Physical property A property of a substance related to its physical, not chemical, characteristics (e.g., density).
pOH A measure of the hydroxide (OH-) ion content of an aqueous solution, defined to be equal to the negative log of the OH- concentration.
Polar covalent bond A covalent bond between atoms with different electronegativities in which electron density is unevenly distributed, giving the bond positive and negative ends.
Polar molecule A molecule possessing one or more polar covalent bond(s) and a geometry that allows the bond dipole moments to add up to a net dipole moment (e.g., H2O).
Polyprotic acid An acid capable of donating more than one proton (e.g., H2CO3).
Potential energy diagram An energy diagram that relates the potential energy of the reactants and products of a reaction to details of the reaction pathway. By convention, the x-axis shows the progression of the reaction, and the y-axis shows potential energy.
Precipitate An insoluble solid that separates from a solution, generally the result of mixing two or more solutions or of a temperature change.
Pressure Average force per unit area measured in atmospheres, torr (mm Hg), or pascals (Pa); 1 atm = 760 torr = 760 mm Hg = 1.01 × 102 kPa.
Principal quantum number (n) The first quantum number, defining the energy level or shell occupied by an electron.
Proton (H+) A subatomic particle that carries a single positive charge and has a mass defined as 1 or the hydrogen ion, H+, which is simply a hydrogen nucleus consisting of one proton. These species are considered to be equivalent.
Quantum number A number used to describe the energy levels available to electrons. The state of any electron is described by four quantum numbers. See azimuthal quantum number; magnetic quantum number; principal quantum number; spin quantum number.
Radioactivity A phenomenon exhibited by certain unstable isotopes in which they undergo spontaneous nuclear transformation via emission of one or more particle(s).
Raoult’s law A law stating that the partial pressure of a component in a solution is proportional to the mole fraction of that component in the solution (i.e., PA = XAPtotal). See Vapor pressure depression.
Rate constant The proportionality constant in the rate law of a reaction; specific to a particular reaction under particular conditions.
Rate-determining step The slowest step of a reaction mechanism. The rate of this step limits the overall rate of the reaction.
Rate law A mathematical expression giving the rate of a reaction as a function of the concentrations of the reactants. The rate law of a given reaction must be determined experimentally.
Reaction intermediate A species that does not appear among the final products of a reaction but is present temporarily during the course of the reaction.
Reaction mechanism The series of steps that occurs in the course of a chemical reaction, often including the formation and destruction of reaction intermediates.
Reaction rate The speed at which a substance is produced or consumed by a reaction.
Real gas A gas that exhibits deviations from the Ideal Gas law.
Redox reaction A reaction combining reduction and oxidation processes. Also called oxidation-reduction reaction.
Reducing agent In a redox reaction, a species that loses electrons and is thereby oxidized.
Reduction A reaction involving the net gain of electrons or, equivalently, a decrease in oxidation number.
s subshell Subshell corresponding to the angular momentum quantum number l = 0 and containing one spherical orbital; found in all energy levels.
Salt An ionic substance (i.e., one consisting of anions and cations but not hydrogen or hydroxide ions). Any salt can be formed by the reaction of the appropriate acid and base (e.g., KBr from HBr and KOH).
Saturated solution A solution containing the maximum amount of solute that can be dissolved in a particular solvent at a particular temperature.
Second law of thermodynamics The law stating that all spontaneous processes lead to an increase in the entropy of the universe.
Semimetal See metalloid.
Semipermeable A quality of a membrane allowing only some components of a solution, usually including the solvent, to pass through, while limiting the passage of other species.
Simple cubic structure A crystalline unit cell with atoms at each corner of a cube.
Solid The phase of matter possessing the greatest order, in which molecules are fixed in a rigid structure.
Solubility A measure of the amount of solute that can be dissolved in a solvent at a certain temperature.
Solubility product (Ksp) The equilibrium constant for the ionization reaction of a slightly soluble electrolyte.
Solute The component of a solution that is present in lesser amount than the solvent.
Solution A homogeneous mixture of two or more substances. It may be solid (e.g., brass), liquid (e.g., HCl(aq)), or gas (e.g., air).
Solvent The component of a solution present in the greatest amount; the substance in which the solute is dissolved.
Specific heat The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.
Spectrum The characteristic wavelengths of electromagnetic radiation emitted or absorbed by an object, atom, or molecule.
Spin quantum number (ms) The fourth quantum number, indicating the orientation of the intrinsic angular momentum of an electron in an atom. The spin quantum number can only assume values of +
Spontaneous process A process that will occur on its own without energy input from the surroundings.
Standard conditions Conditions defined as 25°C and 1 M concentration for each reactant in solution and a partial pressure of 1 atm for each gaseous reactant. Used for measuring the standard Gibb’s free energy, enthalpy, entropy, and cell emf.