This glossary of chemistry terms is a list of terms and definitions relevant to chemistry, including chemical laws, diagrams and formulae, laboratory tools, glassware, and equipment. Chemistry is a physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions; it features an extensive vocabulary and a significant amount of jargon. Note: All periodic table references refer to the IUPAC Style of the Periodic Table.
A
absolute zero A theoretical condition concerning a system at the lowest limit of the thermodynamic temperature scale, or zero kelvins, at which the system does not emit or absorb energy (i.e. all atoms are at rest). By extrapolating the ideal gas law, the internationally agreed-upon value for absolute zero has been determined as −273.15 °C (−459.67 °F; 0.00 K).
absorbance
absorption 1. The physical or chemical process by which a substance in one state becomes incorporated into and retained by another substance of a different state. Absorption differs from adsorption in that the first substance permeates the entire bulk of the second substance, rather than just adhering to the surface. 2. The process by which matter (typically electrons bound in atoms) takes up the energy of electromagnetic radiation and transforms it into any of various types of internal energy, such as thermal energy. This type of absorption is the principle on which spectrophotometry is based.
abundance
accuracy How close a measured value is to the actual or true value. Compare precision.
acetyl
achiral (of a molecule) Having the geometric symmetry of being indistinguishable from its own mirror image; lacking chirality.
acid 1. (Brønsted–Lowry acid) Any chemical species or molecular entity that acts as a proton donor when reacting with another species, because it loses at least one proton (H+) which is then transferred or 'donated' to the other species, which by definition is a Brønsted–Lowry base. When dissolved in an aqueous solution, a proton donor which increases the concentration of hydronium ion (H3O+) by transferring protons to water molecules may also be called an Arrhenius acid. The term "acid", when not otherwise qualified, often refers implicitly to a Brønsted–Lowry acid. 2. (Lewis acid) Any chemical species or molecular entity that acts as an electron pair acceptor when reacting with another species, forming a covalent bond by accepting a lone pair of electrons donated by the other species, which is known as a Lewis base. This definition was intended as a generalization of the Brønsted–Lowry definition by proposing that acid-base reactions are best viewed as reorganizations of electrons rather than transfers of protons, with the acid being a species that accepts electron pairs from another species either directly or by releasing protons (H+) into the solution, which then accept electron pairs from the other species. The Lewis definition is inclusive of many Brønsted–Lowry acids, though not all: most Lewis acids are not Brønsted–Lowry acids, and most Brønsted–Lowry acids are not Lewis acids. 3. Colloquially, any compound which, when dissolved in water, yields a pH of less than 7.0. The term "acid" is commonly used to refer to the entire aqueous solution, whereas stricter definitions refer only to the acidic solute.
acid anhydride Any chemical compound derived by the removal of water molecules from an acid. Contrast base anhydride.
acid dissociation constant (Ka) Also acid ionization constant or acidity constant. A quantitative measure of the strength of an acid in solution expressed as an equilibrium constant for a chemical dissociation reaction in the context of acid-base reactions. It is often given as its base-10 cologarithm, pKa.
acid–base extraction A chemical reaction in which chemical species are separated from other acids and bases.
acid–base reaction
acidic
actinides Also actinoids. The periodic series of metallic elements with atomic numbers 89 to 103, from actinium through lawrencium.
activated complex A structure that forms because of a collision between molecules while new bonds are formed.
activation energy The minimum energy which must be available to a chemical system with potential reactants in order to result in a particular chemical reaction.
activity series See reactivity series.
actual yield
acyclic Containing only linear structures of atoms (particularly in hydrocarbons).
addition reaction In organic chemistry, a type of chemical reaction in which two or more molecules combine to make a larger one.
adduct A distinct chemical species that is the sole product of an addition reaction between two other distinct reactant species, in which all of the atoms comprising the reactants are retained in the single product. Changes in connectivity may occur, but there is no loss of any of the original atoms and no gain of atoms that are not present in the reactant molecules. Stoichiometries other than 1:1 are also possible, e.g. a bis-adduct (2:1).
adhesion The tendency of dissimilar particles or surfaces to cling to one another as a result of intermolecular forces. Contrast cohesion.
adsorption The chemical adhesion of atoms, ions, or molecules of one substance (either a gas, liquid, or dissolved solid) to the surface of another substance, resulting in a film of the first substance being weakly bonded to the interface between the two substances. Adsorption differs from absorption in that it is exclusively a surface phenomenon, while absorption involves entire volumes of materials.
aeration The mixing of air into a liquid or a solid.
alcohol Any organic compound consisting of at least one hydroxyl group attached to a saturated carbon atom. Alcohols have the general formula R–OH.
aldehyde A functional group and a class of organic compounds consisting of a carbonyl group attached to a hydrogen atom and any other R-group. Aldehydes have the general formula R–C(H)=O.
aliphatic
alkali metal Any of the metallic elements belonging to Group 1 of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).
alkaline
alkaline earth metal Any of the metallic elements belonging to Group 2 of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
alkane Also paraffin. Any fully saturated acyclic hydrocarbon,An acyclic saturated hydrocarbon containing only carbon and hydrogen atoms with only single carbon–carbon bonds; alkanes generally have the formula CnH2n+2. i.e. one in which all carbon–carbon bonds are single bonds.
alkene Also olefin. Any unsaturated hydrocarbon containing at least one carbon–carbon double bond.
alkoxy A functional group of the form R–O–, derived from an alcohol by removal of the hydrogen atom from the hydroxyl group.
alkyl The substituent form of an alkane, i.e. any alkane missing a hydrogen atom. The term may be used to broadly describe many different functional groups, e.g. a methyl, ethyl, or propyl group.
alkyne Also acetylene. Any unsaturated hydrocarbon containing at least one carbon–carbon triple bond.
allomer A substance that differs in chemical composition but has the same crystalline structure as another substance.
allotrope Any of a variety of different structural forms of the same element, as with carbon, whose allotropes include diamonds, graphite, and fullerene.
alloy A mixture of elements where at least one is a metal, which in combination exhibit a metallic bonding character. Common examples include bronze, brass, and pewter.
amalgam Any alloy of mercury with another metal.
ambident A molecule or functional group that has two alternative and interacting reaction sites, to either of which a bond may be made during a reaction.
amide
ammoniacal Describing a solution in which the solvent is aqueous ammonia.
amorphous solid
amount of substance Also enplethy, chemical amount, or simply amount. The number of discrete particles (such as molecules, atoms, ions, electrons, or any other atomic-scale entity) in a given sample of matter, divided by the Avogadro constant. The SI unit for amount of substance is the mole (mol).
amphipathic (of a molecule) Composed of both hydrophilic and hydrophobic groups; e.g. wetting agents and membrane lipids.
amphoteric Also amphiprotic. (of a chemical species) Tending to behave both as an acid and as a base, depending upon the medium in which the species is situated; e.g. sulfuric acid (H2SO4) is a strong acid in water but behaves more like a base in superacids.
amyl A common non-systematic name for a pentyl group.
analyte The specific substance or chemical constituent that is of interest in a chemical analysis.
analytical chemistry The branch of chemistry which studies and makes use of instruments and methods to separate, quantify, and identify chemical substances, both by classical wet chemistry techniques such as precipitation, extraction, distillation, and observational analysis, and by modern instrumental techniques such as chromatography, spectroscopy, and electrochemistry.
ångström (Å) A non-SI, metric unit of length equal to 10−10 metre, i.e. 1⁄10000000000 of a metre or 0.1 nanometre. The angstrom is commonly used in the natural sciences to express microscopic or atomic-scale distances, including the sizes of atomic nuclei, wavelengths of electromagnetic radiation, and lengths of chemical bonds (e.g. the covalent radius of a chlorine atom averages about 1 angstrom).
anhydrous Having or containing no water molecules, referring especially to water of hydration. Because many processes in chemistry are impeded in the presence of water, it is often of critical importance that water-free reagents and techniques are used. Anhydrous compounds tend to gradually absorb water from the atmosphere. Contrast hydrous.
anion A negatively charged ion; i.e. an atom or molecule with a net negative electric charge caused by an excess of electrons compared to protons.
annulation The formation of a cyclic compound or ring structure from one or several acyclic precursors; or a reaction involving the addition of a ring structure to another molecule via two new bonds.
anode 1. An electrode through which the conventional electric current (the flow of positive charges) enters into a polarized electrical circuit. 2. The wire or plate of an electrochemical cell having an excess positive charge. Negatively charged anions always move toward the anode. Contrast cathode.
anomer Either of a pair of cyclic hemiacetal or hemiketal saccharides that are epimers of each other, differing at only one carbon stereocenter, specifically the carbon that bears the aldehyde or ketone functional group in the compound's acyclic, open-chain configuration, known as the anomeric carbon.
aprotic (of a chemical species) Not protic; i.e. not capable of acting as a proton donor or readily yielding of protons (H+) in solution.
aqua regia A liquid mixture of nitric acid (HNO3) and hydrochloric acid (HCl), optimally in a molar ratio of 1:3, so named by historical alchemists because it is capable of dissolving the noble metals gold and platinum.
aquation The process by which water molecules solvate or form coordination complexes with ions.
aqueous solution A solution in which the solvent is water. It is denoted in chemical equations by appending (aq) to a chemical formula.
aromatic
aromaticity A chemical property of conjugated rings of atoms, such as benzene, which results in unusually high stability. Such rings are said to be aromatic.
Arrhenius acid Any substance that, when dissolved in water, increases the concentration of H+ ions, or, more correctly, of hydronium ions (H3O+), in the resulting aqueous solution. The definition is similar to that of a Brønsted–Lowry acid. Contrast Arrhenius base.
Arrhenius base Any substance that, when dissolved in water, increases the concentration of OH− ions, or, alternatively, decreases the concentration of hydronium ions (H3O+), in the resulting aqueous solution. The definition is similar to that of a Brønsted–Lowry base. Contrast Arrhenius acid.
arrow pushing
aryl Any functional group or substituent derived from an aromatic ring, such as phenyl or naphthyl. The symbol Ar is often used as a placeholder for a generic aryl group in structural diagrams.
atmolysis The separation of a mixture of gases by exploiting their different rates of diffusion, usually by allowing the gases to diffuse through the walls of a porous partition or membrane.
atom A chemical element in its smallest form, made up of protons and neutrons within the nucleus and electrons circling the nucleus.
atomic mass The mass of an atom, typically expressed in daltons and nearly equivalent to the mass number multiplied by one dalton.
atomic mass unit See dalton.
atomic number (Z) Also proton number. The number of protons found in the nucleus of an atom of a given chemical element. It is identical to the charge number of the nucleus and is used in the periodic table to uniquely identify each chemical element.
atomic orbital Any region in which one or more electrons may be found in an individual atom (as opposed to that within a molecule).
atomic radius
atomic weight See relative atomic mass.
atomicity The total number of atoms present in a single molecule of a given substance; e.g. ozone (O3) has an atomicity of 3, while benzene (C6H6) has an atomicity of 12.
autoignition temperature Also kindling point. The lowest temperature at which a given substance will spontaneously ignite in a normal atmosphere without an external source of ignition such as a flame or spark, i.e. when the ambient temperature is sufficiently high to provide the activation energy needed for combustion. Substances which spontaneously ignite at naturally occurring temperatures are termed pyrophoric. Compare ignition temperature.
Avogadro constant (NA) The ratio of the number of discrete constituent particles (such as molecules, atoms, or ions) to the amount of a substance, defined as exactly 6.02214076×1023 mol−1.
Avogadro number The number of discrete constituent particles in one mole of a substance, defined as exactly 6.02214076×1023. This dimensionless number differs from the Avogadro constant in that it has no unit.
Avogadro's law
azeotrope A mixture of liquids whose chemical composition is unchanged by distillation.
B
balance
backbone Also main chain. The primary or most structurally significant portion of a molecule with respect to its other parts, functional groups, moieties, or substituents; or, in the case of a polymer, that linear chain of atoms to which all other chains, long or short or both, may be regarded as being pendant or as side chains. Where two or more chains might equally be considered the backbone, the one which permits the simplest representation of the molecule in chemical formulae and nomenclature is considered the backbone.
barometer A device used to measure atmospheric pressure.
base A substance that accepts a proton and has a pH above 7.0. A common example is sodium hydroxide (NaOH).
base anhydride An oxide of a group I or II metal element.
basic
basicity
battery
beaker A cylindrical vessel or container with a flat bottom, most commonly a type of glassware, widely used in laboratories for a variety of purposes, such as preparing, holding, containing, collecting, or volumetrically measuring chemicals, samples, or solutions, or as a chamber in which a chemical reaction occurs. Beakers are distinguished from flasks by having straight rather than sloping sides; most beakers also have a small spout in the rim to aid pouring.
Beer–Lambert law Also Beer–Lambert–Bouguer law or simply Beer's law. A chemical law stating that the amount of light absorbed by a solution is proportional to the solution's concentration; or more specifically that the absorbance ( A {\displaystyle A} ) of a beam of radiation by a homogeneous isotropic medium is directly proportional to the absorption path length ( l {\displaystyle l} ) and to the concentration ( c {\displaystyle c} ) or the pressure ( p {\displaystyle p} ) of the absorbing species. This is usually expressed with the equation A = ϵ c l {\displaystyle A=\epsilon cl} , in which ϵ {\displaystyle \epsilon } is a proportionality constant called the molar absorption coefficient.
biochemistry The study of the chemistry of biological systems and organisms.
Bohr model Also Rutherford–Bohr model. A model of the general structure of the atom proposed by Niels Bohr and Ernest Rutherford in 1913, featuring a small, dense nucleus of positively charged particles surrounded by orbiting electrons, which are attracted to the nucleus by electrostatic forces. This interpretation replaced several earlier hypotheses and quickly became the prevailing standard model for depicting atomic structure.
boiling Also ebullition. A more rapid, highly energetic form of vaporization, in which a substance undergoes a phase transition from liquid to gas, as contrasted with the much slower process of vaporization. Boiling occurs when a liquid is heated to its boiling point, above which the liquid's internal vapor pressure exceeds the pressure exerted upon it by the surrounding atmosphere, causing the gaseous phase to rapidly and often violently separate from the liquid phase.
boiling flask Also Florence flask. A type of flask, usually made of glass, with a large round body, long neck, and flat bottom, designed especially for heating, boiling, and distilling liquids and to make swirling easy. See also round-bottom flask.
boiling point Also vaporization point. The temperature at which a substance changes state from a liquid to a gas (or vapor). It depends on pressure and is usually specified for a given substance under standard conditions.
boiling-point elevation The process by which a substance's boiling point is elevated by adding another substance.
bond Any persistent attraction between atoms, ions, or molecules that enables the formation of chemical compounds. Bonds are created as a result of a wide variety of electrochemical forces, whose strengths can vary considerably; they are broken when these forces are overcome by other forces. The types, strengths, and quantities of bonds holding together chemical substances dictate the structure and bulk properties of matter.
bond angle
Boyle's law For a given mass of gas at constant temperature, the volume varies inversely with the pressure.
Bragg's law
bridge A chemical bond between valence electrons, or an atom or unbranched chain of atoms connecting two different parts of the same molecule; i.e. an intramolecular bond linking different moieties or functional groups.
bridgehead Either of the two tertiary atoms which by bonding to each other form an intramolecular bridge.
Brønsted–Lowry acid Any chemical species that readily donates a proton.
Brønsted–Lowry acid–base reaction
Brønsted–Lowry base Any chemical species that readily accepts a proton.
Brownian motion
Büchner flask
buffered solution Also simply called a buffer. An aqueous solution consisting of a weak acid and its conjugate base or a weak base and its conjugate acid that resists changes in pH when strong acids or bases are added.
bumping A phenomenon in which a homogeneous liquid raised to its boiling point becomes superheated and, upon nucleation, rapidly boils to the gas phase, resulting in a violent expulsion of the liquid from the container; in extreme cases, the container itself may shatter. Frequent stirring, the use of an appropriate container, and the use of boiling chips can help prevent bumping.
bung Also stopper or cork. A cylindrical or conical plug or closure used to seal a container such as a bottle, tube, flask, or barrel.
burette Also spelled buret. Glassware used to dispense specific amounts of liquid when precision is necessary (e.g. during titrations and resource-dependent reactions).
butyl The alkyl functional group derived from either of the two isomers of butane, with the generic chemical formula –C4H9. It may occur as a substituent in organic compounds or exist independently as an ion or radical. In IUPAC nomenclature, the presence of a butyl substituent is indicated with the prefix butyl in the name of the compound, or with the abbreviation Bu in chemical formulae; e.g. butyl alcohol (butanol), which may occur in any of five different isomeric forms depending on the arrangement of the four carbon atoms, is often written with the generic formula CH4CH9OH or BuOH.
C
calorific value A measure of the heat per unit mass produced by complete combustion of a given substance, usually expressed in megajoules per kilogram (MJ/kg) or in kilojoules per gram (kJ/g).
calorimeter Any of various devices used to measure thermal properties (i.e. heat), such as calorific values or heats of chemical reactions.
calx A metal oxide formed by heating an ore in air.
carbanion Any organic ion with a negative charge on a carbon atom, i.e. an ion of the general formula R3C−. Carbanions are frequently intermediate species in certain organic reactions. Contrast carbocation.
carbide A class of interstitial compounds composed of carbon bonded to a particular metal (usually a large-radius transition metal) in a densely packed crystal lattice, where the carbon atoms occupy interstices between the metal atoms; e.g. tungsten carbide (WC).
carbocation
carbon
carbonic acid
carbonization 1. The conversion of organic compounds, such as those found in biological organisms, into other forms of carbon or carbonic residues by heating or burning, or during fossilization. 2. The process of coating a substance with carbon residues such as charcoal, or of causing a substance to become scorched, blackened, or charred.
carbonyl 1. A functional group composed of a carbon atom double-bonded to an oxygen atom, with the formula C = O {\displaystyle {\ce {C=O}}} . Carbonyl groups are common to many classes of organic compounds and are also a part of many larger functional groups. 2. An inorganic or organometallic coordination complex with carbon monoxide as a ligand (e.g. a metal carbonyl).
carboxyl
carboxylic acid A class of organic acids and a functional group consisting of a carboxyl group attached to a substituent group. Carboxylic acids have the general formula R − COOH {\displaystyle {\ce {R-COOH}}} (also written as R − CO 2 H {\displaystyle {\ce {R-CO2H}}} ), where R {\displaystyle {\ce {R}}} can be an alkyl, alkenyl, aryl, or any other carbon-containing substituent.
CAS Registry Number (CAS RN) Also simply CAS Number. A unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature, including more than 182 million organic and inorganic compounds, minerals, isotopes, alloys, polymers, and mixtures, as well as so-called "UVCBs" (substances of unknown or variable composition, complex reaction products, or biological origin). CAS numbers are an internationally recognized standard used by scientists, industries, and regulatory bodies.
catalyst Any element or compound that facilitates an increase in the rate of a chemical reaction but which is not consumed or destroyed during the reaction. It is considered both a reactant and a product of the reaction.
cathode An electrode from which the conventional electric current (the flow of positive charges) exits a polarized electrical circuit. Positively charged cations always move toward the cathode, though the cathode's polarity can be positive or negative depending on the type of electrical device and how it is being operated. Contrast anode.
cation A positively charged ion.
cell potential The force in a galvanic cell that pulls electrons through a reducing agent to an oxidizing agent.
centrifugation A laboratory technique which involves the application of centrifugal force to separate particles from a solution according to their size, shape, and density. Larger and/or denser substances migrate away from the axis of a centrifuge, while smaller and/or less dense substances migrate towards the axis.
centrifuge A device used to separate substances based on size, shape, and density by centrifugation, or the rotation of vessels containing the substances around a centred axis at extremely high velocities.
chain reaction
charge number A quantized value of electric charge calculated as the electric charge in coulombs divided by the elementary-charge constant, or z = q/e. Charge numbers for ions are denoted in superscript (e.g. Na+ indicates a sodium ion with a charge number of positive one). Atomic numbers are charge numbers of atomic nuclei.
Charles's law A classical gas law which states that when the pressure on a sample of a dry gas is held constant, the Kelvin temperature is directly proportional to its volume.
chelating agent
chelation A type of bonding involving the formation of two separate coordinate covalent bonds between a polydentate ligand and a single central metal ion. The ligand is usually an organic compound called a chelant or chelating agent.
chemical See chemical species and chemical compound.
chemical bond See bond.
chemical composition The identity and relative number of the elements that make up a chemical compound, which can often be expressed with a chemical formula.
chemical compound See compound.
chemical decomposition The breakdown of a single particle or entity (such as a molecule or reactive intermediate) into two or more fragments, or a chemical reaction in which two or more products are formed from a single reactant. Contrast chemical synthesis.
chemical element See element.
chemical formula Any of various means of concisely displaying information about the chemical composition of a compound or molecule using letters, numbers, and/or typographical symbols. Chemical formulas, such as empirical and molecular formulas, can only indicate the identities and numerical proportions of the atoms in a compound and are therefore more limited in descriptive power than chemical names and structural formulas.
chemical law A law of nature relevant to chemistry, such as the law of conservation of mass.
chemical nomenclature
chemical physics
chemical process 1. Any method or means of changing one or more chemicals or chemical compounds in any way, either naturally or artificially, spontaneously or by the actions of external forces. 2. In chemical engineering, any method used on an industrial scale (especially in manufacturing) to change the composition of one or more chemicals or materials.
chemical reaction The change of one or more substances into one or more different substances.
chemical species Also simply called a chemical. A chemical substance or ensemble of substances composed of chemically identical molecular entities which can explore the same set of molecular energy levels on a characteristic or delineated time scale.
chemical substance Also pure substance or simply substance. A form of matter that has constant chemical composition and characteristic properties and which cannot be separated into simpler components by purely physical methods (i.e. without breaking chemical bonds). It is often called a pure substance to distinguish it from a mixture.
chemical synthesis The artificial execution of one or more chemical reactions in order to obtain one or more products. In modern laboratory contexts, specific chemical syntheses are both reliable and reproducible.
chemistry The scientific discipline that studies chemical substances, compounds, and molecules composed of atoms of various chemical elements, as well as their compositions, structures, properties, behaviors, and the changes they undergo during reactions with other substances.
chirality A property of asymmetry in which a molecule or ion is distinguishable from its mirror image such that it cannot be superposed upon it by any combination of geometric rotations, translations, or some conformational changes. Such a molecule or ion is said to be chiral, and exists in two forms, known as enantiomers, which are stereoisomers of each other; these forms are distinguished as either "right-handed" or "left-handed" by their absolute configuration or some other criterion. Several different types of asymmetry can give rise to chirality, most commonly when molecules possess stereogenic elements such as one or more stereocenters (central chirality), a stereogenic axis (axial chirality), or a stereogenic plane (planar chirality); additionally, the inherent curvature of a molecule can cause it to possess inherent chirality.
chromatography Any of a variety of laboratory methods designed to separate a heterogeneous mixture into its component chemical species by first dissolving the mixture in a gas or liquid solvent called the mobile phase and then passing it through a system such as a chromatography column or a capillary tube upon which a material called the stationary phase is fixed. The different species present in the mixture have different affinities for and retention times upon the stationary phase, causing them to separate from the rest of the species in the moving fluid at characteristic rates. This phenomenon is exploited both to detect or measure the relative proportions of analytes present in a mixture, and to purify specific compounds.
chromometer See colorimeter.
cis–trans isomerism
closed system
cluster
cohesion The tendency of similar particles or surfaces to cling to one another as a result of intermolecular forces. Contrast adhesion.
colligative property Any property of a solution that depends upon the ratio of the number of solute particles to the number of solvent particles in the solution, and not on the nature of the chemical species present. Examples include osmotic pressure, freezing-point depression, and boiling-point elevation.
colloid A mixture in which microscopic insoluble particles are suspended within and evenly dispersed throughout another substance, usually a liquid but sometimes inclusive of aerosols and gels. Thus a colloid contains a dispersed phase and a continuous phase. Many milks are colloids.
color standard A liquid solution of known chemical composition and concentration, and hence of known and standardized color, used as a reference in the optical analysis of samples of unknown strength.
color test The quantitative analysis of a substance by comparing the intensity of the color produced when the substance is exposed to a reagent with a standard color produced similarly in a solution of known strength.
colorimeter Also chromometer. Any instrument used for color measurement based on optical comparison with standard colors, particularly a device used in colorimetry that measures the absorbance of specific wavelengths of light by a given solution in order to determine the concentration of a known solute in the solution, by application of the principle that solute concentration is directly proportional to absorbance.
combustion An exothermic reaction between an oxidant and a fuel that produces large amounts of heat and often light.
Commission on Isotopic Abundances and Atomic Weights (CIAAW)
complex A molecular entity formed by loose association between two or more component molecular entities (ionic or uncharged), or the corresponding chemical species. The bonding between the components is normally weaker than in a covalent bond. See also coordination complex.
compound A substance that is made up of two or more chemically bonded elements.
Compton rule An empirical law of physical chemistry which states that the heat of fusion of a given element multiplied by its atomic weight and then divided by its melting point in kelvin is always equal to approximately 2.
concatemer
concentration The quantity or abundance of a constituent of a mixture per unit quantity of that mixture; e.g. the amount, in moles, of a dissolved solute per unit volume of a solution, a measure known as molar concentration or molarity. Several different definitions of concentration are widely used in chemistry, including molar concentration, mass concentration, and volume concentration.
condensation The phase transition of a substance from a gas to a liquid.
condosity A comparative measurement of the electrical conductivity of a solution defined as the molar concentration of a sodium chloride (NaCl) solution that has the same specific electrical conductance as the solution under test. It is typically expressed in units of moles per litre (or per some other unit of volume).
conduction
conductivity See electrical conductivity and thermal conductivity.
conductor Any object or material that allows the flow of an electric current in one or more directions. Contrast insulator.
conformation The spatial arrangement of atoms affording distinction between stereoisomers which can be interconverted by rotations about formally single bonds.
conjugate acid
conjugate base
conjugated system A molecule that contains double or triple bonds separated by one single bond; e.g. the compound buta-1,3-diene, with the chemical structure H2C=CH−CH=CH2, has conjugated double bonds. In such molecules, there is some delocalization of electrons in the pi orbitals between the carbon atoms linked by the single bond.
constitutional isomer See structural isomer.
constitutional unit An atom or group of atoms (including pendant atoms or groups, if any) comprising part of the structure of a macromolecule, oligomer, polymer, block, or chain.
convection
cooling curve A line graph representing the change between different phases of matter, typically from a gas to a solid or a liquid to a solid, as a function of time and temperature; e.g. showing how the temperature of a liquid substance changes over time as it condenses below its freezing point.
coordinate chemistry
coordinate covalent bond See dipolar bond.
coordination complex A chemical compound consisting of a central atom or ion, usually metallic and known as the coordination center, bonded to a surrounding array of other groups of atoms, e.g. molecules or ions, which are known as ligands or complexing agents. Many metal-containing compounds, especially those of the transition metals, are coordination complexes. See also complex.
corrosion An irreversible interfacial chemical reaction of a material, especially a metal, with its environment, which results in consumption of the material or dissolution into the material of an external component of the environment.
coulomb (C) The SI unit of electric charge, defined as the charge transported by a constant current of one ampere in one second.
counterion The ion that is the counterpart to an oppositely charged ion in a dissociated ionic species; the cation that pairs with a given anion, or vice versa. For example, Na+ is the counterion to Cl−, and vice versa, in solutions of sodium chloride (NaCl).
covalent bond Also molecular bond. A bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces that occurs between atoms when they share electrons is known as covalent bonding.
critical point The end point of a phase equilibrium curve or pressure-temperature curve at which conditions are such that phase boundaries vanish and a substance's different phases, such as liquid and vapor, can coexist. The critical point is defined by the intersection of a critical temperature, Tc, and a critical pressure, pc; above this temperature and pressure, all distinction between phases disappears and the substance becomes a supercritical fluid.
crucible A ceramic or metal dish or other vessel in which substances can be melted or otherwise subjected to very high temperatures.
crystal A solid whose constituent particles (such as atoms, ions, or molecules) are arranged in an orderly periodic microscopic structure, forming a lattice with definite geometry that extends in all directions. Such materials are often described as crystalline.
crystallization
crystallization point See freezing point.
crystallography The branch of chemistry concerned with the study of crystalline solids, including determining their structure and properties.
cuvette A type of small container used in spectroscopy experiments, usually made of plastic, glass, or quartz and designed to hold a sample (typically a liquid) for measurement inside a spectrometer. Cuvettes should be as clean and transparent as possible to minimize interference with the beams of light on which spectroscopic techniques rely.
cyclic
D
dalton (Da) Also unified atomic mass unit (u). A unit of mass defined as 1⁄12 of the mass of a free unexcited atom of carbon-12 at rest. It is approximately equal to the mass of one nucleon.
Dalton's law of partial pressures An empirical law which states that in a mixture of non-reacting gases, the total pressure exerted by all of the gases combined is equal to the sum of the partial pressures exerted by each gas individually.
d-block
dative bond See dipolar bond.
debye (D) A non-SI unit of measurement of electric dipole moment, defined as 10−18 statcoulomb-centimetres. See also electric dipole moment.
deionization The removal of ions from a solution by any method. In the case of water, this typically refers to mineral ions such as sodium, iron, and calcium.
deliquescence A substance's affinity for water, often characterized as its tendency to absorb moisture from the atmosphere to form aqueous solutions. Most strongly deliquescent substances are salts, such as calcium chloride and potassium carbonate.
delocalized electron Any electron in a molecule, ion, or solid metal that is not associated with an individual atom or covalent bond. The term may refer to electrons involved in resonance in conjugated systems or aromatic compounds; to free electrons which facilitate electrical conductivity; or to electrons within delocalized molecular orbitals encompassing several adjacent atoms.
density An intensive property of a substance defined as mass per unit volume and expressed by the equation d = m/V.
denticity The number of donor groups in a single ligand that bind to a central atom in a coordination complex.
deposition The settling of particles within a solution or mixture.
depression of freezing point See freezing-point depression.
desiccant Also drying agent. A hygroscopic substance used to induce or sustain a state of dryness or desiccation (i.e. the absence of moisture) in its vicinity by abstracting water molecules from other substances. Desiccants come in many different forms and work by many different principles, ranging from simple absorption to the chemical bonding of water molecules.
desiccation
deuterium Also hydrogen-2 or heavy hydrogen, and symbolized 2H or D. One of two stable isotopes of a hydrogen atom, the nucleus of which contains one proton and one neutron. Deuterium is both heavier and much less abundant in nature than the other stable isotope, known as protium (1H).
deuteron The nucleus of a deuterium atom (an isotope of hydrogen), containing one proton and one neutron.
Dewar flask See vacuum