelengths carry lower frequency, and in visible light they correspond to different colors.

The smallest unit of life that can carry out basic life functions. Cells have a membrane, genetic material, and cytoplasm, and all living organisms are made of one or more cells.

A specialized structure inside a cell that performs a particular job. Examples include the nucleus, which stores genetic material, and mitochondria, which release energy from food.

The process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to make sugar and release oxygen. It stores light energy as chemical energy in food.

A model showing how energy and matter pass from one organism to another through eating. It begins with producers and continues through consumers and decomposers, with energy lost at each step.

A community of living things interacting with each other and with nonliving parts of their environment, such as water, soil, air, and sunlight. Energy flows and matter cycles within it.

A trait or behavior that helps an organism survive and reproduce in its environment. Adaptations arise over generations through natural selection acting on inherited variations.

The process by which individuals with helpful inherited traits tend to survive and reproduce more, so those traits become more common in later generations. It explains how populations change over time.

A characteristic of an organism, such as eye color, height, or beak shape. Traits are influenced by genes, by the environment, or by a combination of both.

The stages an organism passes through from the start of life to producing offspring of its own. Examples include seed to seedling to adult plant and egg to larva to adult insect.

The ongoing set of processes that change rock among igneous, sedimentary, and metamorphic forms. Melting, cooling, weathering, erosion, compacting, and heat and pressure drive these changes.

The breakdown of rock into smaller pieces by water, wind, ice, plants, or chemical reactions. It happens in place and produces sediment that erosion can then move away.

The transport of weathered rock and soil by moving water, wind, ice, or gravity. Erosion shapes landforms and carries sediment that may later be deposited elsewhere.

The theory that Earth's outer shell is broken into large plates that move slowly over the mantle. Their interactions cause earthquakes, volcanoes, mountain building, and ocean-floor features.

A sudden release of built-up energy in Earth's crust that sends seismic waves outward. It is most common near plate boundaries and is recorded by instruments called seismographs.

The continuous movement of water among oceans, atmosphere, land, and living things through evaporation, condensation, precipitation, and runoff. The Sun provides the energy that drives it.

The Sun and all the objects bound to it by gravity, including planets, moons, asteroids, and comets. Planets orbit the Sun in nearly circular paths and differ in size and composition.

A force of attraction between any two objects that have mass. It keeps planets in orbit, causes objects to fall toward Earth, and shapes the large-scale structure of the universe.

A recurring period of the year marked by particular weather and daylight patterns. Seasons result mainly from the tilt of Earth's axis as it orbits the Sun, not from distance to the Sun.

A repeating set of steps engineers use to solve problems: define the problem, gather information, plan, build a model, test, evaluate, and improve. Constraints and criteria guide each decision.

A limit on a design, such as available materials, budget, time, safety rules, or size. Constraints, together with criteria for success, shape which solutions are acceptable.

An early working version of a design built to test ideas and find problems before final production. Prototypes are often revised repeatedly based on test results and feedback.

A factor in an experiment that can change or be changed. Scientists control variables so they can tell which factor actually caused the observed result.

An experiment in which only one variable is changed at a time while all other conditions are kept the same. This allows a valid conclusion about cause and effect.

Information collected through observation or measurement, which can be numbers or descriptions. Data are organized, displayed in tables and graphs, and analyzed to support or reject a claim.

Observations or data that support or contradict a scientific claim. Strong evidence is repeatable, measurable, and gathered through fair testing or careful observation.

A statement that something is true or that answers a question, which must be supported by evidence and reasoning. In science, a claim is accepted only when evidence holds up under testing.

A simplified representation of a real object, process, or system used to explain, predict, or test ideas. Models have limits and may need revision as new evidence appears.