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Chemical 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")

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.

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.

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.

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.

A gel made from seaweed used to make salt bridges.

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.

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.

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.

A measure of a material's ability to neutralize acids. Alkalinities are usually determined using titration.

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).

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.

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).

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.

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