Praxis Middle School Science key terms

A testable, falsifiable statement that predicts the outcome of an experiment based on prior observation or reasoning. It must be worded so that data could show it to be wrong.

The factor that the experimenter deliberately changes or manipulates between trials. Its effect on the measured outcome is what the study is designed to reveal.

The factor that is measured or observed in an experiment; its value is expected to respond to changes in the manipulated factor. It is the outcome the researcher records as data.

The comparison group in an experiment that does not receive the treatment being tested. It shows what would happen without the manipulation, allowing researchers to attribute changes to the treatment.

A study in which all variables except the one being tested are held constant, so any difference in results can be attributed to that single factor rather than to outside influences.

A broad, well-tested explanation for a large body of observations and experimental results, supported by converging evidence. It is not a guess and can be revised if new evidence demands it.

A concise statement, often mathematical, describing a consistent pattern observed in nature under specified conditions. It describes what happens rather than explaining why.

The process in which independent experts evaluate a study's methods, analysis, and conclusions before publication. It screens for flaws and helps maintain the reliability of the scientific literature.

How close a measured or calculated value is to the true or accepted value. A measurement can be accurate without being precise if errors happen to cancel out.

How closely repeated measurements of the same quantity agree with one another, reflecting consistency rather than correctness. Repeated values may cluster tightly yet all be far from the true value.

The digits in a measurement that carry meaningful information about its precision, including all certain digits plus one estimated digit. Rules govern how they are counted and how results are rounded.

A way of writing very large or very small numbers as a coefficient between 1 and 10 multiplied by a power of ten. It makes the scale of quantities easier to compare and compute.

A measure of how far an experimental result deviates from an accepted value, calculated as the absolute difference divided by the accepted value, times 100. It quantifies accuracy.

A logical conclusion drawn from observations and prior knowledge rather than from direct measurement. Inferences go beyond what is seen and must be supported by evidence.

Observations described with words or categories rather than numbers, such as color, texture, or odor. Such data can guide hypotheses but is harder to analyze statistically.

Observations recorded as numbers with units, such as mass in grams or time in seconds. This data can be graphed, averaged, and tested statistically.

A systematic error that pushes results consistently in one direction, often from flawed sampling, measurement, or study design. Unlike random error, it does not average out with repetition.

Unpredictable variation in measurements caused by chance factors such as fluctuations in instruments or human reading. Repeating measurements and averaging reduces its effect.

A subset of a population selected so that its characteristics mirror those of the whole group. Conclusions drawn from it can reasonably be generalized to the population.

A statistical relationship in which two variables change together in a patterned way. Correlation alone does not establish that one variable causes the other.

A simplified representation of a system or process used to explain, predict, or test ideas. Models omit some detail by design and are revised as evidence accumulates.

The principle that all living things are made of one or more cells, that the cell is the basic unit of life, and that new cells arise from existing cells.

A specialized structure within a cell that carries out a particular function, such as energy release, protein synthesis, or storage. Different organelles are separated by membranes or located in the cytoplasm.

A membrane-bound organelle where cellular respiration converts the chemical energy of food molecules into ATP, the cell's usable energy currency. Cells with high energy demands typically contain many of them.

A plant-cell organelle containing chlorophyll where photosynthesis captures light energy and uses it to build sugars from carbon dioxide and water. It releases oxygen as a byproduct.

The process by which plants, algae, and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen. It stores energy in chemical bonds and underpins most food chains.

The set of reactions in cells that break down glucose, using oxygen in aerobic cases, to release energy stored as ATP along with carbon dioxide and water. It is essentially the reverse of photosynthesis.

The division of a cell's nucleus that produces two daughter cells with the same chromosome number as the parent. It underlies growth, repair, and asexual reproduction in many organisms.

A two-stage form of cell division that halves the chromosome number and produces genetically varied reproductive cells. It is the basis of sexual reproduction and genetic diversity.

One of the alternative versions of a gene that can occupy the same position on a chromosome. Different alleles produce variations in the trait the gene controls.

The particular combination of alleles an organism carries for a given gene or set of genes. It is the genetic makeup that, together with environment, shapes observable traits.

The observable characteristics of an organism, such as appearance, behavior, or biochemistry. It results from the interaction of genotype with environmental conditions.

The process by which individuals with inherited traits that improve survival and reproduction in a given environment leave more offspring, so those traits become more common over generations.

An inherited trait or behavior that improves an organism's ability to survive and reproduce in its environment. Adaptations arise over generations through natural selection.

A community of interacting organisms together with the nonliving factors of their environment, such as water, soil, and sunlight. Energy and matter move among its living and nonliving parts.

A diagram showing the overlapping feeding relationships among organisms in an ecosystem, with arrows tracing the flow of energy from producers to various consumers. It reveals multiple interconnected pathways.

The maximum population size of a species that a given environment can support indefinitely with its available resources. Populations may overshoot it temporarily and then decline.

The principle that whenever one object exerts a force on a second, the second exerts an equal and opposite force on the first. Forces always occur in interacting pairs.

The relationship stating that the net force on an object equals its mass times its acceleration. It explains how force, mass, and acceleration are quantitatively linked.

A push or pull on an object that can change its motion or shape, measured in newtons. Forces are vectors, so direction matters as much as size.

The energy an object possesses because of its motion, calculated as one-half its mass times the square of its speed. Doubling speed quadruples kinetic energy.

Stored energy an object has because of its position or condition, such as height above the ground or a stretched spring. It can be converted into other forms when released.

The principle that energy cannot be created or destroyed but only transferred or transformed between forms. The total energy of a closed system stays constant.

The smallest unit of an element that retains that element's chemical properties, consisting of a nucleus of protons and neutrons surrounded by electrons. Atoms combine to form compounds.

A pure substance made of only one kind of atom, defined by its atomic number. Elements cannot be broken down into simpler substances by ordinary chemical means.

An arrangement of the elements in order of increasing atomic number so that elements with similar properties fall into the same columns. It organizes patterns in reactivity and structure.

A group of two or more atoms bonded together, which may be of the same element or of different elements. Molecules are the smallest units of many compounds.

A lasting attraction between atoms that holds them together in a compound, formed by sharing or transferring electrons. Bond type influences a substance's properties.

A process in which substances are transformed into new substances with different properties by breaking and forming bonds. Atoms are rearranged but neither created nor destroyed.

The principle that the total mass of the substances involved in a chemical reaction stays the same before and after the change. It reflects that atoms are only rearranged, not destroyed.

The mass of a substance per unit volume, often expressed in grams per cubic centimeter. It is an intensive property useful for identifying materials and predicting floating or sinking.

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

A sudden release of energy in Earth's crust, usually along a fault, that produces seismic waves. Its location and magnitude are recorded by instruments called seismographs.

The ongoing series of processes by which igneous, sedimentary, and metamorphic rocks are formed, broken down, and transformed over geologic time. Heat, pressure, weathering, and deposition drive it.

The continuous movement of water among the ocean, atmosphere, and land through evaporation, condensation, precipitation, and runoff. Solar energy and gravity drive the cycle.

The Sun and all the objects bound to it by gravity, including planets, moons, asteroids, and comets. Its structure reflects how it formed from a rotating cloud of gas and dust.

The changing portion of the Moon's sunlit side visible from Earth as it orbits, completing a cycle of about 29.5 days. Phases result from changing viewing angles, not Earth's shadow.

The attractive force between objects that have mass, increasing with mass and decreasing with distance. It governs falling bodies, orbits, and the large-scale structure of the universe.