PSSA Grade 8 Science key terms

A testable statement that predicts what will happen in an experiment, based on observations and prior knowledge. It must be worded so that evidence could show it to be wrong.

The one factor in an experiment that the researcher deliberately changes or sets at different levels so that its effect on the outcome can be measured.

The factor that is measured or observed in an experiment. It is the result that may change when the researcher changes the independent variable.

The part of an experiment that does not receive the treatment being tested. It is kept under the same conditions so the results can be compared fairly.

Information gathered directly through the senses or with instruments, such as a measurement, a color, or a sound. Observations describe what actually happens.

A conclusion drawn from observations plus prior knowledge rather than from direct sensing. For example, seeing wet pavement and concluding that it recently rained.

A well-tested explanation for a broad set of observations, supported by much evidence and accepted by most scientists. Theories can be revised if new evidence appears.

A statement describing a consistent pattern in nature, often in words or mathematics. It tells what happens under certain conditions, not always why it happens.

The smallest unit of life that can carry out all the basic activities of living things. Some organisms are a single cell; others are made of many cells.

A thin, flexible barrier around a cell that controls which materials enter and leave. It helps the cell keep a stable internal environment.

The process by which plants, algae, and some bacteria use sunlight, water, and carbon dioxide to make sugar and release oxygen. The sugar stores energy that came from sunlight.

The process cells use to release energy from sugar by combining it with oxygen, producing carbon dioxide and water. This energy powers life activities in almost all organisms.

The long molecule inside cells that carries instructions for building proteins and for passing traits to offspring. Its information is coded in a sequence of chemical units.

A section of DNA that holds instructions for one trait or for making a particular protein. Genes are passed from parents to their offspring.

The process by which organisms with traits that help them survive and reproduce in their environment tend to pass those traits to more offspring, so populations change over generations.

All the living things in an area together with the nonliving parts such as water, soil, air, and sunlight, plus the interactions among them.

A model showing how energy and matter move through an ecosystem as many connected food chains, with arrows pointing from the organism eaten to the one that eats it.

The ability of a living thing to keep internal conditions, such as body temperature and water level, within a stable range even when conditions outside change.

The smallest particle of an element that still has the properties of that element. Atoms combine to form molecules and all other substances.

A pure substance made of only one kind of atom, such as oxygen or iron, that cannot be broken into simpler substances by ordinary chemical means.

A substance formed when atoms of two or more different elements are chemically bonded together in a fixed ratio, such as water, which is H2O.

A process in which substances break apart or combine to form new substances with different properties. Atoms are rearranged, but none are created or destroyed.

The principle that the total mass before a chemical reaction equals the total mass after it, because atoms are only rearranged, never lost or gained.

A push or a pull that can change an object's speed, direction, or shape. Force is measured in newtons and has both a size and a direction.

The tendency of an object to keep doing what it is already doing, staying still or moving in a straight line at constant speed, unless a net force acts on it.

The energy an object has because of its motion. A faster-moving or more massive object has more kinetic energy.

Stored energy that depends on an object's position or condition, such as a ball held above the ground or energy stored in chemical bonds. It can change into kinetic energy.

How much mass is packed into a given volume of a substance, often calculated as mass divided by volume. Denser materials sink in less dense fluids.

The distance between two matching points on neighboring waves, such as from one crest to the next. It helps determine a wave's color or pitch.

The number of wave cycles that pass a point in one second, measured in hertz. Higher frequency means more cycles per second and, for light, more energy.

The ongoing set of processes that change rocks among the three main types, igneous, sedimentary, and metamorphic, through melting, cooling, weathering, erosion, and pressure.

The breaking down of rocks into smaller pieces by water, wind, ice, temperature changes, or living things. The pieces stay where they form unless something moves them.

The movement of weathered rock, soil, and sediment from one place to another by water, wind, ice, or gravity.

The theory that Earth's outer shell is broken into large plates that move slowly over the mantle, causing earthquakes, volcanoes, and mountain building at their boundaries.

The continuous movement of water among the ocean, atmosphere, and land through evaporation, condensation, precipitation, and runoff.

The warming of Earth's surface as certain gases in the atmosphere trap heat that would otherwise escape to space. It keeps Earth warm enough for life.

The average pattern of weather in a region over many years, including typical temperatures and rainfall. It differs from weather, which is the day-to-day condition.

The changing apparent shapes of the Moon as seen from Earth, caused by how much of the sunlit side faces us as the Moon orbits Earth.

A huge system of billions of stars, gas, dust, and dark matter held together by gravity. Earth's Sun is one star in the Milky Way galaxy.