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Chemical Industry Definitions and acronyms ab initio A calculation or prediction that is based purely on theory rather than on experimental data. Accurate ab initio predictions are an important test of a theory. (Lat., "from first principles")
abrasive. A very hard, brittle, heat-resistant substance that is used to grind the edges or rough surfaces of an object. boron carbide, diamond, and corundum are abrasives.
absolute error. absolute uncertainty. Compare with relative error. The uncertainty in a measurement, expressed with appropriate units. For example, if three replicate weights for an object are 1.00 g, 1.05 g, and 0.95 g, the absolute error can be expressed as ± 0.05 g. Absolute error is also used to express inaccuracies; for example, if the "true value" is 1.11 g and the measured value is 1.00 g, the absolute error could be written as 1.00 g - 1.11 g = -0.11 g. Note that when absolute errors are associated with indeterminate errors, they are preceded with "±"; when they are associated with determinate errors, they are preceded by their sign.
absolute temperature. Temperature measured on a scale that sets absolute zero as zero. In the SI system, the kelvin scale is used to measure absolute temperature.
absolute zero. (0 K) The temperature at which the volume of an ideal gas becomes zero; a theoretical coldest temperature that can be approached but never reached. Absolute zero is zero on the Kelvin scale, -273.15°C on the Celsius scale, and -459.67°F on the Fahrenheit scale.
absorbance. (A, D, E) optical density; extinction; decadic absorbance. A measure of the amount of light absorbed by a sample. The absorbance (A) equals minus the base-10 log of the transmittance.
absorption. absorb; absorbent. Compare with adsorption and sorption. 1. Penetration of molecules into the bulk of a solid or liquid, forming either a solution or compound. Absorption can be a chemical process (a strong solution of NaOH absorbs CO2 from the air) or a physical process (palladium absorbs hydrogen gas). 2. Capture and transformation of energy by a substance; for example, copper looks reddish because it absorbs blue light. An absorbent captures another material and distributes it throughout; an adsorbent captures another material and distributes it on its surface only.
absorption spectroscopy. Compare with absorption spectrum. A technique for determining the concentration and structure of a substance by measuring the amound of electromagnetic radiation the sample absorbs at various wavelengths.
absorption spectrum. absorption spectra. Compare with absorption spectroscopy. A plot that shows how much radiation a substance absorbs at different wavelengths. Absorption spectra are unique for each element and compound and they are often used as chemical "fingerprints" in analytical chemistry. The spectrum can represented by a plot of either absorbance or transmittance versus wavelength, frequency, or wavenumber.
absorptivity. (a) extinction coefficient; absorption cross section; decadic absorptivity. Compare with molar absorptivity and absorbance. The absorbance of a solution per unit of path length and per unit concentration; a = A/(bc) where a, A, b, and c are the absorptivity, absorbance, path length, and concentration, respectively. Absorptivity varies with wavelength of the incident light.
accelerator. 1. A substance that makes vulcanization of rubber occur more quickly or at a lower temperature. 2. A substance that makes crosslinking in a polymer occur more quickly or at a lower temperature, e. g., accelerators are added to Super Glue to make it set up quickly.
accuracy. Compare with precision and trueness. Accuracy is the correctness of a single measurement. The accuracy of a measurement is assessed by comparing the measurement with the true or accepted value, based on evidence independent of the measurement. The closeness of an average to a true value is referred to as "trueness".
acetate. (CH3COO-, C2H3O2-) acetate ion. 1. an ion formed by removing the acidic hydrogen of acetic acid, HC2H3O2. 2. a compound derived by replacing the acidic hydrogen in acetic acid. 3. A fiber made of cellulose acetate.
acetic acid (CH3COOH, HC2H3O2) ethanoic acid; vinegar acid; methanecarboxylic acid.A simple organic acid that gives vinegar its characteristic odor and flavor. Glacial acetic acid is pure acetic acid.
acid. ([Lat. acidus, sour]) Compare with base. 1. a compound which releases hydrogen ions (H+) in solution (Arrhenius). 2. a compound containing detachable hydrogen ions (Bronsted-Lowry). 3. a compound that can accept a pair of electrons from a base (Lewis)..
acid anhydride. Compare with acid. Nonmetallic oxides or organic compounds that react with water to form acids. For example, SO2, CO2, P2O5, and SO3 are the acid anhydrides of sulfurous, carbonic, phosphoric, and sulfuric acids, respectively. Acetic anhydride (CH3CO)2O) reacts with water to form acetic acid.
acid-base indicator. A weak acid that has acid and base forms with sharply different colors. Changes in pH around the acid's pKa are "indicated" by color changes.
acid dissociation constant. (Ka) acid ionization constant. Compare with base hydrolysis constant. The equilibrium constant for the dissociation of an acid into a hydrogen ion and an anion. For example, the acid dissociation constant for acetic acid is the equilibrium constant for HC2H3O2(aq) H+(aq) + C2H3O2-(aq), which is Ka = [H+][C2H3O2-]/[HC2H3O2].
acid error. Compare with alkaline error. A systematic error that occurs when glass pH electrodes are used in strongly acidic solutions. Glass electrodes give pH readings that are consistently too high in these solutions.
acid halide. acid chloride; acyl halide; acyl chloride. Compounds containing a carbonyl group bound to a halogen atom.
acidic solution. A solution in which the hydrogen ion activity is higher than that of the hydroxide ion, when the solvent is water.
acidulant. A substance added to food or beverages to lower pH and to impart a tart, acid taste. Phosphoric acid is an acidulant added to cola drinks.
actinide. Elements 89-102 are called actinides. Electrons added during the Aufbau construction of actinide atoms go into the 5f subshell. Actinides are unstable and undergo radioactive decay. The most common actinides on Earth are uranium and thorium.
activated charcoal. activated carbon; active carbon. A porous form of carbon that acts as a powerful adsorbent, used to decolorize liquids, recover solvents, and remove toxins from water and air.
activated complex. transition state. An intermediate structure formed in the conversion of reactants to products. The activated complex is the structure at the maximum energy point along the reaction path; the activation energy is the difference between the energies of the activated complex and the reactants.
activation energy. (Ea) The minimum energy required to convert reactants into products; the difference between the energies of the activated complex and the reactants.
active site. A pocket or crevice on an enzyme molecule that fits reactant molecules like a hand in a glove. The active site lowers the activation energy for reaction.
activity. (a) An effective concentration used in thermodynamic calculations in place of the actual concentration to allow equations developed for ideal solutions to be used to treat real solutions.
activity coefficient. () The ratio of activity to concentration; a = c where a, , and c are the activity, activity coefficient, and concentrations, respectively. Activity coefficients are usually obtained from measurements of the emf of electrochemical cells or the colligative properties of solutions.
adiabat. adiabatic line. Compare with adiabatic. A line on an indicator diagram that represents an adiabatic process.
adiabatic. adiabatic process; isentropic process. A process that neither absorbs nor releases energy into the surroundings. For example, a chemical reaction taking place in a closed thermos bottle can be considered adiabatic. Very fast processes can often be considered adiabatic with respect to heat exchange with the surroundings, because heat exchange is not instantaneous.
adiabatic ionization energy. Compare with vertical ionization energy. The lowest energy required to remove an electron from an atom, ion, or molecule in the gas phase. The adiabatic ionization energy is the difference between the ground state energy of the ion formed and the energy of the original atom, molecule, or ion.
addition compound. complex compound. Compare with hydrate. An addition compound contains two or more simpler compounds that can be packed in a definite ratio into a crystal. A dot is used to separate the compounds in the formula. For example, ZnSO4·7 H2O is an addition compound of zinc sulfate and water. This represents a compound, and not a mixture, because there is a definite 1:7 ratio of zinc sulfate to water in the compound. Hydrates are a common type of addition compound.
adhesion. (cohesion) Attraction between different substances on either side of a phase boundary.
adsorb. adsorbed; adsorbing. To collect molecules of a substance on a surface.
adsorbent. Compare with absorbent. A substance that collects molecules of another substance on its surface. For example, gases that make water taste bad are strongly adsorbed on activated charcoal granules in water filters.
adsorption. adsorb; adsorbed. Compare with absorption and sorption. Adsorption is collection of a substance on the surface of a solid or a liquid. For example, gases that make water taste bad are strongly adsorbed on charcoal granules in water filters.
adsorption chromatography. A technique for separating or analyzing mixtures that contain at least one component that is preferentially adsorbed by the stationary phase as it moves over it.
adsorption indicator. A substance that indicates an excess of a reactant in a precipitation reaction. For example, dichlorofluorescein is added to an NaCl solution being titrated with silver nitrate. Before the endpoint, excess chloride ions make the surface of the AgCl precipitate negative, and dichlorofluorescein anions remain in solution. After the endpoint, the excess silver ions make the surface of the AgCl precipitate positive, and the dichlorofluorescein anions are adsorbed onto their surface. Adsorption changes the color of the indicator from yellow-green to pink.
aeration. aerate. Preparation of a saturated solution of air gases by either spraying the solution in air or by bubbling air through it.
aerosol. Compare with colloid. A colloid in which solid particles or liquid droplets are suspended in a gas. Smoke is an example of a solid aerosol; fog is an example of a liquid aerosol.
agar. A gel made from seaweed used to make salt bridges.
-al. A suffix added to the systematic names of organic compounds that contain an aldehyde group -(C=O)-H. For example, the systematic name of acetaldehyde, CH3CHO, is ethanal.
alanine. (A, CH3CH(NH2)COOH) Ala; alpha-aminopropionic acid. A naturally occurring aliphatic amino acid which is required for protein synthesis but is not essential in the diet. Beta-alanine (NH2CH2CH2COOH) also occurs naturally.
alcohol. (ROH) Compare with phenol and hydroxide. An alcohol is an organic compound with a carbon bound to a hydroxyl group. Examples are methanol, CH3OH; ethanol, CH3CH2OH; propanol, CH3CH2CH2OH. Compounds with -OH attached to an aromatic ring are called phenols rather than alcohols.
aldehyde. (RCHO) An aldehyde is an organic compound with a carbon bound to a -(C=O)-H group. Examples are formaldehyde (HCHO), acetaldehyde, CH3CHO, and benzaldehyde, C6H6CHO.
aliphatic. Compare with aromatic. An organic compound that does not contain ring structures.
aliquot. A sample of precisely determined amount taken from a material.
alkali metal. (alkaline earth metal) alkali metal element. The Group 1 elements, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr) react with cold water for form strongly alkaline hydroxide solutions, and are referred to as "alkali metals". Hydrogen is not considered an alkali metal, despite its position on some periodic tables.
alkaline. Having a pH greater than 7.
alkaline earth. An oxide of an alkaline earth metal, which produces an alkaline solution in reaction with water.
alkaline earth metal. (alkali metal) The Group 2 elements, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) form alkaline oxides and hydroxides and are called "alkaline earth metals".
alkaline error. Compare with acid error. A systematic error that occurs when glass electrodes are used to read the pH of an extremely alkaline solution; the electrode responds to sodium ions as though they were hydrogen ions, giving a pH reading that is consistently too low.
alkalinity. A measure of a material's ability to neutralize acids. Alkalinities are usually determined using titration.
alkaloid. A class of bitter-tasting, basic organic compounds with nitrogen-containing rings. Alkaloids often have powerful effects on living things. Examples are cocaine, nicotine, strychnine, caffeine, and morphine.
alkane. paraffin. Compare with hydrocarbon and alkene. A series of organic compounds with general formula CnH2n+2. Alkane names end with -ane. Examples are propane (with n=3) and octane (with n=8).
alkene. A compound that consists of only carbon and hydrogen, that contains at least one carbon-carbon double bond. Alkene names end with -ene. Examples are ethylene (CH2=CH2); 1-propene (CH2=CH2CH3), and 2-octane (CH3CH2=CH2(CH2)4CH3).
alkoxide. (RO- M+) alkoxide ion. An ionic compound formed by removal of hydrogen ions from the hydroxyl group in an alcohol using reactive metals, e. g. sodium. For example, potassium metal reacts with methanol (CH3OH) to produce potassium methoxide (KOCH3).
alkyl. (-CnH2n+1) alkyl group. A molecular fragment derived from an alkane by dropping a hydrogen atom from the formula. Examples are methyl (CH3) and ethyl (CH2CH3).
alkyl halide. An alkyl group attached to a halogen atom.
alkyne. A compound that consists of only carbon and hydrogen, that contains at least one carbon-carbon triple bond. Alkyne names end with -yne. Examples are acetylene (CHCH); 1-propyne (CH2CH2CH3), and 2-octyne (CH3CH2CH2(CH2)4CH3).
allo-. A prefix that designates the more stable of a pair of geometric isomers. allo- is sometimes used less precisely to designate isomers or close relatives of a compound.
allobar. A form of an element that has isotopic abundances that are different from the naturally occuring form. For example, "depleted" uranium has had most of the uranium-235 removed, and is an allobar of natural uranium.
allomer. allomerism. Substances with different chemical composition but the same crystalline form.
allosteric effect. allosteric interaction. A change in the behavior of one part of a molecule caused by a change in another part of the molecule.
allotrope. allotropy; allotropic; allotropism. Compare with isotope and polymorph. Some elements occur in several distinct forms called allotropes. Allotropes have different chemical and physical properties. For example, graphite and diamond are allotropes of carbon.
alloy. alloying; alloyed. Compare with amalgam. A mixture containing mostly metals. For example, brass is an alloy of copper and zinc. Steel contains iron and other metals, but also carbon.
allyl. allylic; allyl group; allyl radical. A molecular fragment derived by removing a methyl hydrogen from propene (-CH2-CH2=CH2). For example, "allyl chloride" is 3-chloropropene, Cl-CH2-CH2=CH2.
alpha particle. (42He) A particle that is commonly ejected from radioactive nuclei, consisting of two protons and two neutrons. Alpha particles are helium nuclei. Alpha particles have a mass of 6.644 655 98 × 10-27 kg or 4.001 506 1747 atomic mass units. [1998 CODATA values]
alpha ray. (-ray) alpha radiation. A stream of alpha particles. Alpha rays rapidly dissipate their energy as they pass through materials, and are far less penetrating than beta particles and gamma rays.
amalgam. Compare with alloy. An alloy that contains mercury.
American Chemical Society ACS. A large and influential professional society for professionals and students in chemistry and related fields.
amide. An amide is an organic compound that contains a carbonyl group bound to nitrogen: . The simplest amides are formamide (HCONH2) and acetamide (CH3CONH2).
amine. Compare with ammine. An amine is an organic compound that contains a nitrogen atom bound only to carbon and possibly hydrogen atoms. Examples are methylamine, CH3NH2; dimethylamine, CH3NHCH3; and trimethylamine, (CH3)3N.
amino acid. Amino acids are molecules that contain at least one amine group (-NH2) and at least one carboxylic acid group (-COOH). When these groups are both attached to the same carbon, the acid is an -amino acid. -amino acids are the basic building blocks of proteins.
ammine. Compare with amine. A metal ion complex containing ammonia as a ligand. The ammonia nitrogen is bound directly to a metal ion in ammines; amines differ in that the ammonia nitrogen is directly bound to a carbon atom.
ammonia. (NH3) Compare with ammonium. Pure NH3 is a colorless gas with a sharp, characteristic odor. It is easily liquified by pressure, and is very soluble in water. Ammonia acts as a weak base. Aqueous solutions of ammonia are (incorrectly) referred to as "ammonium hydroxide".
ammonium ion. (NH4+) ammonium. NH4+ is a cation formed by neutralization of ammonia, which acts as a weak base.
amorphous. amorphous solid. Compare with crystal. A solid that does not have a repeating, regular three-dimensional arrangement of atoms, molecules, or ions.
amperage. The amount of charge moved per second by an electric current, measured in amperes.
ampere. (A) amp. The SI unit of electric current, equal to flow of 1 coulomb of charge per second. An ampere is the amount of current necessary to produce a force of 0.2 micronewtons per meter between two arbitrarily long, arbitrarily thin wires, placed parallel in a vacuum and exactly 1 m apart. Named for 19th century physicist André Marie Ampčre.
amperometry. amperometric. Determining the concentration of a material in a sample by measuring electric current.
amphi-. A prefix used to name certain members of a series of geometric isomers or stereoisomers.
amphiprotic solvent. Compare with aprotic solvent. Solvents that exhibit both acidic and basic properties; amphiprotic solvents undergo autoprotolysis. Examples are water, ammonia, and ethanol.
amphoteric. ampholyte. A substance that can act as either an acid or a base in a reaction. For example, aluminum hydroxide can neutralize mineral acids ( Al(OH)3 + 3 HCl = AlCl3 + 3 H2O ) or strong bases ( Al(OH)3 + 3 NaOH = Na3AlO3 + 3 H2O).
amplitude. The displacement of a wave from zero. The maximum amplitude for a wave is the height of a peak or the depth of a trough, relative to the zero displacement line.
amylopectin. Compare with amylose. A form of starch made of glucose molecules linked in a branching pattern.
amylose. Compare with amylopectin. A form of starch made of long, unbranched chains of -D-glucose molecules.
aprotic solvent. Compare with amphiprotic solvent. A solvent that does not act as an acid or as a base; aprotic solvents don't undergo autoprotolysis. Examples are pentane, pet ether, and toluene.
analysis. chemical analysis. Determination of the composition of a sample.
analyte. An analyte is the sample constituent whose concentration is sought in a chemical analysis.
Angstrom. (Ĺ) Ĺngstrom; Ĺngstrom units. A non-SI unit of length used to express wavelengths of light, bond lengths, and molecular sizes. 1 Ĺ = 10-10 m = 10-8 cm.
angular momentum quantum number. () azimuthal quantum number; orbital angular momentum quantum number. A quantum number that labels the subshells of an atom. Sometimes called the orbital angular momentum quantum number, this quantum number dictates orbital shape. can take on values from 0 to n-1 within a shell with principal quantum number n.
anhydrous. anhydrous compound; anhydride. Compare with hydrate. A compound with all water removed, especially water of hydration. For example, strongly heating copper(II) sulfate pentahydrate (CuSO4·5H2O) produces anhydrous copper(II) sulfate (CuSO4).
anion. Compare with cation. An anion is a negatively charged ion. Nonmetals typically form anions.
anode. Compare with cathode. The electrode at which oxidation occurs in a cell. Anions migrate to the anode.
anodize. To coat a metal with a protective film by electrolysis.
anthocyanin. anthocyan. A family of pigments that give flowers, fruits, and leaves of some plants their red or blue coloring. Anthocyanins consist of sugar molecules bound to a benzopyrylium salt (called anthocyanidin). See Water to Wine for more about anthocyanins.
antibonding orbital. antibonding; antibonding molecular orbital. A molecular orbital that can be described as the result of destructive interference of atomic orbitals on bonded atoms. Antibonding orbitals have energies higher than the energies its constituent atomic orbitals would have if the atoms were separate.
antichlor. A chemical compound that reacts with chlorine-based bleaches to stop the bleaching. Thiosulfate compounds are antichlors.
antioxidant. Antioxidants are compounds that slow oxidation processes that degrade foods, fuels, rubber, plastic, and other materials. Antioxidants like butylated hydroxyanisole (BHA) are added to food to prevent fats from becoming rancid and to minimize decomposition of vitamins and essential fatty acids; they work by scavenging destructive free radicals from the food.
antiozonant. antiozidant. Substances that reverse or prevent severe oxidation by ozone. Antiozonants are added to rubber to prevent them from becoming brittle as atmospheric ozone reacts with them over time. Aromatic amines are often used as antiozonants.
antipyretic. A substance that can lessen or prevent fever.
Antoine equation Antoine's equation. A simple 3-parameter fit to experimental vapor pressures measured over a restricted temperature range: log P = A - B T + C
where A, B, and C are "Antoine coefficients" that vary from substance to substance. Sublimations and vaporizations of the same substance have separate sets of Antoine coefficients, as do components in mixtures. The Antoine equation is accurate to a few percent for most volatile substances (with vapor pressures over 10 Torr).
aqua regia. A mixture of nitric and hydrochloric acids, usually 1:3 or 1:4 parts HNO3 to HCl, used to dissolve gold.
aqueous. (aq) aqueous solution. A substance dissolved in water.
arene. A hydrocarbon that contains at least one aromatic ring.
arginine. (R, C6H14N4O2) Arg. An essential amino acid and building block of proteins. Arginine acts as a base under physiological conditions; the double-bonded nitrogen on the end of the side chain readily captures a hydrogen ion, becoming positively charged. This charged side group makes arginine hydrophilic.
aromatic ring. (Ar) An exceptionally stable planar ring of atoms with resonance structures that consist of alternating double and single bonds, e. g. benzene:
aromatic compound. A compound containing an aromatic ring. Aromatic compounds have strong, characteristic odors.
Arrhenius equation. In 1889, Svante Arrhenius explained the variation of rate constants with temperature for several elementary reactions using the relationship k = A exp(-Ea/RT)
where the rate constant k is the total frequency of collisions between reaction molecules A times the fraction of collisions exp(-Ea/RT) that have an energy that exceeds a threshold activation energy Ea at a temperature of T (in kelvins). R is the universal gas constant.
aryl. (Ar) aryl group. A molecular fragment or group attached to a molecule by an atom that is on an aromatic ring.
asparagine. Asn. A natural amino acid that is the amide of aspartic acid.
aspartic acid. (D,HOOCCH2CH(NH2)COOH) Asp. A nonessential amino acid that is abundant in molasses. The carboxylic acid group on the side chain is ionized under physiological conditions, making aspartic acid residues in proteins hydrophilic.
atmosphere. (atm) A unit of pressure, equal to a barometer reading of 760 mm Hg. 1 atmosphere is 101325 pascals and 1.01325 bar.
atomic mass unit. (amu,u) amu; dalton. A unit of mass equal to 1/12 the mass of a carbon-12 nucleus, which is 1.660 538 73 × 10-27 kg ± 0.000 000 13 × 10-27 kg [1998 CODATA values]. Abbreviated as amu or u. Sometimes called the dalton, after John Dalton, architect of the first modern atomic theory.
atomic nucleus. nucleus; nuclei; atomic nuclei. A tiny, incredibly dense positively charged mass at the heart of the atom. The nucleus is composed of protons and neutrons (and other particles). It contains almost all of the mass of the atom but occupies only a tiny fraction of the atom's volume.
atomic number. (Z) The number of protons in an atomic nucleus. The atomic number and the element symbol are two alternate ways to label an element. In nuclide symbols, the atomic number is a leading subscript; for example, in 126C, the "6" is the atomic number.
atomic orbital. A wavefunction that describes the behavior of an electron in an atom.
atomic radius. metallic radius; covalent radius; atomic radii. Compare with ionic radius. One half the distance between nuclei of atoms of the same element, when the atoms are bound by a single covalent bond or are in a metallic crystal. The radius of atoms obtained from covalent bond lengths is called the covalent radius; the radius from interatomic distances in metallic crystals is called the metallic radius.
atomic theory. An explanation of chemical properties and processes that assumes that tiny particles called atoms are the ultimate building blocks of matter.
atomic unit. Compare with Bohr radius and hartree. A system of non-SI units used in quantum chemistry to simplify calculations and mathematical expressions. The definitions of atomic units include physical constants (like the speed of light, the rest mass of the electron, and other quantities that never change), so that all constants drop out of expressions when atomic units are used.
atomic weight. atomic mass. The average mass of an atom of an element, usually expressed in atomic mass units. The terms mass and weight are used interchangeably in this case. The atomic weight given on the periodic table is a weighted average of isotopic masses found in a typical terrestrial sample of the element.
atom. Compare with molecule and ion. An atom is the smallest particle of an element that retains the chemical properties of the element. Atoms are electrically neutral, with a positively charged nucleus that binds one or more electrons in motion around it.
atto-. (a) Prefix used in the SI system meaning "multiply by 10-18". For example, 3 am means 3× 10-18 meters.
aufbau principle. aufbau construction; building-up principle. An approximate procedure for writing the ground state electronic configuration of atoms. The configuration of an atom is obtained by inserting one electron into the configuration of the atom immediately to its left on the periodic table. The electron is inserted into the subshell indicated by the element's period and block.
auto-ignition temperature. Compare with flash point. Minimum temperature at which the vapor/air mixture over a liquid spontaneously catches fire.
autoxidation. autooxidation; autoxidize; autoxidizing. Oxidation caused by exposure to air. Rust is an example of autoxidation. Autoxidation makes ether taken from half-filled bottles very dangerous, because air oxidizes ether to highly explosive organic peroxides.
autoprotolysis. autoionization; autoionization constant; autoprotolysis constant. Transfer of a hydrogen ion between molecules of the same substance, e. g. the autoprotolysis of methanol (2 CH3OH = CH3OH2+ + CH3O-). Autoprotolysis of water into hydronium ions and hydroxide ions results in equilibrium concentrations that satisfy Kw = [H3O+][OH-], where the autoprotolysis constant Kw is equal to 1.01 × 10-14 at 25°C.
auxochrome. Compare with chromophore. A group or substructure in a molecule that influences the intensity of absorption of the molecule.
average bond enthalpy. Compare with bond enthalpy. Average enthalpy change per mole when the same type of bond is broken in the gas phase for many similar substances
Avogadro. Amadeo Avogadro. Italian chemist and physicist Amadeo Avogadro (1776-1856) proposed a correct molecular explanation for Gay-Lussac's law of combining volumes. His work provided a simple way to determine atomic weights and molecular weights of gases.
Avogadro number. (NA, L) Avogadro's number; Avogadro constant. The number of particles in one mole, equal to 6.02214199 × 1023 mol-1 (± 0.00000047 mol-1) [1998 CODATA values]
Avogadro's law. Equal volumes of an ideal gas contain equal numbers of molecules, if both volumes are at the same temperature and pressure. For example, 1 L of ideal gas contains twice as many molecules as 0.5 L of ideal gas at the same temperature and pressure.
axial. 1. An atom, bond, or lone pair that is perpendicular to equatorial atoms, bonds, and lone pairs in a trigonal bipyramidal molecular geometry.
azeotrope. azeotropic mixture; azeotropy. A solution that does not change composition when distilled. For example, if a 95% (w/w) ethanol solution in water is boilled, the vapor produced also is 95% ethanol- and it is not possible to obtain higher percentages of ethanol by distillation.
azo. azo compound; azo group; azo dye. The azo group has the general structure Ar-N=N-Ar', where Ar and Ar' indicate substituted aromatic rings. Compounds containing the azo compounds are often intensely colored and are economically important as dyes. Methyl orange is an example of an azo dye.
back titration. indirect titration. Determining the concentration of an analyte by reacting it with a known number of moles of excess reagent. The excess reagent is then titrated with a second reagent. The concentration of the analyte in the original solution is then related to the amount of reagent consumed.
balanced equation. balanced. A description of a chemical reaction that gives the chemical formulas of the reactants and the products of the reaction, with coefficients introduced so that the number of each type of atom and the total charge is unchanged by the reaction. For example, a balanced equation for the reaction of sodium metal (Na(s)) with chlorine gas (Cl2(g)) to form table salt (NaCl(s)) would be 2 Na(s) + Cl2(g) = 2 NaCl(s), NOT Na(s) + Cl2(g) = NaCl(s).
Balmer series. Balmer lines. A series of lines in the emission spectrum of hydrogen that involve transitions to the n=2 state from states with n>2.
band. 1. A set of closely spaced energy levels in an atom, molecule, or metal. 2. A set of closely spaced lines in an absorption spectrum or emission spectrum. 3. A range of frequencies or wavelengths.
band spectrum. band spectra. Compare with line spectrum and continuous spectrum. An emission spectrum that contains groups of sharp peaks that are so close together that they are not distinguishable separately, but only as a "band".
bar. Unit of pressure. 1 bar = 105 pascals = 1.01325 atmospheres.
barometer. Compare with manometer. An instrument that measures atmospheric pressure. A mercury barometer is a closed tube filled with mercury inverted in a mercury reservoir. The height of the mercury column indicates atmospheric pressure (with 1 atm = 760 mm of mercury). An aneroid barometer consists of an evacuated container with a flexible wall. When atmospheric pressure changes, the wall flexes and moves a pointer which indicates the changing pressure on a scale.
base hydrolysis constant. (Kb) base ionization constant; basic hydrolysis constant. Compare with acid dissociation constant. The equilibrium constant for the hydrolysis reaction associated with a base. For example, Kb for ammonia is the equilibrium constant for NH3(aq) + H2O() NH4+(aq) + OH-(aq), or Kb = [NH4+][OH-]/[NH3].
base unit. Base units are units that are fundamental building blocks in a system of measurement. There are seven base units in the SI system.
base. alkali; alkaline; basic. Compare with acid. 1. a compound that reacts with an acid to form a salt. 2. a compound that produces hydroxide ions in aqueous solution (Arrhenius). 3. a molecule or ion that captures hydrogen ions.(Bronsted-Lowry). 4. a molecule or ion that donates an electron pair to form a chemical bond.(Lewis).
basis function. A mathematical function that can be used to build a description of wavefunctions for electrons in atoms or molecules.
basis set. A set of mathematical functions that are combined to approximate the wavefunctions for electrons in atoms and molecules.
battery acid. A solution of approximately 6M sulfuric acid used in the lead storage battery.
Baumé. (, be°Bé, °B) Baumé scale; degrees Baumé; Baume; Baumé scale. A, Be scale related to specific gravities, devised by the French chemist Antoine Baumé for marking hydrometers. At 60°F, specific gravity can be calculated from degrees Baumé by the following formulas: liquids lighter than water: sp. gr. = 140/(°Bé + 130) liquids heavier than water: sp. gr. = 145/(145 - °Bé)
Beer's law. (A=abc or A=bc) Beer-Lambert law. In absorption spectroscopy, the absorbance of a dilute solution is equal to its absorptivity times the path length times the concentration of the absorbing solute.
beryllium. (Be) Be; glucinium. Element 4, atomic weight 9.0122, an extremely toxic metal used as a neutron source and in phosphors.
beta particle. (ß-) An electron emitted by an unstable nucleus, when a neutron decays into a proton and an electron. In some cases, beta radiation consists of positrons ("antielectrons" which are identical to electrons but carry a +1 charge.") Note that beta particles are created in nuclear decay; they do not exist as independent particles within the nucleus.
bidentate. A ligand that has two "teeth" or atoms that coordinate directly to the central atom in a complex. For example, 1,10-phenanthroline is a bidentate ligand of iron.
binary compound. Compare with compound. A compound that contains two different elements. NaCl is a binary compound; NaClO is not.
biochemistry. The chemistry of living things, including the structure and function of biological molecules and the mechanism and products of their reactions.
bleach. A dilute solution of sodium hypochlorite or calcium hypochlorite which kills bacteria and destroys colored organic materials by oxidizing them.
block. A region of the periodic table that corresponds to the type of subshell (s, p, d, or f) being filled during the Aufbau construction of electron configurations.
Bohr atom. Bohr's theory; Bohr's atomic theory; Bohr model. A model of the atom that explains emission and absorption of radiation as transitions between stationary electronic states in which the electron orbits the nucleus at a definite distance. The Bohr model violates the Heisenberg uncertainty principle, since it postulates definite paths and momenta for electrons as they move around the nucleus. Modern theories usually use atomic orbitals to describe the behavior of electrons in atoms.
Bohr radius. (a0) bohr. Compare with atomic unit. The atomic unit of length, equal to 0.529 177 2083 × 10-10 m, with an uncertainty of 0.000 000 0019 × 10-10 m [1998 CODATA values]
boiling point. (bp) standard boiling point; normal boiling point. The temperature at which the vapor pressure of a liquid is equal to the external pressure on the liquid. The standard boiling point is the temperature at which the vapor pressure of a liquid equals standard pressure.
boiling point elevation. The boiling point of a solution is higher than the boiling point of the pure solvent. Boiling point elevation is a colligative property.
Boltzmann constant. (k) Boltzmann's constant. A fundamental constant equal to the ideal gas law constant divided by Avogadro's number, equal to 1.3805 × 10-23 J K-1.
Boltzmann equation. A statistical definition of entropy, given by S = k ln W, where S and k are the entropy and Boltzmann's constant, respectively, and W is the probability of finding the system in a particular state.
bond energy. Compare with bond enthalpy. Energy change per mole when a bond is broken in the gas phase for a particular substance.
bond enthalpy. Compare with average bond enthalpy. Enthalpy change per mole when a bond is broken in the gas phase for a particular substance.
bond length. The average distance between the nuclei of two bonded atoms in a stable molecule.
bond order. 1. In Lewis structures, the number of electron pairs shared by two atoms. 2. In molecular orbital theory, the net number of electron pairs in bonding orbitals (calculated as half the difference between the number of electrons in bonding orbitals and the number of electrons in antibonding orbitals.
bond strength. Some measure of how difficult it is to break a chemical bond, for example, a bond energy or a bond enthalpy.
boron. (B) B. Element 5, atomic weight 10.811. Hard yellow crystals or brown amorphous powder, used as a neutron absorber in nuclear chemistry and as a hardener in alloys.
Boyle's law. The pressure of a ideal gas is inversely proportional to its volume, if the temperature and amount of gas is held constant. Doubling gas pressure halves gas volume, if temperature and amount of gas don't change. If the initial pressure and volume are P1 and V1 and the final pressure and volume are P2V2, then P1V1 = P2V2 at fixed temperature and gas amount.
brass. A shiny yellow to yellow-orange alloy that contains about two parts copper for every one part zinc.
bronze. A yellow to yellow-brown alloy that contains mostly copper and tin, with small amounts of other metals.
Brösted acid. Compare with acid. A material that gives up hydrogen ions in a chemical reaction.
Brösted base. Compare with base. A material that accepts hydrogen ions in a chemical reaction.
Brownian motion. Brownian movement. Small particles suspended in liquid move spontaneously in a random fashion. The motion is caused by unbalanced impacts of molecules on the particle. Brownian motion provided strong circumstantial evidence for the existence of molecules.
buckminsterfullerene. (C60) C60; fullerene; buckyball. A form of carbon consisting of 60 carbon atoms bound together to make a roughly spherical "buckyball" (which looks rather like a soccer ball).
buffer. pH buffer; buffer solution. A solution that can maintain its pH value with little change when acids or bases are added to it. Buffer solutions are usually prepared as mixtures of a weak acid with its own salt or mixtures of salts of weak acids. For example, a 50:50 mixture of 1 M acetic acid and 1 M sodium acetate buffers pH around 4.7.
Bunsen burner. A gas burner with adjustable air intake, commonly used in laboratories.
buret. burette. A cylindrical glass tube closed by a stopcock on one end and open on the other, with volume gradations marked on the barrel of the tube, used to precisely dispense a measured amount of a liquid.
butanol. An alcohol containing four carbon atoms. Example: 1-butanol.
caffeine. (C8H10N4O2) methyltheobromine; guaranine; 1,3,7-trimethylxanthine; 1,3,7-trimethyl-2,6-dioxopurine. A substance found in tea, coffee, and cola that acts as a stimulant. It is extremely soluble in supercritical fluid carbon dioxide and somewhat soluble in water; aqueous solutions of caffeine quickly break down.
calibration. Calibration is correcting a measuring instrument by measuring values whose true values are known. Calibration minimizes systematic error.
calorie. The amount of heat required to raise the temperature of 1 g of water at 14.5°C to 15.5°C. One calorie is equivalent to exactly 4.184 J.
calorimeter. An insulated vessel for measuring the amount of heat absorbed or released by a chemical or physical change.
calorimetry. Experimental determination of heat absorbed or released by a chemical or physical change.
calutron. A device that separates isotopes (e. g. 235U from 238U) by ionizing the sample, accellerating the ions in a strong electric field, and then passing them through a strong magnetic field. The magnetic field bends the trajectories of the ions with high charge-to-mass ratio more, allowing ions to be separated by mass and collected.
capacitor. (A device for storing electric charge, consisting of two metal plates separated by an insulating material. )
carbohydrate. carb. A class of organic compounds including sugars and starches. The name comes from the fact that many (but not all) carbohydrates have empirical formula CH2O.
carbon. C. An element with atomic number 6. Carbon is a nonmetal found in all organic compounds. Carbon occurs naturally as diamond, graphite, and buckminsterfullerene.
carbon dioxide. (CO_2) A colorless, odorless gas produced by respiration and combustion of carbon-containing fuels.
carbon monoxide. (CO) A colorless, odorless, poisonous gas produced by incomplete combustion.
carbonate. (CO32-) 1. an inorganic ion with a charge of -2, containing carbon bound directly to three oxygens in a in a flat triangular arrangement. 2. A compound containing CO32- ions.
carbonate hardness. carbonate water hardness. Compare with water hardness. Water hardness due to the presence of calcium and magnesium carbonates and bicarbonates. The "noncarbonate hardness" is due mostly to calcium and magnesium sulfates, chlorides, and nitrates.
carbonyl. carbonyl group. A divalent group consisting of a carbon atom with a double-bond to oxygen. For example, acetone (CH3-(C=O)-CH3) is a carbonyl group linking two methyl groups. Also refers to a compound of a metal with carbon monoxide, such as iron carbonyl, Fe(CO)5.
carboxylic acid. carboxyl; carboxyl group. A carboxylic acid is an organic molecule with a -(C=O)-OH group. The group is also written as -COOH and is called a carboxyl group. The hydrogen on the -COOH group ionizes in water; carboxylic acids are weak acids. The simplest carboxylic acids are formic acid (H-COOH) and acetic acid (CH3-COOH).
carboy. A very large bottle. Glass carboys are usually encased in a wire mesh or wooden box for protection.
carotene. Carotene is an unsaturated hydrocarbon pigment found in many plants. Carotene is the basic building block of vitamin A.
catalyst. catalyze; catalysis. A substance that increases the rate of a chemical reaction, without being consumed or produced by the reaction. Catalysts speed both the forward and reverse reactions, without changing the position of equilibrium. Enzymes are catalysts for many biochemical reactions.
cathode ray. A negatively charged beam that emanates from the cathode of a discharge tube. Cathode rays are streams of electrons.
cathode. Compare with anode. The electrode at which reduction occurs.
cation. Compare with anion. A cation is a positively charged ion. Metals typically form cations.
cellulose. A polysaccharide made of linked glucose molecules that strengthens the cell walls of most plants. See also: What is cellulose?
Celsius. (°C) Celsius temperature scale; Celsius scale. A common but non-SI unit of temperature, defined by assigning temperatures of 0°C and 100°C to the freezing and boiling points of water, respectively.
centi-. (c) Prefix used in the SI system meaning "one hundredth of". For example 1 cm means "one hundredth of a meter"; 2.3 cg could also be written "2.3 × 10-2 g" or "0.023 g".
cgs. Compare with SI. An older metric system of units that uses centimeters, grams, and seconds as base units.
Charles' law. The volume of a gas is directly proportional to its temperature in kelvins, if pressure and amount of gas remain constant. Doubling the kelvin temperature of a gas at constant pressure will double its volume. If V1 and T1 are the initial volume and temperature, the final volume and temperature ratio V2/T2 = V1/T1 if pressure and moles of gas are unchanged.
A stable complex of a metal with one or more polydentate ligands. For example, calcium complexes with EDTA to form a chelate.
chelating agent. chelator. A ligand that binds to a metal using more than one atom; a polydentate ligand.
chemical. 1 of or pertaining to chemistry. 2. a substance.
chemical bond. bond; bonding; chemical bonding. A chemical bond is a strong attraction between two or more atoms. Bonds hold atoms in molecules and crystals together. There are many types of chemical bonds, but all involve electrons which are either shared or transferred between the bonded atoms.
chemical change. reaction; chemical reaction. Compare with physical change. A chemical change is a dissociation, recombination, or rearrangement of atoms.
chemical equation. A compact notation for describing a chemical change. The formulas of the reactants are added together on the left hand side of the equation; the formulas of the products are added together on the right side. Coefficients are inserted before the formulas to ensure that the equation is balanced. The phase in which each substance is found is usually indicated in parentheses after each formula. For example, 2 H2(g) + O2(g) = 2 H2O(g) indicates that 2 moles of hydrogen gas combine with one mole of oxygen gas to produce two moles of steam.
chemical potential. (µ) The chemical potential is a partial molar Gibbs free energy, defined as µi = (G/ni)T,P,nj. The definition means that the chemical potential is the change in Gibbs free energy when one mole of a substance is added to a very large amount of a sample. Chemical potential is a measure of chemical stability that can be used to predict and interpret phase changes and chemical reactions. Substances with higher chemical potential will react or move from one phase to another to lower the overall Gibbs free energy of the system. For example, consider a mixture of ice and water. If the ice melts, the chemical potential of the water was lower than that of the ice. If the water freezes, the chemical potential of the ice was lower.
chemical property. chemical properties. Compare with physical property. Measurement of a chemical property involves a chemical change. For example, determining the flammability of gasoline involves burning it, producing carbon dioxide and water.
chemiluminescence. A chemical reaction that releases energy as electromagnetic radiation.
chemistry. The study of matter and its transformations. See What is chemistry? for other definitions.
chiral. chirality. Having nonsuperimposable mirror images. For example, a shoe or a glove is chiral.
chiral center. asymmetric center. An atom in a molecule that causes chirality, usually an atom that is bound to four different groups. A molecule can have chirality without having a chiral center, and a molecule may also have more than one chiral centers.
chromatography. Chromatography is a method for separating mixtures based on differences in the speed at which they migrate over or through a stationary phase.
chromophore. Compare with auxochrome. A group or substructure on a molecule that is responsible for the absorption of light.
Clausius-Clapeyron equation. The Clausius-Clapeyron equation predicts the temperature dependence of vapor pressures of pure liquids or solids: ln (P/P°) = H -------------------------------------------------------------------------------- R ( 1 T° - 1 T )
where P is the vapor pressure, P° is a vapor pressure at a known temperature T°, H is an enthalpy of vaporization if the substance is a liquid or an enthalpy of sublimation if it's a solid, R is the ideal gas law constant, and T is the temperature (in kelvins).
cohesion. Compare with adhesion. Attraction between like molecules.
colligative property. colligative; colligative properties. Properties of a solution that depend on the number of solute molecules present, but not on the nature of the solute. Osmotic pressure, vapor pressure, freezing point depression, and boiling point elevation are examples of colligative properties.
collision frequency. collision frequencies; frequency of collision. The average number of collisions that a molecule undergoes each second.
collision theory. collision model. A theory that explains reaction rates in terms of collisions between reactant molecules.
colloid. A colloid is a heterogeneous mixture composed of tiny particles suspended in another material. The particles are larger than molecules but less than 1 µm in diameter. Particles this small do not settle out and pass right through filter paper. Milk is an example of a colloid. The particles can be solid, tiny droplets of liquid, or tiny bubbles of gas; the suspending medium can be a solid, liquid, or gas (although gas-gas colloids aren't possible).
colorimetry. A method for chemical analysis that relates color intensity to the concentration of analyte.
column chromatography. Column chromatography is a method for separating mixtures. A solution containing the mixture is passed through a narrow tube packed with a stationary phase. Different substances in the mixture have different affinities for the stationary phase, and so move through the tube at different rates. This allows the substances in the mixture to be detected or collected separately as they reach the end of the tube.
combination reaction. A reaction in which two or more substances are chemically bonded together to produce a product. For example, 2 Na(s) + Cl2(g) 2 NaCl(s) is a combination reaction.
combustion. combustion reaction. A chemical reaction between a fuel and an oxidizing agent that produces heat (and usually, light). For example, the combustion of methane is represented as CH4(g) + 2 O2(g) = CO2(g) + 2 H2O().
complete combustion. Compare with incomplete combustion. A combustion reaction that converts all of the fuel's carbon, hydrogen, sulfur, and nitrogen into carbon dioxide, water, sulfur dioxide, and N2 respectively.
complete ionic equation. total ionic equation. Compare with net ionic equation. A balanced equation that describes a reaction occurring in solution, in which all strong electrolytes are written as dissociated ions.
complexing agent. complexant. A ligand that binds to a metal ion to form a complex.
complexometric titration. chelometric titration. A titration based on a reaction between a ligand and a metal ion to form a complex. For example, free Ca2+ in milk powder can be determined by titrating a milk powder sample with EDTA solution, which chelates calcium ion. Endpoints in complexometric titrations are often determined using organochromic indicators.
complex ion. An ion formed by combination of simpler ions or molecules; for example, Co2+ combines with six molecules of water to form the complex ion Co(H2O)62+.
component. 1. A substance whose concentration must be specified to describe the state of a mixture in which reactions are occurring. 2. A substance present in a mixture in which no reactions occur.
compound Compare with element and mixture. A compound is a material formed from elements chemically combined in definite proportions by mass. For example, water is formed from chemically bound hydrogen and oxygen. Any pure water sample contains 2 g of hydrogen for every 16 g of oxygen.
computer-assisted drug design. Using computational chemistry to discover, enhance, or study drugs and related biologically active molecules.
computational chemistry. A branch of chemistry concerned with the prediction or simulation of chemical properties, structures, or processes using numerical techniques.
concentrate. Compare with dilute. To increase the amount of substance present in a unit amount of mixture. For example, allowing solvent to evaporate from a solution concentrates the solution.
concentrated. Compare with diluted. Having a relatively large amount of substance present in a unit amount of mixture. For example, a 12 M HCl solution is more concentrated than an 0.001 M HCl solution.
concentration. Compare with dilution. 1. A measure of the amount of substance present in a unit amount of mixture. The amounts can be expressed as moles, masses, or volumes. 2. The process of increasing the amount of substance in a given amount of mixture.
condensation. condensed. 1. The conversion of a gas into a liquid is called condensation. Condensation usually occurs when a gas is cooled below its boiling point. 2. A reaction that involves linking of two molecules with the elimination of water (or another small molecule).
conformers. conformation. Molecular arrangements that differ only by rotations around single bonds. For example, the "boat" and "chair" forms of cyclohexane are conformers.
congener. 1. Elements belonging to the same group on the periodic table. For example, sodium and potassium are congeners. 2. Compounds produced by identical synthesis reactions and procedures.
constructive interference. Compare with destructive interference. When the peaks and troughs of two interfering waves match, the amplitudes add to give the resultant wave a higher amplitude.
continuous spectrum. Compare with line spectrum and band spectrum. A plot of the relative absorbance or intensity of emitted light vs. wavelength or frequency that shows a smooth variation, rather than a series of sharp peaks or bands.
conversion factor. A conversion factor is a fraction that relates one unit to another. Multiplying a measurement by a conversion factor changes the units of the measurement. For example, since 1 in = 2.54 cm, to convert 10 inches to centimeters, (10 in) 2.54 cm 1 in = 25.4 cm
coordination number. The number of bonds formed by the central atom in a metal-ligand complex.
copolymer. A polymer composed of two or more different monomers. The different monomers can be linked randomly, or in repeating sequences, or in blocks, or as side chains off the main chain.
core electron. Compare with valence electron. Electrons occupying completely filled shells under the valence shell.
corrosion. corrode. Corrosion is a reaction that involves action of an oxidizing agent on a metal. The oxidizing agent is often oxygen dissolved in water. See How Iron Rusts for examples.
coulomb. (C) The SI unit of electric charge, equal to the amount of charge delivered by a current of 1 ampere running for 1 second. One mole of electrons has a charge of about 96487 C.
coulombic interactions. electrostatic interactions. Attractions between opposite charges or repulsions between like charges that grow stronger as the charges become closer to each other.
covalent bond. covalent; covalently bound. Compare with covalent compound and ionic bond. A covalent bond is a very strong attraction between two or more atoms that are sharing their electrons. In structural formulas, covalent bonds are represented by a line drawn between the symbols of the bonded atoms.
covalent compound. molecular compound. Compare with ionic bond and ionic compound. A compound made of molecules- not ions. The atoms in the compound are bound together by shared electrons. Also called a molecular compound.
critical point. critical state. State at which two phases of a substance first become indistinguishable. For example, at pressures higher than 217.6 atm andtemperatures above 374°C, the meniscus between steam and liquid water will vanish; the two phases become indistinguishable and are referred to as a supercritical fluid.
critical molar volume. (Vc) The molar volume at the critical point.
critical pressure. (Pc) The pressure at the critical point.
critical temperature. (Tc) The temperature at the critical point. A gas above the critical temperature will never condense into a liquid, no matter how much pressure is applied. Most substances have a critical temperature that is about 1.5 to 1.7 times the standard boiling point, in kelvin.
cryogen. cryogenic gas. A gas that has been liquified by lowering temperature, usually to a temperature under about -100°C.
crystal. A sample of a crystalline solid that has a regular shape bound by plane surfaces (facets) that intersect at characteristic angles. The shape results from the arrangement of the substances atoms, ions, or molecules. Most crystals contain defects that can strongly affect their optical and electrical properties.
crystallite. Compare with crystal. A perfect crystalline part of a larger imperfect crystal. Real crystals are usually built of a large number of crystallites.
crystallization. recrystallization. Production of a purer sample of a substance by slow precipitation of crystals from a solution of the substance.
crystal field splitting energy. () Ligands complexed to a metal ion will raise the energy of some of its d orbitals and lower the energy of others. The difference in energy is called the crystal field splitting energy.
crystal field theory. crystal field. The color, spectra, and magnetic properties of metal-ligand complexes can be explained by modeling the effect of ligands on metal's d orbital energies.
crystalline solid. crystalline. Compare with amorphous. A solid that has a repeating, regular three-dimensional arrangement of atoms, molecules, or ions.
crystallization. fractional crystallization; crystallisation. The process of forming pure crystals by freezing a liquid, evaporating a solution, or precipitating a solid from solution. Impurities remain in the liquid, so crystallization is often to purify solid substances.
cupric. (Cu2+) cupric ion. Deprecated. 1. the copper(II) ion, Cu2+. 2. A compound that contains copper in the +2 oxidation state.
cuprous. (Cu+) cuprous ion. Deprecated. 1. the copper(I) ion, Cu+. 2. A compound that contains copper in the +1 oxidation state.
Curie point. Temperature above which a ferromagnetic material loses its ferromagnetism.
cyanide. (CN-) 1. An ion with a -1 charge containing one atom of carbon bound to one atom of nitrogen. 2. A compound that contains CN- ions.
cyanide process. A method for separating a metal from an ore. Crushed ore is treated with cyanide ion to produce a soluble metal cyanide complex. The complex is washed out of the ore and reduced to metallic form using an active metal (usually zinc).
cysteine. Compare with cystine. A naturally occuring amino acid with an SH group on its side chain.
cystine. dicysteine. Compare with cysteine. A naturally occuring amino acid with a disulfide bridge group on its side chain, formed by condensation of two cysteine residues.
D-. D-isomer. Compare with L-. Prefix used to designate a dextrorotatory enantiomer.
Dalton's law. Dalton's law of partial pressure. The total pressure exerted by a mixture of gases is the sum of the pressures that each gas would exert if it were alone. For example, if dry oxygen gas at 713 torr is saturated with water vapor at 25 torr, the pressure of the wet gas is 738 torr.
Debye. (D) Debye unit. Compare with dipole moment. A common non-SI unit of dipole moment, named for Dutch physical chemist Peter Debye. A charge separation equal to one electron charge placed one Ĺngstrom unit apart has a dipole moment of 4.8 D. In SI units, 1 D = 3.338 × 10-30 coulomb meters.
decomposition. decompose; decomposable; decomposition reaction. Compare with synthesis. A reaction in which a compound is broken down into simpler compounds or elements. Compounds sometimes decompose if heated strongly or if subjected to a strong electric current (electrolysis).
degenerate. degenerate orbital. A set of orbitals are said to be degenerate if they all have the same energy. This degeneracy can sometimes be "lifted" by external electric or magnetic fields.
deliquescent. deliquesce; deliquescence. Deliquescent compounds absorb so much moisture from the air that they dissolve. Examples are calcium chloride and sodium hydroxide.
denature. denatured; denaturation. 1. A loss of chemical function, usually due to some heat or chemically-induced structural change. For example, heating a protein causes it to lose its three dimensional form and it no longer functions correctly.
density. (,d) Compare with specific gravity. Mass of a substance per unit volume. Saying "the density of mercury is 13.55 g/cm3 " is the same as saying "the mass of exactly 1 cm3 of mercury is 13.55 g".
density functional. density functional theory; density functional model. A model that describes the electronic structure of an atom or molecule by approximating the total energy as a function of electron density.
dependent variable. Compare with independent variable. A dependent variable changes in response to changes in independent variables. For example, in an experiment where the vapor pressure of a liquid is measured at several different temperatures, temperature is the independent variable and vapor pressure is the dependent variable.
derived unit. Derived units are units constructed from the SI system's base units. For example, the SI unit for density is kg/m3, derived from the base units kg and m.
desalination. Removal of dissolved salts from seawater.
destructive interference. Compare with constructive interference. When the peaks of one wave match the troughs of another, the waves interfere destructively. The amplitudes of the interfering waves cancel to give the resultant wave a lower amplitude.
deuterium. (D, 2H) An isotope of hydrogen that contains one neutron and one proton in its nucleus.
dextrorotatory. dextrorotary. Compare with levorotatory. Having the property of rotating plane-polarized light clockwise.
dialysis. Dialysis is the separation of components in a mixture by passing them across a semipermeable membrane.
diamagnetism. diamagnetic. Compare with paramagnetism. Diamagnetic materials are very weakly repelled by magnetic fields. The atoms or molecules of diamagnetic materials contain no unpaired spins.
diamond. A crystalline form of carbon, made of a network of covalent, tetrahedrally bound carbon atoms.
diastereomer. diastereomeric. Compare with enantiomer. Stereoisomers which are not mirror images of each other. Diastereomers are chemically similar but distinguishable; they have different melting points and boiling points and they react at different rates.
diatomic molecule. Compare with binary compound and polyatomic molecule. A molecule that contains only two atoms. All of the noninert gases occur as diatomic molecules; e. g. hydrogen, oxygen, nitrogen, fluorine, and chlorine are H2, O2, N2, F2, and Cl2, respectively.
diazonium salt. A diazonium salt is a compound with general form Ar-NN+X-, where Ar represents a substituted benzene ring and X- is a halide ion such as chloride. Diazonium salts are unstable and explosive in dry form. They are used to manufacture many different organic compounds, including azo dyes. See also diazotization.
diazotization. Diazotization is a reaction that converts an -NH2 group connected to a phenyl ring to a diazonium salt. For example,
Diazotization reactions are extremely useful in organic synthesis. The nitrous acid provides NO+ which replaces a hydrogen on the -NH3+ group to produce -NH2NO+ and water; a second water is eliminated to produce the -N2+ group.
dichloromethane. (CH2Cl2) Dichloromethane (CH2Cl2) is an organic solvent often use to extract organic substances from samples. It is toxic but much less so than chloroform or carbon tetrachloride, which were previously used for this purpose.
differential thermal analysis. (DTA) A technique that is often used to analyze materials that react or decompose at higher temperatures. The difference in temperature between the sample and an inert reference material is monitored as both are heated in a furnace. Phase transitions and chemical reactions taking place in the sample on heating cause the temperature difference to become larger, at temperatures that are characteristic of the sample.
diffraction. diffract. Compare with effusion. The ability of a wave to bend around the edges of obstacles or holes. The effect is most noticeable when the obstacle or hole is comparable to the size of the wavelength.
diffusion. diffuse. Compare with effusion. The mixing of two substances caused by random molecular motions. Gases diffuse very quickly; liquids diffuse much more slowly, and solids diffuse at very slow (but often measurable) rates. Molecular collisions make diffusion slower in liquids and solids.
diffusion rate. rate of diffusion. Compare with effusion. The number of randomly moving molecules that pass through a unit area per second. Diffusion rates are fastest when a large concentration difference exists on either side of the unit area. Diffusion rates increase with temperature, and decrease with increasing pressure, molecular weight, and molecular size.
dilatometer. A device for measuring volume changes.
dilute. diluted; diluting. Compare with concentrate. Having a relatively low concentration.
dilution. Adding solvent to a solution to lower its concentration.
dipole-dipole interaction. dipole-dipole force. Electrostatic attraction between oppositely charged poles of two or more dipoles.
dissolved oxygen. DO. The amount of oxygen dissolved in a solvent (usually water). Dissolved oxygen levels are used as a general indicator of water quality.
displacement. displacement reaction; replacement reaction; replacement. A reaction in which a fragment of one reactant is replaced by another reactant (or by a fragent of another reactant). Displacement reactions have the same number of products as reactants, and are described by equations of the form A + BC AB + C (single displacement) or AB + CD AC + BD (double displacement).
disproportionation. disproportion; disproportionate; disproportionating. A reaction involving a substance that produces two different forms of the substance, one more oxidized and the other more reduced than the original.
distillate. The vapor collected and condensed from a distillation.
distillation. Distillation is a technique for separating components of a mixture on the basis of differing boiling points. The mixture is heated, vaporizing some of the components. The vapor is collected and condensed to isolate the components with the lowest boiling points.
divalent. Binds to two other things (which may be other atoms, molecules, ions, or electrons). See also divalent anion and divalent cation.
divalent anion. An ion with a charge of -2.
divalent cation. An ion with a charge of +2.
DNA. deoxyribonucleic acid. Compare with nucleic acid and nucleotide. A nucleic acid with 2-deoxy-D-ribose as the sugar in its nucleotides. DNA contains encoded genetic information, specifically templates for the synthesis of all of an organism's proteins and enzymes.
domoic acid. Domoic acid is a toxic amino acid produced by certain species of algae. Domoic acid binds to a receptor that helps nerve cells control the flow of ions across their cell membranes. The receptor no longer works correctly, and the uncontrolled flux of ions damages and eventually kills the nerve cell.
double displacement. double displacement reaction; double replacement; double replacement reaction; double exchange; exchange; metathesis. A double displacement or metathesis is a reaction in which two reactants trade fragments: AB + CD = AC + BD Most commonly, the fragments are ions, e. g. AgNO3(aq) + NaCl(aq) = AgCl(s) + NaNO3(aq)
drug. A biologically active compound or mixture used to cure, prevent, or detect disease, to control biological processes, or to alter mental state.
dry cell. Leclanché cell. A electrolytic cell that uses a moist paste rather than a liquid as an electrolyte. Flashlight batteries are dry cells with a zinc cup for an anode, a carbon rod for a cathode, and a paste made of powdered carbon, NH4Cl, ZnCl2, and MnO2 for an electrolyte.
ductile. ductility. Compare with malleable. Capable of being drawn into wire. Metals are typically ductile materials.
dynamic equilibrium. equilibrium. Compare with position of equilibrium. Dynamic equilibrium is established when two opposing processes are occuring at precisely the same rate, so that there is no apparent change in the system over long periods of time.
dyne. (dyn) The unit of force in the obsolete cgs system of units. A dyne is the force required to accelerate a 1 g mass by 1 cm/s per second.
ebulliometry. ebulliometric. Determination of average molecular weight of a dissolved substance from the boiling point elevation of the solution.
EDTA. ethylenediaminetetracetic acid; versine. A polydentate ligand that tightly complexes certain metal ions. EDTA is used as a blood preservative by complexing free calcium ion (which promotes blood clotting). EDTA's ability to bind to lead ions makes it useful as an antidote for lead poisoning.
effective nuclear charge. (Zeff) Compare with atomic number. The nuclear charge experienced by an electron when other electrons are shielding the nucleus.
efflorescent. efflorescence; efflorescing. Compare with deliquescent and hygroscopic. Efflorescent substances lose water of crystallization to the air. The loss of water changes the crystal structure, often producing a powdery crust.
effusion. effuse. Compare with diffusion and diffraction. Gas molecules in a container escape from tiny pinholes into a vacuum with the same average velocity they have inside the container. They also move in straight-line trajectories through the pinhole.
electric charge. charge. A property used to explain attractions and repulsions between certain objects. Two types of charge are possible: negative and positive. Objects with different charge attract; objects with the same charge repel each other.
electric current. current; electrical current. A flow of electric charges. The SI unit of electric current is the ampere.
electric dipole. dipole. An object whose centers of positive and negative charge do not coincide. For example, a hydrogen chloride (HCl) molecule is an electric dipole because bonding electrons are on average closer to the chlorine atom than the hydrogen, producing a partial positive charge on the H end and a partial negative charge on the Cl end.
electric dipole moment. (µ) dipole moment. A measure of the degree of polarity of a polar molecule. Dipole moment is a vector with magnitude equal to charge separation times the distance between the centers of positive and negative charges. Chemists point the vector from the positive to the negative pole; physicists point it the opposite way. Dipole moments are often expressed in units called Debyes.
electric field. A field of forces that act on any electric charge placed within it. The stronger the field, the stronger the force that acts on the charge. For example, the positive charge on an atomic nucleus creates an electric field that traps electrons.
electrical conductivity. conductivity; electric conductivity; electrical conductance; conductance. Compare with resistance. A measure of how easily an electric current can pass through a material. The conductivity is the reciprocal of the resistance. The SI unit of conductance is the siemens.
electrical resistance. resistance. Compare with conductivity. The ability of a material to oppose the flow of an electric current, converting electrical energy into heat. The SI unit of resistance is the ohm.
electrochemical cell. electric cell. A device that uses a redox reaction to produce electricity, or a device that uses electricity to drive a redox reaction in the desired direction.
electrode. An electrically conducting surface that allows electrons to be transferred between reactants in an electrochemical cell.
electrolytic cell. A device that uses electricity from an external source to drive a redox reaction.
electrolysis. The process of driving a redox reaction in the reverse direction by passage of an electric current through the reaction mixture.
electrolyte. A substance that dissociates fully or partially into ions when dissolved in a solvent, producing a solution that conducts electricity. See strong electrolyte, weak electrolyte.
electromagnetic radiation. electromagnetic wave. A wave that involves perpendicular oscillations in the electric and magnetic fields, moving at a speed of 2.99792458×108 m/s in a vacuum away from the source. gamma rays, x-rays, ultraviolet light, visible light, infrared radiation, and radio waves are all electromagnetic waves.
electron. (e-) Compare with proton and neutron. A fundamental consituent of matter, having a negative charge of 1.602 176 462 × 10-19 coulombs ± 0.000 000 063 × 10-19 coulombs and a mass of 9.109 381 88 × 10-31 kg ± 0.000 000 72 × 10-31 kg [1998 CODATA values].
electron affinity. The enthalpy change for the addition of one electron to an atom or ion in the gaseous state. For example, the electron affinity of hydrogen is H in the reaction H(g) + e- H-(g)H = -73 kJ/mol.
electron configuration. electronic configuration. A list showing how many electrons are in each orbital or subshell. There are several notations. The subshell notation lists subshells in order of increasing energy, with the number of electrons in each subshell indicated as a superscript. For example, 1s2 2s2 2p3 means "2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 3 electrons in the 2p subshell.
electronegativity Compare with ionization energy and electron affinity. Electronegativity is a measure of the attraction an atom has for bonding electrons. Bonds between atoms with different electronegativities are polar, with the bonding electrons spending more time on average around the atom with higher electronegativity.
electron volt. Energy required to move an electron through a potential difference of 1 volt. An electron volt is equivalent to 1.6×10-19 J.
electrorefining. Electrorefining is a method for purifying a metal using electrolysis. An electric current is passed between a sample of the impure metal and a cathode when both are immersed in a solution that contains cations of the metal. Metal is stripped off the impure sample and deposited in pure form on the cathode.
element Compare with compound and mixture. An element is a substance composed of atoms with identical atomic number. The older definition of element (an element is a pure substance that can't be decomposed chemically) was made obsolete by the discovery of isotopes.
element symbol. An international abbreviation for element names, usually consisting of the first one or two distinctive letters in element name. Some symbols are abbreviations for ancient names.
elementary reaction. Compare with net chemical reaction. A reaction that occurs in a single step. Equations for elementary reactions show the actual molecules, atoms, and ions that react on a molecular level.
emission spectrum. emission spectra. Compare with absorption spectrum. A plot of relative intensity of emitted radiation as a function of wavelength or frequency.
emollient. A substance added to a formulation that gives it softening ability. For example, oils that can soften skin are added as emollients in some skin creams.
empirical formula. simplest formula. Compare with molecular formula. Empirical formulas show which elements are present in a compound, with their mole ratios indicated as subscripts. For example, the empirical formula of glucose is CH2O, which means that for every mole of carbon in the compound, there are 2 moles of hydrogen and one mole of oxygen.
empirical temperature. A property that is the same for any two systems that are in thermodynamic equilibrium with each other.
emulsion. Compare with colloid. A colloid formed from tiny liquid droplets suspended in another, immiscible liquid. Milk is an example of an emulsion.
enantiomer. enantiomeric. Compare with diasteromer. Two molecules that are nonsuperimposable mirror images of each other. One enantiomer rotates plane-polarized light to the left; the other rotates it to the right.
endothermic. endothermic reaction; endothermic process. Compare with exothermic. A process that absorbs heat. The enthalpy change for an endothermic process has a positive sign.
endpoint. end point. Compare with equivalence point. The experimental estimate of the equivalence point in a titration.
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