ILTS Science: Chemistry (240) key terms

The cyclical process of asking testable questions, forming hypotheses, designing controlled investigations, collecting and analyzing data, and revising explanations based on evidence. It emphasizes evidence over authority and replication over single results.

A testable, falsifiable prediction about the relationship between variables, stated before data collection. It must be phrased so that observations could contradict it, distinguishing it from a guess or an unfalsifiable claim.

The comparison condition in an experiment that receives no treatment or a placebo, holding all other variables constant. Differences between it and the experimental group are attributed to the manipulated variable rather than confounding factors.

The factor deliberately manipulated or selected by the investigator, plotted on the x-axis of a graph. Its effect on the measured outcome is what the experiment is designed to detect.

Evaluation of a manuscript by independent experts in the field before publication, checking methods, evidence, and interpretation. It filters weak claims but does not guarantee correctness, so published findings still require replication.

A broad, well-tested explanation supported by extensive converging evidence that organizes laws and observations, such as atomic theory or kinetic molecular theory. It is not a tentative guess and can be revised as evidence accumulates.

The digits in a measurement that carry meaningful information about precision, including one estimated digit. Addition and subtraction round to the least precise decimal place; multiplication and division round to the fewest significant figures.

The absolute difference between an experimental value and an accepted value, divided by the accepted value and multiplied by 100. It quantifies accuracy, not precision, and is reported with the sign convention the course requires.

Accuracy describes how close a measurement is to the true value, while precision describes how closely repeated measurements agree with one another. A set can be precise yet inaccurate because of systematic error.

The smallest unit of an element that retains its chemical identity, composed of a nucleus of protons and neutrons surrounded by electrons. Atoms combine in fixed ratios to form compounds.

Atoms of the same element with identical proton counts but different neutron counts, giving different mass numbers. Isotopes share chemical behavior but differ in mass and nuclear stability.

The sum of protons and neutrons in a nucleus, written as a superscript before the element symbol. It is an integer and differs from atomic mass, which is a weighted average of isotope masses.

The weighted mean of the masses of an element's naturally occurring isotopes, weighted by relative abundance. It is the value shown on the periodic table and is expressed in atomic mass units.

An electron in the outermost principal energy level of an atom that participates in bonding and determines chemical properties. Main-group elements show periodic trends tied to their valence electron count.

The energy required to remove one mole of electrons from one mole of gaseous atoms or ions. It generally increases across a period and decreases down a group as outer electrons become more shielded and farther from the nucleus.

A measure of atomic size, typically half the distance between nuclei of bonded identical atoms. It decreases across a period because effective nuclear charge rises and increases down a group as energy levels are added.

A relative measure of how strongly an atom attracts shared electrons in a bond. Differences between bonded atoms determine whether a bond is nonpolar covalent, polar covalent, or ionic.

Attractions between separate molecules, including London dispersion, dipole-dipole, and hydrogen bonding. They are much weaker than covalent bonds but control boiling point, viscosity, surface tension, and solubility.

The amount of substance containing Avogadro's number, 6.022 times ten to the twenty-third, of representative particles. It links mass in grams to particle count and to molar volume of gases at standard conditions.

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 stoichiometric calculations.

The reactant consumed first because it is present in insufficient stoichiometric proportion, capping the theoretical yield. The excess reactant remains after the reaction stops.

The actual mass of product obtained divided by the theoretical mass predicted from stoichiometry, multiplied by 100. Values below 100 reflect side reactions, incomplete reaction, or transfer losses.

A balanced equation showing only the species that actually change, omitting spectator ions present identically on both sides. It is written after assigning charges and checking both mass and charge balance.

The heat content of a system at constant pressure, with change measured by calorimetry or calculated from Hess's law. Negative enthalpy change indicates an exothermic process releasing heat to the surroundings.

The minimum energy colliding particles must possess for a reaction to proceed, forming the transition state. Catalysts lower it without changing the overall enthalpy change of the reaction.

The change in concentration of a reactant or product per unit time, influenced by concentration, temperature, surface area, and catalysts. Rate laws express its dependence on reactant concentrations.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium for a given temperature. Its magnitude indicates whether products or reactants are favored.

A system at equilibrium subjected to a stress such as concentration change, pressure change, or temperature change shifts in the direction that partially offsets that stress. Catalysts do not shift the position of equilibrium.

The negative base-ten logarithm of hydrogen ion concentration, expressing acidity on a scale near 0 to 14 in water at room temperature. A change of one unit corresponds to a tenfold change in concentration.

A solution of a weak acid with its conjugate base, or a weak base with its conjugate acid, that resists pH change when small amounts of strong acid or base are added.

The point in a titration where the moles of titrant added exactly match the stoichiometric moles required to react with the analyte. It is detected with an indicator or pH meter and differs from the endpoint.

A loss of electrons by a species, accompanied by an increase in oxidation number. Oxidation and reduction always occur together in a redox reaction, conserving total electrons transferred.

A gain of electrons by a species, accompanied by a decrease in oxidation number. The reducing agent donates electrons and is itself oxidized.

A device converting chemical energy into electrical energy, as in a galvanic cell, or the reverse in an electrolytic cell. Electrons flow through an external circuit while ions migrate through a salt bridge.

A specific arrangement of atoms within an organic molecule that gives it characteristic chemical reactivity and physical properties, such as hydroxyl, carbonyl, carboxyl, and amino groups.

Compounds with the same molecular formula but different structural arrangements or spatial orientations, producing distinct properties. Structural, geometric, and optical isomers are the main categories.

A large molecule built by covalently linking many small repeating units called monomers, through addition or condensation reactions. Proteins, starch, and polyethylene are examples with different linkage chemistry.

The time required for half of a radioactive sample to decay, independent of starting quantity. It underlies radiometric dating and radiation-safety calculations involving exponential decay.

A standardized document listing a chemical's hazards, handling precautions, storage requirements, and emergency procedures. Teachers must review it before preparing solutions or planning any student laboratory activity.

Barriers worn to reduce exposure, including splash goggles, lab coats or aprons, and nitrile gloves. Goggles, not safety glasses, are standard for chemistry labs with liquid or vapor hazards.

A ventilated enclosure that draws vapors and fumes away from the user, used for volatile, toxic, or irritating chemicals. It is not a storage cabinet and its sash should remain low while working.

In tort law, the improper or negligent performance of a lawful act, such as a teacher supervising a lab but failing to enforce safety rules. It differs from nonfeasance, the failure to act at all.

The legal and professional obligation of teachers to anticipate foreseeable harm and take reasonable steps to prevent injury to students. It requires adequate supervision, instruction, and safe equipment.

An ongoing check of student understanding during instruction, used to adjust teaching rather than to assign a grade. Exit tickets, whiteboard probes, and concept inventories are common chemistry examples.

A persistent, plausible but incorrect idea students hold, such as the belief that atoms expand when heated or that mass is lost in a gas-producing reaction. It must be surfaced and confronted with discrepant evidence.