NYSTCE Chemistry key terms

A cyclical process of asking questions, forming testable hypotheses, designing controlled experiments, analyzing data, and revising explanations. Conclusions are provisional and must be replicated before being accepted as reliable knowledge.

A proposed, falsifiable explanation for an observation that generates specific predictions. A useful hypothesis must be testable through experiment or further observation and can be rejected by conflicting evidence.

The comparison group in an experiment that receives no treatment or a placebo, so the effect of the manipulated variable can be isolated. All other conditions are held constant between control and experimental groups.

The digits in a measurement that convey meaningful precision, including all certain digits plus one estimated digit. Addition and subtraction keep the fewest decimal places; multiplication and division keep the fewest total significant figures.

A problem-solving technique that tracks units through calculations so unwanted units cancel and the desired unit remains. It catches setup errors and converts between unit systems without memorizing separate formulas.

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

The amount of substance containing 6.022 × 10²³ representative particles, such as atoms, molecules, or formula units. It bridges the microscopic particle count and the macroscopic mass used in the laboratory.

The quantitative relationship between amounts of reactants and products in a balanced chemical equation, based on mole ratios from the coefficients. It predicts expected yields and required masses or volumes.

The reactant consumed first in a reaction because it is present in insufficient stoichiometric amount, capping the maximum product formed. The excess reactant remains partially unreacted after the reaction stops.

The actual experimental mass of product divided by the theoretical maximum mass predicted by stoichiometry, multiplied by 100. Values below 100 percent reflect side reactions, incomplete reaction, or handling losses.

A concentration unit equal to moles of solute per liter of solution, abbreviated M. Because solution volume varies with temperature, molarity is temperature-dependent, unlike molality.

A logarithmic scale of hydrogen ion activity defined as the negative base-10 logarithm of the hydrogen ion concentration. Each unit change represents a tenfold change in acidity; pH 7 is neutral at 25 °C.

A solution of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists large pH changes on addition of small amounts of strong acid or base.

A volumetric procedure in which a solution of known concentration is added to a measured volume of analyte until the equivalence point is signaled, often by a color change from an indicator.

The point in a titration where the moles of added titrant exactly match the stoichiometric moles required to react with the analyte. It is determined by stoichiometry, not by the indicator color change.

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

When a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the system shifts in the direction that partially offsets the disturbance and reestablishes equilibrium.

The minimum kinetic energy colliding particles must possess for a reaction to occur. Catalysts lower it by providing an alternate pathway; temperature increases the fraction of collisions exceeding it.

A substance that increases reaction rate by lowering the activation energy without being consumed in the overall reaction. It changes the pathway, not the equilibrium position or the thermodynamics of the reaction.

The heat content of a system at constant pressure, with change in enthalpy equal to heat absorbed or released. Negative enthalpy change indicates an exothermic process; positive indicates an endothermic process.

A measure of the dispersal of energy and matter among available microstates, often described as disorder. Spontaneous processes tend to increase total entropy of the universe.

A thermodynamic function whose change predicts spontaneity at constant temperature and pressure. A negative change indicates a spontaneous process; a positive change indicates a nonspontaneous one.

The total enthalpy change of a reaction is the sum of enthalpy changes for any sequence of steps summing to the overall reaction, because enthalpy is a state function.

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 atm. Tabulated values let you compute reaction enthalpies from products minus reactants.

The number of protons in an atom's nucleus, which uniquely identifies the element and determines its position on the periodic table. In a neutral atom it also equals the electron count.

The sum of protons and neutrons in one atom's nucleus. Atoms of the same element with different mass numbers are isotopes and differ in neutron count only.

One of two or more atoms of the same element having identical proton counts but different neutron counts, and therefore different masses. Isotopes share chemical properties but differ in nuclear stability.

The electrons in an atom's outermost principal energy level that participate in bonding and determine chemical reactivity. Main-group elements show periodic trends in valence count across a period.

A measure of how strongly an atom attracts shared electrons in a bond. Large differences produce ionic bonds; moderate differences produce polar covalent bonds with partial charges.

The tendency of main-group atoms to gain, lose, or share electrons until surrounded by eight valence electrons, matching a noble gas configuration. Hydrogen and helium follow a duet instead.

A diagram showing valence electrons as dots and bonds as lines to represent connectivity and lone pairs in a molecule or polyatomic ion. It is a starting point for predicting geometry and polarity.

A model predicting molecular shape from the repulsion of electron domains around a central atom, which arrange to maximize separation. Lone pairs occupy more space than bonding pairs, compressing bond angles.

A bond in which shared electrons are unequally attracted by two atoms of differing electronegativity, creating partial positive and partial negative ends. Molecular polarity also depends on overall geometry.

Attractive forces between molecules, including London dispersion, dipole-dipole, and hydrogen bonding, weaker than covalent bonds. They govern boiling point, vapor pressure, viscosity, and solubility behavior.

A model describing gases as particles in constant random motion whose average kinetic energy is proportional to absolute temperature. It explains pressure, diffusion, and gas law behavior.

The equation PV = nRT relating pressure, volume, moles, and absolute temperature of a gas through the universal gas constant. Real gases approximate it best at low pressure and high temperature.

A graph of pressure versus temperature showing the conditions under which a substance exists as solid, liquid, or gas, including the triple point and critical point.

The loss of electrons by a species, accompanied by an increase in oxidation state. Oxidation and reduction always occur together in a redox reaction, balancing electron transfer.

The gain of electrons by a species, accompanied by a decrease in oxidation state. The oxidizing agent is the species that is reduced and thereby causes another species to be oxidized.

A bookkeeping number assigned to an atom in a compound by distributing bonding electrons to the more electronegative atom. It tracks electron transfer in redox reactions and balancing.

A device converting chemical energy into electrical energy (galvanic) or using electrical energy to drive a nonspontaneous reaction (electrolytic). Electrons flow through an external circuit between two electrodes.

The voltage of a half-reaction measured against the standard hydrogen electrode under standard conditions. Comparing values predicts which species is oxidized and the overall cell potential.

An atom or group of atoms within an organic molecule that confers characteristic chemical reactivity and physical properties, such as hydroxyl, carbonyl, carboxyl, amino, and ester groups.

One of two or more compounds with the same molecular formula but different arrangements of atoms, giving different structures and properties. Structural isomers differ in connectivity; stereoisomers differ in spatial arrangement.

A large molecule formed by covalently linking many small repeating units called monomers, through addition or condensation reactions. Proteins, nucleic acids, and starches are biological examples.

The time required for half the nuclei in a radioactive sample to decay, a constant characteristic of each isotope. After n half-lives, the remaining fraction is one-half raised to n.

The spontaneous emission of particles or energy from an unstable nucleus, transforming it toward a more stable configuration. Alpha, beta, and gamma emissions each change mass number or atomic number differently.

A standardized document summarizing a chemical's hazards, safe handling, storage, exposure limits, and emergency procedures. It must be accessible to anyone working with the substance in a school laboratory.

Barrier gear such as splash goggles, gloves, and lab coats worn to reduce exposure to chemical, thermal, and biological hazards. Selection depends on the specific hazards identified for the procedure.

A ventilated enclosure that draws vapors and airborne particles away from the user while work is performed inside it. It is used for volatile, toxic, or irritating substances, not as a storage cabinet.