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Textbook Site for:
World of Chemistry
Steven S. Zumdahl, University of Illinois
Susan A. Zumdahl, University of Illinois
Donald DeCoste, University of Illinois
Glossary


A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X-Y-Z



A [return to top]

Acid a substance that produces hydrogen ions in aqueous solution; a proton donor.

Acid - base indicator a substance that marks the end point of an acid - base titration by changing color.

Acid rain rainwater with an acidic pH, a result of air pollution by sulfur dioxide and nitrogen oxides.

Acidic oxide a covalent oxide that dissolves in water to give an acidic solution.

Actinide series a group of fourteen elements following actinium on the periodic table, in which the 5f orbitals are being filled.

Activation energy the threshold energy that must be overcome to produce a chemical reaction.

Air pollution contamination of the atmosphere, mainly by the gaseous products of transportation and the production of electricity.

Alcohol an organic compound in which the hydroxyl group is a substituent on a hydrocarbon.

Aldehyde an organic compound containing the carbonyl group bonded to at least one hydrogen atom.

Alkali metal a Group 1 metal.

Alkaline earth metal a Group 2 metal.

Alkane a saturated hydrocarbon with the general formula CnH2n12.

Alkene an unsaturated hydrocarbon containing a carbon - carbon double bond. The general formula is CnH2n.

Alkyne an unsaturated hydrocarbon containing a carbon - carbon triple bond. The general formula is CnH2n22.

Alloy a substance that contains a mixture of elements and has metallic properties.

Alloy steel a form of steel containing carbon plus metals such as chromium, cobalt, manganese, and molybdenum.

Alpha () particle a helium nucleus produced in radioactive decay.

Alpha-particle production a common mode of decay for radioactive nuclides in which the mass number changes.

Amine an organic base derived from ammonia in which one or more of the hydrogen atoms are replaced by organic groups.

-Amino acid an organic acid in which an amino group, a hydrogen atom, and an R group are attached to the carbon atom next to the carboxyl group.

Ampere the unit of measurement for electric current; 1 ampere is equal to 1 coulomb of charge per second.

Amphoteric substance a substance that can behave either as an acid or as a base.

Anion a negative ion.

Anode in a galvanic cell, the electrode at which oxidation occurs.

Aqueous solution a solution in which water is the dissolving medium, or solvent.

Aromatic hydrocarbon one of a special class of cyclic unsaturated hydrocarbons, the simplest of which is benzene.

Arrhenius concept a concept postulating that acids produce hydrogen ions in aqueous solution, whereas bases produce hydroxide ions.

Atmosphere the mixture of gases that surrounds the earth's surface.
Atom the fundamental unit of which elements are composed.

Atomic mass (weight) the weighted average mass of the atoms in a naturally occurring element.

Atomic number the number of protons in the nucleus of an atom; each element has a unique atomic number.

Atomic radius half the distance between the atomic nuclei in a molecule consisting of identical atoms.

Atomic solid a solid that contains atoms at the lattice points.

Aufbau principle a principle stating that as protons are added one by one to the nucleus to build up the elements, electrons are similarly added to hydrogen-like orbitals.

Auto-ionization the transfer of a proton from one molecule to another of the same substance.

Avogadro's law equal volumes of gases at the same temperature and pressure contain the same number of particles (atoms or molecules).

Avogadro's number the number of atoms in exactly 12 grams of pure 12C, equal to 6.02 x 1023.




B [return to top]

Ball-and-stick model a molecular model that distorts the sizes of atoms but shows bond relationships clearly.

Barometer a device for measuring atmospheric pressure.

Base a substance that produces hydroxide ions in aqueous solution; a proton acceptor.

Basic oxide an ionic oxide that dissolves in water to produce a basic solution.

Battery a group of galvanic cells connected in series.

Beta () particle an electron produced in radioactive decay.

Beta-particle production a decay process for radioactive nuclides in which the mass number remains constant and the atomic number increases by one.
The net effect is to change a neutron to a proton.

Binary compound a two-element compound.

Binding energy (nuclear) the energy required to decompose a nucleus into its component nucleons.

Biochemistry the study of the chemistry of living systems.

Biomolecule a molecule that functions in maintaining and/or reproducing life.

Bond (chemical bond) the force that holds two atoms together in a compound.

Bond energy the energy required to break a given chemical bond.

Bond length the distance between the nuclei of the two atoms that are connected by a bond.

Bonding pair an electron pair found in the space between two atoms.

Boyle's law the volume of a given sample of gas at constant temperature varies inversely with the pressure.

Breeder reactor a nuclear reactor in which fissionable fuel is produced while the reactor runs.

Brĝnsted - Lowry model a model proposing that an acid is a proton donor and that a base is a proton acceptor.

Buffer capacity the ability of a buffered solution to absorb protons or hydroxide ions without a significant change in pH.

Buffered solution a solution that resists a change in its pH when either hydroxide ions or protons are added.

Buret a device for the accurate measurement of the delivery of a given volume of liquid.



C [return to top]

Calorie a unit of measurement for energy; one calorie is the quantity of energy required to heat one gram of water by one Celsius degree.

Calorimetry the science of measuring heat flow.

Carbohydrate a polyhydroxyl ketone or polyhydroxyl aldehyde or a polymer composed of these.

Carbon steel an alloy of iron containing up to about 1.5% carbon.

Carboxyl group the COOH group in an organic acid.

Carboxylic acid an organic compound containing the carboxyl group.

Catalyst a substance that speeds up a reaction without being consumed.

Cathode in a galvanic cell, the electrode at which reduction occurs.

Cathode rays the "rays" emanating from the negative electrode (cathode) in a partially evacuated tube; a stream of electrons.

Cathodic protection the connection of an active metal, such as magnesium, to steel to protect the steel from corrosion.

Cation a positive ion.

Cell potential (electromotive force) the driving force in a galvanic cell that pushes electrons from the reducing agent in one compartment to the oxidizing agent in the other.

Chain reaction (nuclear) a self-sustaining fission process caused by the production of neutrons that proceed to split other nuclei.

Charles's law the volume of a given sample of gas at constant pressure is directly proportional to the temperature in kelvins.

Chemical change the change of substances into other substances through a reorganization of the atoms; a chemical reaction.

Chemical equation a representation of a chemical reaction showing the relative numbers of reactant and product molecules.

Chemical equilibrium a dynamic reaction system in which the concentrations of all reactants and products remain constant as a function of time.

Chemical formula a representation of a molecule in which the symbols for the elements are used to indicate the types of atoms present and subscripts are used to show the relative numbers of atoms.

Chemical kinetics the area of chemistry that concerns reaction rates.

Chemical property the ability of a substance to change to a different substance.

Chemical stoichiometry the quantities of materials consumed and produced in a chemical reaction.

Colligative property a solution property that depends on the number of solute particles present.

Collision model a model based on the idea that molecules must collide to react; used to account for the observed characteristics of reaction rates.

Combustion reaction the vigorous and exothermic oxidation - reduction reaction that takes place between certain substances (particularly organic compounds) and oxygen.

Complete ionic equation an equation that shows as ions all substances that are strong electrolytes.

Compound a substance with constant composition that can be broken down into elements by chemical processes.

Condensation the process by which vapor molecules re-form a liquid.

Condensed states of matter liquids and solids.

Conjugate acid the species formed when a proton is added to a base.

Conjugate acid - base pair two species related to each other by the donating and accepting of a single proton.

Conjugate base what remains of an acid molecule after a proton is lost.

Continuous spectrum a spectrum that exhibits all the wavelengths of visible light.

Control rods in a nuclear reactor, rods composed of substances that absorb neutrons.
These rods regulate the power level of the reactor.

Core electron an inner electron in an atom; one that is not in the outermost (valence) principal quantum level.

Corrosion the process by which metals are oxidized in the atmosphere.

Covalent bonding a type of bonding in which atoms share electrons.

Critical mass the mass of fissionable material required to produce a self-sustaining chain reaction.

Critical reaction (nuclear) a reaction in which exactly one neutron from each fission event causes another fission event, thus sustaining the chain reaction.

Crystalline solid a solid characterized by the regular arrangement of its components.



D [return to top]

Dalton's law of partial pressures for a mixture of gases in a container, the total pressure exerted is the sum of the pressures that each gas would exert if it were alone.

Denaturation the breaking down of the three-dimensional structure of a protein, resulting in the loss of its function.

Density a property of matter representing the mass per unit volume.

Deoxyribonucleic acid (DNA) a huge nucleotide polymer having a double-helical structure with complementary bases on the two strands.
Its major functions are pro- tein synthesis and the storage and transport of genetic information.

Diatomic molecule a molecule composed of two atoms.

Dilution the process of adding solvent to lower the concentration of solute in a solution.

Dipole - dipole attraction the attractive force resulting when polar molecules line up such that the positive and negative ends are close to each other.

Dipole moment a property of a molecule whereby the charge distribution can be represented by a center of positive charge and a center of negative charge.

Disaccharide a sugar formed from two monosaccharides joined by a glycoside linkage.

Distillation a method for separating the components of a liquid mixture that depends on differences in the ease of vaporization of the components.

Double bond a bond in which two atoms share two pairs of electrons.

Dry cell battery a common battery used in calculators, watches, radios, and tape players.



E [return to top]

Electrical conductivity the ability to conduct an electric current.

Electrochemistry the study of the interchange of chemical and electrical energy.

Electrolysis a process that involves forcing a current through a cell to cause a nonspontaneous chemical reaction to occur.

Electrolyte a material that dissolves in water to give a solution that conducts an electric current.

Electrolytic cell a cell that uses electrical energy to produce a chemical change that would not otherwise occur.

Electromagnetic radiation radiant energy that exhibits wave-like behavior and travels through space at the speed of light in a vacuum.

Electron a negatively charged particle that occupies the space around the nucleus of an atom.

Electronegativity the tendency of an atom in a molecule to attract shared electrons to itself.

Element a substance that cannot be decomposed into simpler substances by chemical or physical means. It consists of atoms all having the same atomic number.

Empirical formula the simplest whole-number ratio of atoms in a compound.

End point the point in a titration at which the indicator changes color.

Endothermic refers to a reaction in which energy (as heat) flows into the system.

Energy the capacity to do work or to cause the flow of heat.

Enthalpy at constant pressure, the change in enthalpy equals the energy flow as heat.

Enzyme a large molecule, usually a protein, that catalyzes biological reactions.

Equilibrium constant the value obtained when equilibrium concentrations of the chemical species are substituted into the equilibrium expression.

Equilibrium expression the expression (from the law of mass action) equal to the product of the product concentrations divided by the product of the reactant concentrations, each concentration having first been raised to a power represented by the coefficient in the balanced equation.

Equilibrium position a particular set of equilibrium concentrations.

Equivalence point (stoichiometric point) the point in a titration when enough titrant has been added to react exactly with the substance in solution that is being titrated.

Essential elements the elements known to be essential to human life.

Ester an organic compound produced by the reaction between a carboxylic acid and an alcohol.

Exothermic refers to a reaction in which energy (as heat) flows out of the system.

Exponential notation expresses a number in the form N 3 10M; a convenient method for representing a very large or very small number and for easily indicating the number of significant figures.



F [return to top]

Fat (glyceride) an ester composed of glycerol and fatty acids.

Fatty acid a long-chain carboxylic acid.

Filtration a method for separating the components of a mixture containing a solid and a liquid.

First law of thermodynamics a law stating that the energy of the universe is constant.

Fission the process of using a neutron to split a heavy nucleus into two nuclei with smaller mass numbers.

Fossil fuel a fuel that consists of carbon-based molecules derived from decomposition of once-living organisms; coal, petroleum, or natural gas.

Frequency the number of waves (cycles) per second that pass a given point in space.

Fuel cell a galvanic cell for which the reactants are continuously supplied.

Functional group an atom or group of atoms in hydrocarbon derivatives that contains elements in addition to carbon and hydrogen.

Fusion the process of combining two light nuclei to form a heavier, more stable nucleus.



G [return to top]

Galvanic cell a device in which chemical energy from a spontaneous oxidation - reduction reaction is changed to electrical energy that can be used to do work.

Galvanizing a process in which steel is coated with zinc to prevent corrosion.

Gamma () ray a high-energy photon produced in radioactive decay.

Gas one of the three states of matter; has neither fixed shape nor fixed volume.
Geiger - Müller counter (Geiger counter) an instrument that measures the rate of radioactive decay by registering the ions and electrons produced as a radioactive particle passes through a gas-filled chamber.

Gene a given segment of the DNA molecule that contains the code for a specific protein.

Greenhouse effect a warming effect exerted by certain molecules in the earth's atmosphere (particularly carbon dioxide and water).

Ground state the lowest possible energy state of an atom or molecule.

Group (of the periodic table) a vertical column of elements having the same valence electron configuration and similar chemical properties.



H [return to top]

Haber process the manufacture of ammonia from nitrogen and hydrogen, carried out at high pressure and high temperature with the aid of a catalyst.

Half-life (of a radioactive sample) the time required for the number of nuclides in a radioactive sample to reach half the original number of nuclides.

Half-reactions the two parts of an oxidation - reduction reaction, one representing oxidation, the other reduction.

Halogen a Group 7 element.

Hard water water from natural sources that contains relatively large concentrations of calcium and magnesium ions.

Heat energy transferred between two objects because of a temperature difference between them.

Heating/cooling curve a plot of temperature versus time for a substance, where energy is added at a constant rate.

Heisenberg uncertainty principle a principle stating that there is a fundamental limitation to how precisely we can know both the position and the momentum of a particle at a given time.

Herbicide a pesticide applied to kill weeds.

Heterogeneous equilibrium an equilibrium involving reactants and/or products in more than one state.

Heterogeneous mixture a mixture that has different properties in different regions of the mixture.

Heterogeneous reaction reaction involving reactants in different phases.

Homogeneous equilibrium an equilibrium system in which all reactants and products are in the same state.

Homogeneous mixture a mixture that is the same throughout; a solution.

Homogeneous reaction reaction involving reactants in only one phase.

Hydration the interaction between solute particles and water molecules.

Hydrocarbon a compound of carbon and hydrogen.

Hydrocarbon derivative an organic molecule that contains one or more elements in addition to carbon and hydrogen.

Hydrogen bonding unusually strong dipole - dipole attractions that occur among molecules in which hydrogen is bonded to a highly electronegative atom.

Hydronium ion the H3O1 ion; a hydrated proton.

Hypothesis one or more assumptions put forth to explain observed phenomena.



I [return to top]

Ideal gas a hypothetical gas that exactly obeys the ideal gas law.
A real gas approaches ideal behavior at high temperature and/or low pressure.

Ideal gas law an equation relating the properties of an ideal gas, expressed as PV 5 nRT, where P 5 pressure, V 5 volume, n 5 moles of the gas, R 5 the universal gas constant, and T 5 temperature on the Kelvin scale.
This equation expresses behavior closely approached by real gases at high temperature and/or low pressure.

Indicator a chemical that changes color and is used to mark the end point of a titration.

Intermolecular forces relatively weak interactions that occur between molecules.

Internal energy the sum of the kinetic and potential energies of all components of an object.

Intramolecular forces interactions that occur within a given molecule.

Ion an atom or a group of atoms that has a net positive or negative charge.

Ion-product constant (Kw) the equilibrium constant for the auto-ionization of water; Kw 5 [H1][OH2]. At 25 C, Kw equals 1.0 3 10214.

Ionic bonding the attraction between oppositely charged ions.

Ionic compound a compound that results when a metal reacts with a nonmetal to form cations and anions.

Ionic solid a solid containing cations and anions that dissolves in water to give a solution containing the separated ions, which are mobile and thus free to conduct an electric current.

Ionization energy the quantity of energy required to remove an electron from a gaseous atom or ion.

Isomers species that have the same chemical formula but different properties.

Isotopes atoms of the same element (the same number of protons) that have different numbers of neutrons.
They have identical atomic numbers but different mass numbers.



J [return to top]

Joule a unit of measurement for energy; 1 calorie 5 4.184 joules.



K [return to top]

Ketone an organic compound containing the carbonyl group bonded to two carbon atoms.

Kinetic energy 112mv22 energy due to the motion of an object; dependent on the mass of the object and the square of its velocity.

Kinetic molecular theory a model that assumes that an ideal gas is composed of tiny particles (molecules) in constant motion.



L [return to top]

Lanthanide series a group of fourteen elements following lanthanum on the periodic table, in which the 4f orbitals are being filled.

Lattice a three-dimensional system of points designating the positions of the centers of the components of a solid (atoms, ions, or molecules).

Law of chemical equilibrium a general description of the equilibrium condition; it defines the equilibrium expression.

Law of conservation of energy energy can be converted from one form to another but can be neither created nor destroyed.

Law of conservation of mass mass is neither created nor destroyed.

Law of constant composition a given compound always contains elements in exactly the same proportion by mass.

Law of mass action (also called the law of chemical equilibrium) a general description of the equilibrium condition; it defines the equilibrium expression.

Law of multiple proportions a law stating that when two elements form a series of compounds, the ratios of the masses of the second element that combine with one gram of the first element can always be reduced to small whole numbers.

Lead storage battery a battery (used in cars) in which the anode is lead, the cathode is lead coated with lead dioxide, and the electrolyte is a sulfuric acid solution.
Le Châtelier's principle if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce the effect of that change.

Lewis structure a diagram of a molecule showing how the valence electrons are arranged among the atoms in the molecule.

Limiting reactant (limiting reagent) the reactant that is completely consumed when a reaction is run to completion.

Line spectrum a spectrum showing only certain discrete wavelengths.

Linear accelerator a type of particle accelerator in which a changing electrical field is used to accelerate a beam of charged particles along a linear path.

Lipids water-insoluble substances that can be extracted from cells by nonpolar organic solvents.

Liquid one of the three states of matter; has a fixed volume but takes the shape of its container.

London dispersion forces the relatively weak forces, which exist among noble gas atoms and nonpolar molecules, that involve an accidental dipole that induces a momentary dipole in a neighbor.

Lone pair an electron pair that is localized on a given atom; an electron pair not involved in bonding.



M [return to top]

Main-group (representative) elements elements in the groups labeled 1,2,3,4,5,6,7, and 8 on the periodic table.
The group number gives the sum of the valence s and p electrons.

Mass the quantity of matter in an object.

Mass number the total number of protons and neutrons in the atomic nucleus of an atom.

Mass percent the percent by mass of a component of a mixture or of a given element in a compound.

Matter the material of the universe.

Metal an element that gives up electrons relatively easily and is typically lustrous, malleable, and a good conductor of heat and electricity.

Metalloid an element that has both metallic and nonmetallic properties.

Metallurgy the process of separating a metal from its ore and preparing it for use.

Millimeters of mercury (mm Hg) a unit of measurement for pressure, also called a torr; 760 mm Hg 5 760 torr 5 101,325 Pa 5 1 standard atmosphere.

Mixture a material of variable composition that contains two or more substances.

Model (theory) a set of assumptions put forth to explain the observed behavior of matter.
The models of chemistry usually involve assumptions about the behavior of individual atoms or molecules.

Moderator a substance used in a nuclear reactor to slow down the neutrons.

Molar heat of fusion the energy required to melt 1 mol of a solid.

Molar heat of vaporization the energy required to vaporize 1 mol of a liquid.

Molar mass the mass in grams of one mole of a compound.

Molar volume the volume of one mole of an ideal gas; equal to 22.
42 liters at standard temperature and pressure.

Molarity moles of solute per volume of solution in liters.

Mole (mol) the number equal to the number of carbon atoms in exactly 12 grams of pure 12C: Avogadro's number. One mole represents 6.02 x 1023 units.

Mole ratio (stoichiometry) the ratio of moles of one substance to moles of another substance in a balanced chemical equation.

Molecular equation an equation representing a reaction in solution and showing the reactants and products in undissociated form, whether they are strong or weak electrolytes.

Molecular formula the exact formula of a molecule, giving the types of atoms and the number of each type.

Molecular solid a solid composed of small molecules.

Molecular structure the three-dimensional arrangement of atoms in a molecule.

Molecular weight (molar mass) the mass in grams of one mole of a substance.

Molecule a bonded collection of two or more atoms of the same element or different elements.

Monoprotic acid an acid with one acidic proton.



N [return to top]

Natural gas consists of mostly methane and is associated with petroleum deposits.

Natural law a statement that expresses generally observed behavior.

Net ionic equation an equation for a reaction in solution, representing strong electrolytes as ions and showing only those components that are directly involved in the chemical change.

Network solid an atomic solid containing strong directional covalent bonds.

Neutralization reaction an acid - base reaction.

Neutron a particle in the atomic nucleus with a mass approximately equal to that of the proton but with no charge.

Noble gas a Group 8 element.

Nonelectrolyte a substance that, when dissolved in water, gives a nonconducting solution.

Nonmetal an element that does not exhibit metallic characteristics.
Chemically, a typical nonmetal accepts electrons from a metal.

Normal boiling point the temperature at which the vapor pressure of a liquid is exactly one atmosphere; the boiling temperature under one atmosphere of pressure.

Normal melting/freezing point the melting/freezing point of a solid at a total pressure of one atmosphere.

Normality the number of equivalents of a substance dissolved in a liter of solution.

Nuclear atom the modern concept of the atom as having a dense center of positive charge (the nucleus) and electrons moving around the outside.

Nuclear transformation the change of one element into another.

Nucleon a particle in an atomic nucleus, either a neutron or a proton.

Nucleus the small, dense center of positive charge in an atom.

Nuclide the general term applied to each unique atom; represented by AZX, where X is the symbol for a particular element.



O [return to top]

Octet rule the observation that atoms of nonmetals form the most stable molecules when they are surrounded by eight electrons (to fill their valence orbitals).

Orbital a representation of the space occupied by an eletron in an atom; the probability distribution for the electron.

Organic acid an acid with a carbon-atom backbone and a carboxyl group.

Organic chemistry the study of carbon-containing compounds (typically containing chains of carbon atoms) and their properties.

Oxidation an increase in oxidation state (a loss of electrons).

Oxidation - reduction (redox) reaction a reaction in which one or more electrons are transferred.

Oxidation states a concept that provides a way to keep track of electrons in oxidation - reduction reactions according to certain rules.

Oxidizing agent (electron acceptor) a reactant that accepts electrons from another reactant.

Oxyacid an acid in which the acidic proton is attached to an oxygen atom.

Ozone O3, a form of elemental oxygen much less common than O2 in the atmosphere near the earth.



P [return to top]

Partial pressures the independent pressures exerted by different gases in a mixture.

Particle accelerator a device used to accelerate nuclear particles to very high speeds.

Pascal the SI unit of measurement for pressure; equal to one newton per square meter.

Percent yield the actual yield of a product as a percentage of the theoretical yield.

Periodic table a chart showing all the elements arranged in columns in such a way that all the elements in a given column exhibit similar chemical properties.

Petroleum a thick, dark liquid composed mostly of hydrocarbon compounds.
pH scale a log scale based on 10 and equal to 2log[H1]; a convenient way to represent solution acidity.

Phenyl group the benzene molecule minus one hydrogen atom.

Photochemical smog air pollution produced by the action of light on oxygen, nitrogen oxides, and unburned fuel from auto exhaust to form ozone and other pollutants.

Photon a "particle" of electromagnetic radiation.

Physical charge a change in the form of a substance, but not in its chemical nature; chemical bonds are not broken in a physical change.

Physical property a characteristic of a substance that can change without the substance becoming a different substance.

Polar covalent bond a covalent bond in which the electrons are not shared equally because one atom attracts them more strongly than the other.

Polar molecule a molecule that has a permanent dipole moment.

Polyatomic ion an ion containing a number of atoms.

Polyelectronic atom an atom with more than one electron.

Polymer a large, usually chain-like molecule built from many small molecules (monomers).

Polymerization a process in which many small molecules (monomers) are joined together to form a large molecule.

Polyprotic acid an acid with more than one acidic proton.
It dissociates in a stepwise manner, one proton at a time.

Positron production a mode of nuclear decay in which a particle is formed that has the same mass as an electron but opposite charge.
The net effect is to change a proton to a neutron.

Potential energy energy due to position or composition.

Precipitation reaction a reaction in which an insoluble substance forms and separates from the solution as a solid.

Precision the degree of agreement among several measurements of the same quantity; the reproducibility of a measurement.

Primary structure (of a protein) the order (sequence) of amino acids in the protein chain.

Probability distribution (orbital) a representation indicating the probabilities of finding an electron at various points in space.

Product a substance resulting from a chemical reaction.
It is shown to the right of the arrow in a chemical equation.

Protein a natural polymer formed by condensation reactions between amino acids.

Proton a positively charged particle in an atomic nucleus.

Pure substance a substance with constant composition.



Q [return to top]

No entries for this letter



R [return to top]

Radioactive decay (radioactivity) the spontaneous decomposition of a nucleus to form a different nucleus.

Radiocarbon dating (carbon-14 dating) a method for dating ancient wood or cloth on the basis of the radioactive decay of the nuclide 146C.

Radiotracer a radioactive nuclide, introduced into an organism for diagnostic purposes, whose pathway can be traced by monitoring its radioactivity.

Random error an error that has an equal probability of being high or low.

Rate of decay the change per unit time in the number of radioactive nuclides in a sample.

Reactant a starting substance in a chemical reaction.
It appears to the left of the arrow in a chemical equation.

Reactor core the part of a nuclear reactor where the fission reaction takes place.

Reducing agent (electron donor) a reactant that donates electrons to another substance, reducing the oxidation state of one of its atoms.

Reduction a decrease in oxidation state (a gain of electrons).

Rem a unit of radiation dosage that accounts for both the energy of the dose and its effectiveness in causing biological damage (from roentgen equivalent for man).

Resonance a condition occurring when more than one valid Lewis structure can be written for a particular molecule.
The actual electronic structure is represented not by any one of the Lewis structures but by the average of all of them.



S [return to top]

Salt an ionic compound.

Salt bridge a U-tube containing an electrolyte that connects the two compartments of a galvanic cell, allow- ing ion flow without extensive mixing of the different solutions.

Saturated solution a solution that contains as much solute as can be dissolved in that solution.

Scientific method a process of studying natural phenomena that involves making observations, forming laws and theories, and testing theories by experimentation.

Scientific notation see Exponential notation.

Scintillation counter an instrument that measures radioactive decay by sensing the flashes of light that the radiation produces in a detector.

Secondary structure (of a protein) the three-dimensional structure of the protein chain (for example, a-helix, random coil, or pleated sheet).

SI units International System of units based on the metric system and on units derived from the metric system.

Sigma () bond a covalent bond in which the electron pair is shared in an area centered on a line running between the atoms.

Significant figures the certain digits and the first uncertain digit of a measurement.

Silica the fundamental silicon-oxygen compound, which has the empirical formula SiO2 and forms the basis of quartz and certain types of sand.

Silicates salts that contain metal cations and polyatomic silicon - oxygen anions that are usually polymeric.

Single bond a bond in which two atoms share one pair of electrons.

Solid one of the three states of matter; has a fixed shape and volume.

Solubility the amount of a substance that dissolves in a given volume of solvent or solution at a given temperature.

Solubility product the constant for the equilibrium expression representing the dissolving of an ionic solid in water.

Solute a substance dissolved in a solvent to form a solution.

Solution a homogeneous mixture.

Solvent the dissolving medium in a solution.

Somatic damage radioactive damage to an organism resulting in its sickness or death.

Specific heat another name for specific heat capacity.

Specific heat capacity the amount of energy required to raise the temperature of one gram of a substance by one Celsius degree.

Spectator ions ions present in solution that do not participate directly in a reaction.

Standard atmosphere a unit of measurement for pressure equal to 760 mm Hg or 101, 325 Pa.

Standard solution a solution the concentration of which is accurately known.

Standard temperature and pressure (STP) the condition 0 C and 1 atmosphere of pressure.

State function a property that is independent of the pathway.

States of matter the three different forms in which matter can exist: solid, liquid, and gas.

Stoichiometric quantities quantities of reactants mixed in exactly the amounts that result in their all being used up at the same time.

Stoichiometry of a reaction the relative quantities of reactants and products involved in the reaction.

Strong acid an acid that completely dissociates (ionizes) to produce H1 ion and the conjugate base.

Strong base a metal hydroxide compound that completely dissociates into its ions in water.

Strong electrolyte a material that, when dissolved in water, dissociates (ionizes) completely and gives a solution that conducts an electric current very efficiently.

Structural formula the representation of a molecule in which the relative positions of the atoms are shown and the bonds are indicated by lines.

Subcritical reaction (nuclear) a reaction in which fewer than one of the neutrons from each fission event causes another fission event and the process dies out.

Sublimation the process by which a substance goes directly from the solid state to the gaseous state without passing through the liquid state.
Substitution reaction (hydrocarbons) a reaction in which an atom, usually a halogen, replaces a hydrogen atom in a hydrocarbon.

Supercooling the process of cooling a liquid to a temperature below its freezing point without its changing to a solid.

Supercritical reaction (nuclear) a reaction in which more than one of the neutrons from each fission event causes another fission event.
The process rapidly escalates to a violent explosion.

Superheating the process of heating a liquid to a temperature above its boiling point without its boiling.

Surroundings everything in the universe surrounding a thermodynamic system.

System (thermodynamic) that part of the universe on which attention is to be focused.

Systematic error an error that always occurs in the same direction.



T [return to top]

Temperature measure of the random motions (average kinetic energy) of the components of a substance.

Tertiary structure (of a protein) the overall shape of a protein, long and narrow or globular, maintained by different types of intramolecular interactions.

Theoretical yield the maximum amount of a given product that can be formed when the limiting reactant is completely consumed.

Theory (model) a set of assumptions put forth to explain some aspect of the observed behavior of matter.

Thermodynamics a study of energy and its interactions.

Titration a technique in which one solution is used to analyze another.

Torr another name for millimeters of mercury (mm Hg).

Trace elements metals present only in trace amounts in the human body.

Transition metals several series of elements in which inner orbitals (d or f orbitals) are being filled.

Transuranium elements the elements beyond uranium that are made artificially by particle bombardment.

Triple bond a bond in which two atoms share three pairs of electrons.



U [return to top]

Uncertainty (in measurement) the characteristic reflecting the fact that any measurement involves estimates and cannot be exactly reproduced.

Unit factor an equivalence statement between units that is used for converting from one set of units to another.

Universal gas constant the combined proportionality constant in the ideal gas law; 0.08206 L atm/K mol, or 8.314 J/K mol.

Unsaturated solution a solution in which more solute can be dissolved than is dissolved already.



V [return to top]

Valence electrons the electrons in the outermost occupied principal quantum level of an atom.

Valence shell electron pair repulsion (VSEPR) model a model the main postulate of which is that the structure around a given atom in a molecule is determined principally by the tendency to minimize electron-pair repulsions.

Vapor pressure the pressure of the vapor over a liquid at equilibrium in a closed container.

Vaporization (evaporation) the change in state that occurs when a liquid evaporates to form a gas.

Viscosity the resistance of a liquid to flow.

Volt the unit of measurement for electric potential; it is defined as one joule of work per coulomb of charge transferred.



W [return to top]

Wavelength the distance between two consecutive peaks or troughs in a wave.

Weak acid an acid that dissociates only to a slight extent in aqueous solution.

Weak base a base that reacts with water to produce hydroxide ions to only a slight extent in aqueous solution.

Weak electrolyte a material that, when dissolved in water, gives a solution that conducts only a small electric current.

Weight the force exerted on an object by gravity.

Work force acting over a distance.



X-Y-Z [return to top]

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