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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. Ex: boron carbide, diamond, corundum, etc
absolute error or absolute uncertainty : The uncertainty in a measurement, expressed with appropriate units. Absolute error is also used to express inaccuracies 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.
absorbance : A measure of the amount of light absorbed by a sample
absorption : 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. 3. An absorbent captures another material and distributes it throughout whereas an adsorbent captures another material and distributes it on its surface only.
absorption spectroscopy : A technique for determining the concentration and structure of a substance by measuring the amount of electromagnetic radiations the sample absorbs at various wavelengths.
absorption spectrum : 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 wave number.
accuracy : 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 : An ion formed by removing the acidic hydrogen of acetic acid, CH3COOH (CH3COO¯)
acetic acid : (CH3COOH) ethanoic acid, vinegar acid or methane carboxylic acid. A simple organic acid that gives vinegar its characteristic odour and flavour. Glacial acetic acid is pure acetic acid.
acid: ([Lat. acidus, sour]): : 1. A compound which releases hydrogen ions (H+) in solution (Arrhenius). 2. A compound containing donate hydrogen ions (Bronsted-Lowry). 3. A compound that can accept a pair of electrons from a base (Lewis)..
acid anhydride : 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 or weak organic base 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) or acid ionization 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 CH3COOH(aq) = H+(aq) + CH3COO¯(aq), which is Ka = [H+][ CH3COO¯]/[ CH3COOH].
acid error : An 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 or acyl chloride. : Compounds containing a carbonyl group bound to a halogen atom.
acidic solution : A solution in which the hydrogen ion concentration is higher than that of the hydroxide ion in water.
acidulant : A substance added to food or beverages to lower its pH and to impart a tart, acid taste. Ex: phosphoric acid is an acidulant added to soft drinks.
actinide : Elements 89-102 are called actinides. Electrons 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 or active carbon. : A porous form of carbon that acts as a powerful adsorbent used to decolorize liquids, recover solvents, remove toxins from water and air, etc
activated complex or 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.
activation energy: (Ea) : The minimum energy required to convert reactants into products, It is 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 key in a lock. The active site lowers the activation energy for reaction.
activity : An effective concentration used in thermodynamic calculations in place of the actual concentration.
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 or adiabatic line : A line in a graph that represents an adiabatic process.
adiabatic or adiabatic process or isentropic process: : A process that neither absorbs nor releases energy into the surroundings. For example, a chemical reaction taking place in a closed thermos flask can be considered adiabatic. Very fast reactions can be considered adiabatic with respect to heat exchange with the surroundings, because heat exchange is not instantaneous.
adiabatic ionization enthalpy : It is the minimum 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 : 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 : Attraction between different substances on either side of a phase boundary
adsorb : To collect molecules of a substance on a surface.
adsorbent : A substance that collects molecules of another substance on its surface. For example, gases that are present in the mines are strongly adsorbed on activated charcoal
adsorption : Adsorption is collection of a substance on the surface of a solid or a liquid.
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.
aeration : Preparation of a saturated solution of air gases by either spraying the solution in air or by bubbling air through it.
aerosol : 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
alanine : (A, CH3CH(NH2)COOH) or Ala or 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) An alcohol is an organic compound with a carbon bound to a hydroxyl group. Ex: methanol, CH3OH; ethanol, CH3CH2OH; propanol, CH3CH2CH2OH,etc.
aldehyde : (RCHO) An aldehyde is an organic compound with a carbon bound to a -CHO group. Ex: formaldehyde (HCHO), acetaldehyde, CH3CHO, benzaldehyde, C6H6CHO,etc
aliphatic : 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 hence are referred to as "alkali metals". Hydrogen is not considered an alkali metal, despite its position in the same group as it is a non metal.
alkali metal : 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 hence are referred to as "alkali metals". Hydrogen is not considered an alkali metal, despite its position in the same group as it is a non metal.
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 : The Group 2 elements, beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) are called "alkaline earth metals".
alkaline error : An 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. Ex: cocaine, nicotine, strychnine, morphine caffeine, etc.
alkane or paraffin : A series of organic compounds with general formula CnH2n+2. Alkane names end with -ane. Ex: methane(CH4), ethane(C2H6),etc
alkene : A compound that consists of only carbon and hydrogen and contains at least one carbon-carbon double bond. Alkene names end with -ene. Ex: ethylene (CH2=CH2); 1-propene (CH2=CH2CH3), (CH3CH2=CH2(CH2)4CH3),etc.
alkoxide : An ionic compound formed by removal of hydrogen ion from the hydroxyl group in an alcohol using reactive metals, such as sodium.
alkyl : A molecular fragment derived from an alkane by dropping a hydrogen atom from the formula. Ex: methyl (CH3) and ethyl (CH2CH3),etc.
alkyl halide : An alkyl group attached to a halogen atom. Ex: CH3Cl,C2H5Cl,etc.
alkyne : A compound that consists of only carbon and hydrogen & that contains at least one carbon-carbon triple bond. Alkyne names end with -yne. Ex: acetylene (CH CH); 1-propyne (CH2 CH2CH3), etc.
allobar : A form of an element that has isotopic abundance that are different from the naturally occuring form. For example, "depleted" uranium has most of the uranium-235 removed, and is an allobar of natural uranium.
allomer : Substances with different chemical composition but the same crystalline form.
allosteric effect : A change in the behavior of one part of a molecule caused by a change in another part of the molecule.
allotrope : Some elements occur in several distinct forms called allotropes. Allotropes same chemical properties but different physical properties. Ex: graphite and diamond are allotropes of carbon.
alloy : 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 or allylic or allyl group or 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 : 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 4.001u
alpha ray : 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 : An alloy that contains mercury
american chemical society : 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 : An amine is an organic compound that contains a nitrogen atom bound only to carbon and possibly hydrogen atoms. . Ex: methylamine, CH3NH2; dimethylamine,CH3NHCH3; and trimethylamine,(CH3)3N,etc.
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 & are the basic building blocks of proteins.
ammine : 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 : Pure NH3 is a colorless gas with a sharp, characteristic odor. It is easily liquefied 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+ is a cation formed by neutralization of ammonia, which acts as a weak base
amorphous : 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 : The SI unit of electric current equal to flow of 1 coulomb of charge per second. Named for 19th century physicist André Marie Ampere.
amperometry : Determining the concentration of a material in a sample by measuring electric current.
amphiprotic solvent : Solvents that exhibit both acidic and basic properties. They undergo autoprotolysis Ex water, ammonia, ethanol, etc.
amphoteric : A substance that can act as either an acid or a base in a reaction. Ex: 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 : It is insoluble & branched component of starch.
amylose : A water soluble component of starch made of long, un branched chains of -D-glucose molecules.
aprotic solvent : A solvent that does not act as an acid or as a base & they do not undergo autoprotolysis Ex: pentane, petroleum ether, toluene, etc.
anabolism : metabolic reaction sequences that put building blocks together to assemble larger molecules.
analysis : Determination of the composition of a sample.
analyte : An analyte is the sample constituent whose concentration is sought in a chemical analysis.
angstrom : A non-SI unit of length used to express wavelengths of light, bond lengths, and molecular sizes. 1 Å = 10-10 m = 10-8 cm.
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. It can take values from 0 to n-1 within a shell with principal quantum number,n.
anhydrous : A compound with all water removed, especially water of hydration. For example, when copper(II) sulfate pentahydrate (CuSO4•5H2O) is strongly heated produces anhydrous copper(II) sulfate (CuSO4).
anion : An anion is a negatively charged ion. Usually nonmetals form anions.
anode : 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.
antibonding orbital or 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 bleaching agents to stop the bleaching. Ex: Sodium thiosulphate is an antichlor.
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 : 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 reduce or prevent fever.
aqua regia : A mixture of nitric and hydrochloric acids, usually 1:3 or 1:4 parts HNO3 to HCl, used to dissolve gold.
aqueous : A substance dissolved in water.
apoenzyme : the protein part of an enzyme
arene : A hydrogen that contains at least one aromatic ring
aromatic ring : 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 odours.
Arrhenius equation : In 1889, Svante Arrhenius explained the variation of rate constants with temperature for several elementary reaction using the relationship k = A e(-Ea/RT) where the rate constant k is the total frequency collisions between reaction molecules A times the fraction of collisions e(-Ea/RT) that have an energy that exceeds a threshold activation energy Ea at a temperature of T (in Kelvin). R is the universal gas constant
arginine : 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.
arrhenius acid : A substance that provides H+ ions when dissolved in water
arrhenius base : A substance that provides OH- ions when dissolved in water.
aryl : A molecular fragment or group attached to a molecule by an atom that is on an aromatic ring.
asparagine : A natural amino acid that is the amide of aspartic acid
aspartic acid : 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 : A unit of pressure, equal to a barometer reading of 760 mm Hg. 1 atmosphere is 101325 Pascal and 1.01325 bar
atom : The smallest particle that retains the chemical properties of an element
atomic mass unit : A unit of mass equal to 1/12 the mass of a carbon-12nucleus,which is 1.66 × 10-27kg Abbreviated as amu or u. Sometimes called the dalton, after John Dalton architect of the first modern atomic theory
atomic nucleus : 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 : The number of protons in anatomic nucleus. The atomic number and the element symbol are two alternate ways to label an element. In nuclide symbols, the atomic number is a subscript; for example, in 126C, the "6" is the atomic number.
atomic orbital : A wave function that describes the behavior of an electron in an atom.
atomic 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 : 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 the 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 or 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.
atto : Prefix used in the SI system meaning "multiply by 10-18". For example, 2 am means 2× 10-18 meters.
Aufbau 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 : Minimum temperature at which the vapor/air mixture over a liquid spontaneously catches fire.
autoxidation : 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 : Transfer of a hydrogen ion between molecules of the same substance. Ex: the autoprotolysis of methanol (2 CH3OH = CH3OH2+ + CH3O-). Autoprotolysis of water into hydronium ions and hydroxide ions
auxochrome : A group or substructure in a molecule that influences the intensity of absorption of the molecule.
average bond enthalpy : Average enthalpy change per mole when the same type of bond is broken in the gas phase for many similar substances.
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 : The number of particles in one mole, equal to 6.022 × 1023 mol-1
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, 2 L of ideal gas contains twice as many molecules as 1 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 : A solution that does not change composition when distilled. For example, if a 95% (w/w) ethanol solution in water is boiled, the vapor produced also is 95% ethanol.
azo : The azo group has the general structure Ar-N=N-Ar', where Ar and Ar' indicate substituted aromatic compounds. Compounds containing the azo compounds are often intensely coloured and are economically important as dyes. Methyl orange is an example of an azo dye.


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