AEPA Chemistry NT306 key terms

A cyclical process of observing, asking questions, forming testable hypotheses, designing controlled experiments, analyzing data, and revising explanations based on evidence. Conclusions are provisional and open to replication and challenge.

A tentative, testable, and falsifiable statement proposing a relationship between variables. It must generate predictions that experiments or observations could confirm or contradict.

A procedure in which all variables except the independent variable are held constant, so that observed changes in the dependent variable can be attributed to the manipulated factor.

The factor that the experimenter deliberately manipulates or selects across treatment groups. It is the presumed cause whose effect on the measured outcome is being tested.

The measured outcome that responds to changes in the independent variable. It is the observed effect recorded as data during an experiment.

Evaluation of a study's methods, data, and conclusions by independent experts before publication. It screens for flaws, unsupported claims, and inadequate documentation, though it does not guarantee correctness.

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

How close a measured value is to the true or accepted value. Accuracy differs from precision and is often assessed by percent error.

How closely repeated measurements agree with one another under the same conditions. Precise data can still be inaccurate if a systematic error shifts all values in one direction.

A measure of accuracy calculated as the absolute difference between an experimental and accepted value, divided by the accepted value, then multiplied by 100.

A consistent, repeatable deviation in measurements caused by faulty instruments, flawed technique, or miscalibration. It shifts results in one direction and is not reduced by averaging repeats.

Unpredictable variation in measurements arising from uncontrolled fluctuations in instruments, environment, or observer judgment. Averaging many trials tends to reduce its effect.

A problem-solving technique that uses unit conversion factors written as ratios to cancel unwanted units and arrive at a desired unit, helping verify that calculations are set up correctly.

A compact way to write very large or very small numbers as a coefficient between 1 and 10 multiplied by a power of ten, preserving significant figures and simplifying comparisons of magnitude.

Anything that has mass and occupies volume. Matter exists as elements, compounds, or mixtures and can undergo physical or chemical changes.

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

A pure substance formed when two or more elements combine chemically in fixed proportions, with properties distinct from those of the constituent elements.

A physical combination of two or more substances that retain their individual identities and can be separated by physical means. Mixtures may be homogeneous or heterogeneous.

A change in a substance's form, state, or appearance that does not alter its chemical identity, such as melting, dissolving, or grinding.

A process in which substances are transformed into new substances with different compositions and properties, typically accompanied by energy change, gas formation, precipitate, or color change.

The principle that mass is neither created nor destroyed in a chemical reaction, so total mass of reactants equals total mass of products in a closed system.

The SI unit for amount of substance, defined as exactly 6.022 x 10^23 representative particles. It links particle counts to measurable masses through molar mass.

The number of representative particles in one mole of a substance, approximately 6.022 x 10^23 per mole. It converts between particle counts and moles.

The mass in grams of one mole of a substance, numerically equal to its formula mass in atomic mass units. It converts between mass and moles in calculations.

The quantitative relationship among amounts of reactants and products in a balanced chemical equation, used to predict masses, moles, or volumes consumed and formed.

The reactant that is entirely consumed first and therefore determines the maximum amount of product that can form in a reaction. Other reactants are in excess.

The ratio of actual product obtained to the theoretical yield predicted by stoichiometry, expressed as a percentage. It measures reaction efficiency.

A symbolic representation of a reaction showing formulas of reactants and products with coefficients that satisfy conservation of atoms and charge.

The principle that properties of elements recur periodically when elements are arranged by increasing atomic number, reflecting repeating electron configurations.

The number of protons in an atom's nucleus, which uniquely identifies the element and determines the number of electrons in a neutral atom.

One of two or more atoms of the same element having the same number of protons but different numbers of neutrons, and therefore different mass numbers.

The weighted average of the masses of an element's naturally occurring isotopes, weighted by their relative abundances. It is the value shown on the periodic table.

A predictable pattern in properties such as atomic radius, ionization energy, and electronegativity across periods and down groups, explained largely by effective nuclear charge and shielding.

A measure of an atom's tendency to attract shared electrons in a chemical bond. Differences in electronegativity help classify bonds as nonpolar covalent, polar covalent, or ionic.

The energy required to remove an electron from a gaseous atom or ion. It generally increases across a period and decreases down a group.

The arrangement of electrons among an atom's orbitals, written to show principal energy levels and subshells. It underlies chemical behavior and periodic patterns.

An attractive force holding atoms together, formed by transferring or sharing valence electrons. Bond type influences melting point, conductivity, and reactivity.

The minimum energy that colliding particles must possess for a reaction to occur. It corresponds to the energy barrier between reactants and the transition state.

A substance that increases reaction rate by providing an alternative pathway with lower activation energy, without being consumed in the overall reaction.

The change in concentration of a reactant or product per unit time. It depends on concentration, temperature, surface area, and the presence of a catalyst.

The model stating that reactions occur when particles collide with sufficient energy and proper orientation. It explains how concentration and temperature affect reaction rates.

A state in a reversible reaction where forward and reverse rates are equal, so concentrations remain constant even though both reactions continue at the molecular level.

A rule stating that when a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, it shifts in the direction that partially offsets the disturbance.

A thermodynamic quantity representing the heat content of a system at constant pressure. The change in enthalpy indicates whether a reaction absorbs or releases heat.

A reaction that releases energy to the surroundings, giving a negative enthalpy change and typically a temperature rise in the surroundings.

A reaction that absorbs energy from the surroundings, giving a positive enthalpy change and typically a temperature drop in the surroundings.

The principle that the overall enthalpy change of a reaction is the sum of enthalpy changes for its individual steps, regardless of the pathway taken.

A measure of the dispersal of energy or the number of accessible microstates in a system. Spontaneous processes tend to increase total entropy of the universe.

A thermodynamic function combining enthalpy and entropy, whose sign predicts spontaneity at constant temperature and pressure: negative values indicate a spontaneous process.