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Glossary of chemistry

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This glossary of chemistry terms is a list of terms and definitions relevant to chemistry, including chemical laws, diagrams and formulae, laboratory tools, glassware, and equipment. Chemistry is a physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions; it features an extensive vocabulary and a significant amount of jargon.

Note: All periodic table references refer to the IUPAC Style of the Periodic Table.

Table of contents:

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A

  • absolute zero A theoretical condition concerning a system at the lowest limit of the thermodynamic temperature scale, or zero kelvins, at which the system does not emit or absorb energy (i.e. all atoms are at rest). By extrapolating the ideal gas law, the internationally agreed-upon value for absolute zero has been determined as −273.15 °C (−459.67 °F; 0.00 K).  
  • absorption 1.  The physical or chemical process by which a substance in one state becomes incorporated into and retained by another substance of a different state. Absorption differs from adsorption in that the first substance permeates the entire bulk of the second substance, rather than just adhering to the surface.2.  The process by which matter (typically electrons bound in atoms) takes up the energy of electromagnetic radiation and transforms it into any of various types of internal energy, such as thermal energy. This type of absorption is the principle on which spectrophotometry is based.  
  • accuracy How close a measured value is to the actual or true value. Compare precision.  
  • acid A compound which, when dissolved in water, gives a pH of less than 7.0, or donates a hydrogen ion.  
  • activated complex A structure that forms because of a collision between molecules while new bonds are formed.  
  • addition reaction In organic chemistry, when two or more molecules combine to make a larger one.   The tendency of dissimilar particles or surfaces to cling to one another as a result of intermolecular forces. Contrast cohesion.  
  • adsorption The chemical adhesion of atoms, ions, or molecules of one substance (either a gas, liquid, or dissolved solid) to the surface of another substance, resulting in a film of the first substance being weakly bonded to the interface between the two substances. Adsorption differs from absorption in that it is exclusively a surface phenomenon, while absorption involves entire volumes of materials.  
  • allomer A substance that differs in chemical composition but has the same crystalline structure as another substance.  
  • anion A negatively charged ion. I.e. an atom that has an excess of electrons compared to protons.  
  • aromaticity A chemical property of conjugated rings of atoms, such as benzene, which results in unusually high stability. Such rings are said to be aromatic.  
  • barometer A device used to measure atmospheric pressure.  
  • beaker A cylindrical vessel or container with a flat bottom, most commonly a type of glassware, widely used in laboratories for a variety of purposes, such as preparing, holding, containing, collecting, or volumetrically measuring chemicals, samples, or solutions, or as a chamber in which a chemical reaction occurs. Beakers are distinguished from flasks by having straight rather than sloping sides; most beakers also have a small spout in the rim to aid pouring.  
  • biochemistry The study of the chemistry of biological systems and organisms.  
  • bond Any persistent attraction between atoms, ions, or molecules that enables the formation of chemical compounds. Bonds are created as a result of a wide variety of electrochemical forces, whose strengths can vary considerably; they are broken when these forces are overcome by other forces. The types, strengths, and quantities of bonds holding together chemical substances dictate the structure and bulk properties of matter.  
  • Boyle's law For a given mass of gas at constant temperature, the volume varies inversely with the pressure.  
  • bumping A phenomenon in which a homogeneous liquid raised to its boiling point becomes superheated and, upon nucleation, rapidly boils to the gas phase, resulting in a violent expulsion of the liquid from the container; in extreme cases, the container itself may shatter. Frequent stirring, the use of an appropriate container, and the use of boiling chips can help prevent bumping.  
  • catalyst Any element or compound that facilitates an increase in the speed of a chemical reaction but which is not consumed or destroyed during the reaction. It is considered both a reactant and a product of the reaction.  
  • cathode An electrode from which the conventional electric current (the flow of positive charges) exits a polarized electrical circuit. Positively charged cations always move toward the cathode, though the cathode's polarity can be positive or negative depending on the type of electrical device and how it is being operated. Contrast anode.  
  • centrifugation A laboratory technique which involves the application of centrifugal force to separate particles from a solution according to their size, shape, and density. Larger and/or denser substances migrate away from the axis of a centrifuge, while smaller and/or less dense substances migrate towards the axis.  
  • centrifuge A device used to separate substances based on size, shape, and density by centrifugation, or the rotation of vessels containing the substances around a centred axis at extremely high velocities.  
  • Charles's law When the pressure on a sample of a dry gas is held constant, the Kelvin temperature is directly proportional to its volume.  
  • compression An area in a longitudinal wave where the particles are closer and pushed in.  
  • corrosion An irreversible interfacial chemical reaction of a material with its environment which results in consumption of the material or dissolution into the material of an external component of the environment.  
  • covalent bond Also molecular bond. A bond that involves the sharing of electron pairs between atoms. The stable balance of attractive and repulsive forces that occurs between atoms when they share electrons is known as covalent bonding. [[]]   The diatomic hydrogen molecule, H2 (right), is formed by a covalent bond when two single hydrogen atoms share two electrons  
  • critical point The end point of a phase equilibrium curve or pressure-temperature curve at which conditions are such that phase boundaries vanish and a substance's different phases, such as liquid and vapor, can coexist. The critical point is defined by the intersection of a critical temperature, Tc, and a critical pressure, pc; above this temperature and pressure, all distinction between phases disappears and the substance becomes a supercritical fluid.  
  • crystal A solid whose constituent particles (such as atoms, ions, or molecules) are arranged in an orderly periodic microscopic structure, forming a lattice that extends in all directions. Such materials are often described as crystalline.  
  • cuvette A type of glassware used in spectroscopic experiments. It is usually made of plastic, glass, or quartz and should be as clean and clear as possible.
  • deionization The removal of ions, and in water's case, mineral ions such as sodium, iron, and calcium.  
  • deposition The settling of particles within a solution or mixture.  
  • diffusion The net movement of atoms or molecules from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in chemical potential of the diffusing species and depends on the random walk of particles; hence it results in mixing or mass transport without required directed bulk motion.  
  • dimer An oligomer consisting of two monomers joined by chemical bonds that may variably be strong or weak, covalent or intermolecular. A homodimer consists of two identical molecules; a heterodimer consists of two different molecules.  
  • dipole The electric or magnetic separation of charge.  
  • distillation The process of separating the component substances of a liquid mixture by exploiting differences in the relative volatility of the mixture's components through selective boiling and subsequent condensation. The apparatus used to distill a substance is called a still, and the re-condensed substance yielded by the process is called the distillate.  
  • ductility Also malleability. A measure of a material's ability to undergo significant plastic deformation before rupturing, typically expressed as percent elongation or percent area reduction from a tensile test and popularly characterized by the material's ability to be stretched into a wire.
  • electrolyte A solution that conducts a certain amount of electric current and can be split categorically into weak and strong electrolytes.  
  • electromagnetic radiation A type of wave that can go through vacuums as well as material and is classified as a self-propagating wave.  
  • electromagnetism Fields with an electric charge and electrical properties that change the way that particles move and interact.  
  • electrophile Any atom or molecule which can accept an electron pair. Most electrophiles carry a net positive charge, include an atom carrying a partial positive charge, or include a neutral atom that does not have a complete octet of electrons, and therefore they attract electron-rich regions of other species; an electrophile with vacant orbitals can accept an electron pair donated by a nucleophile, creating a chemical bond between the two species. Because they accept electrons, electrophiles are Lewis acids by definition.  
  • entropy The amount of energy not available for work in a closed thermodynamic system, usually symbolized by S.  
  • empirical formula Gives the simplest whole-number ratio of the atoms of each element present in a compound.  
  • equilibrium Universally, it is the condition of a system in which all competing influences are balanced. Chemical equilibrium is the state in which the concentrations of the reactants and products have stopped changing in time.  
  • Faraday's laws of electrolysis A set of two laws pertaining to electrolysis which hold that: a) the mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode; and b) the mass of an elemental material altered at an electrode is directly proportional to the element's equivalent weight.  
  • filtration Any physical, biological, or chemical operation that separates large particles (often solid matter) from smaller particles (often a fluid) by passing the mixture through a complex lattice structure through which only particles of a sufficiently small size can pass, called a filter. The fluid and small particles which successfully pass through the filter are called the filtrate.  
  • flask A vessel or container, most commonly a type of glassware, widely used in laboratories for a variety of purposes, such as preparing, holding, containing, collecting, or volumetrically measuring chemicals, samples, or solutions, or as a chamber in which a chemical reaction occurs. Flasks come in a number of shapes and sizes but are typically characterized by a wider vessel "body" and one or more narrower tubular sections with an opening at the top.  
  • frequency A measurement of the number of cycles of a given process per unit of time. The SI unit for measuring frequency is the hertz (Hz), with 1 Hz = 1 cycle per second.  
  • gas One of the four fundamental states of matter, characterized by high-energy particles which fill their container but have no definite shape or volume.  
  • Gay-Lussac's law A chemical law used for each of the two relationships derived by French chemist Joseph Louis Gay-Lussac and which concern the properties of gases, though the name is more usually applied to his law of combining volumes.  
  • Gibbs energy A value that indicates the spontaneity of a reaction. Usually symbolized as G.  
  • gram-atom One gram-atom of an element is defined as a collection of 6.023X10^23 atoms.  
  • hadron A subatomic particle of a type including the baryons and mesons that can take part in the strong interaction.  
  • heat Energy transferred from one system to another by thermal interaction.  
  • hydrate Any substance that contains water or its constituent elements, or any compound formed by the addition of water or its elements to another molecule.  
  • indicator A special compound added to a solution that changes color depending on the acidity of the solution. Different indicators have different colors and are effective within different pH ranges.  
  • induced radioactivity radioactivity caused by bombarding a stable isotope with elemental particles, forming a radioactive isotope  
  • isoelectronicity The phenomenon of two or more chemical species (atoms, molecules, ions, etc.) being composed of different elements but having the same number of valence electrons and the same structural arrangement (i.e. the same number of atoms with the same connectivity). Isoelectronic species typically show useful consistency and predictability in their chemical properties.  
  • ketone An organic compound with a carbonyl group between two carbon atoms. [[]]   The skeletal formula for a generic ketone, with R and R' denoting variable carbon-containing substituent groups  
  • leveling effect The effect of a solvent on the chemical properties of acids or bases which are dissolved in the solvent. The strength of a strong acid is limited or "leveled" by the basicity of the solvent, and likewise the strength of a strong base is limited by the acidity of the solvent, such that the effective pH of the solution is higher or lower than might be suggested by the acid's or base's dissociation constant.  
  • liquid One of the four fundamental states of matter, characterized by nearly incompressible fluid particles that retain a definite volume but no fixed shape.  
  • mass number (A) Also atomic mass number or nucleon number. The total number of protons and neutrons (together known as nucleons) within the nucleus of an atom. It determines the atomic mass of the atom. Mass number varies between different isotopes of the same chemical element, and is often included either after the element's name (as in carbon-12) or as a superscript to the left of the element's symbol (as in 12C) to identify a specific isotope.  
  • mass spectrometry (MS) An analytical technique that measures the mass-to-charge ratio of ions in a chemical sample by bombarding the sample with electrons to the point of ionization and then separating the charged fragments by subjecting them to an electric or magnetic field, typically in order to determine the elemental or isotopic signatures of an unknown substance, the masses of its constituent particles, and/or the identities or structures of the molecules within it. The results are presented as a mass spectrum, a plot of the intensity of ion signals as a function of the mass-to-charge ratio.  
  • mixture A material made up of two or more different substances which are mixed physically but are not combined chemically (i.e. a chemical reaction has not taken place which has changed the molecules of the substances into new substances).  
  • moiety Any named characteristic group, branch, or other part of a large molecule that may be identified within other kinds of molecules as well. Functional groups are typically smaller and more generic than moieties, whereas substituents and side chains may often be classified as moieties and vice versa.  
  • mole (mol) A unit (symbol: mol) that measures the amount of a substance in terms of the absolute number of particles or entities composing the substance. A single mole contains approximately 6.022×1023 particles or entities.  
  • molecule A number of atoms that are chemically bonded together and collectively electrically neutral.  
  • neat Conditions with a liquid reagent or gas performed with no added solvent or cosolvent.  
  • nucleophile Any atom or molecule which can donate an electron pair to another atom or molecule. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles, by which they are attracted to electron-deficient regions of other species; a chemical reaction involving a nucleophile donating an electron pair to an electrophile may be referred to as nucleophilic attack. Because they donate electrons, nucleophiles are Lewis bases by definition.  
  • nuclear magnetic resonance spectroscopy A technique that exploits the magnetic properties of certain nuclei, useful for identifying unknown compounds. Nuclear magnetic resonance is often abbreviated NMR.  
  • nuclide A species of atom characterized by its mass number, atomic number, and nuclear energy state, provided that the mean life in that state is long enough to be observable.  
  • octet rule Also Lewis octet rule. A classical rule for describing the electron configuration of atoms in certain molecules: the maximum number of electron pairs that can be accommodated in the valence shell of an element in the first row of the periodic table is four (or eight total electrons). For elements in the second and subsequent rows, there are many exceptions to this rule.  
  • oxidation state Also oxidation number. 1.  The degree of oxidation of an individual atom in a chemical compound, measured as the decrease in the number of electrons relative to the atom's naturally occurring elemental state.2.  The hypothetical electric charge (positive, negative, or zero) that an atom would have if all bonds to atoms of different elements were 100% ionic, with no covalent component.  
  • oxoacid Also oxyacid or oxacid. 1.  Any acid having oxygen in the acidic group.2.  Any compound which contains oxygen, at least one other element, and at least one hydrogen atom bound to oxygen, and which produces a conjugate base by the loss of positive hydrogen ions.  
  • phase A region of space throughout which all physical properties of a substance are essentially uniform, or a region of material that is chemically uniform, physically distinct, and often mechanically separable. The term phase may have several different uses in chemistry contexts; colloquially, it is often used interchangeably with state of matter, but many distinct phases may exist within a single state of matter.  
  • physical chemistry The branch of chemistry that studies chemical systems in terms of the principles, practices, and concepts of physics, such as motion, energy, force, time, thermodynamics, chemical equilibrium, and statistical mechanics, among others. In contrast to chemical physics, physical chemistry is predominantly (though not entirely) a macroscopic science that studies the physical and chemical interactions of bulk quantities of matter.  
  • pipette Also spelled pipet. A laboratory tool commonly used in chemistry, biology, and medicine to transfer and dispense a precisely measured volume of liquid.  
  • plasma One of the four fundamental states of matter, in which very high-energy particles are partially or fully ionized to the point that they display unique properties and behaviors unlike those of the other three states. Plasma does not exist freely on the Earth's surface under natural conditions.  
  • potential energy The stored energy in a body or in a system due to its position in a force field or due to its configuration.  
  • precipitate The formation of a solid in a solution or inside another solid during a chemical reaction or by diffusion in a solid.  
  • precision How close the results of multiple experimental trials or observations are to each other. Compare accuracy.  
  • pressure The force applied perpendicular to the surface of an object per unit area. The SI unit for pressure is the pascal (Pa), though many other units of pressure are also commonly used in chemistry.  
  • quark An elementary particle and a fundamental constituent of matter.  
  • racemate An equimolar mixture of a pair of enantiomers which does not exhibit optical activity. The chemical name or formula of a racemate is distinguished from those of the enantiomers by the prefix (±)- or by the symbols RS and SR.  
  • radical Also free radical. Any atom, molecule, or ion that has at least one unpaired valence electron. With few exceptions, such unpaired electrons make radicals highly chemically reactive, and therefore organic radicals are usually short-lived.  
  • radioactive decay The process of an unstable atomic nucleus losing energy by emitting radiation.  
  • reactivity series Also activity series. An empirical, calculated, and structurally analytical progression of a series of metals, arranged by their general reactivity from highest to lowest and used to summarize information about their reactions with acids and water and the methods used to extract them from ores.  
  • reagent Also another name for a reactant. A test substance that is added to a system in order to bring about a chemical reaction, or to see whether a reaction occurs.  
  • retort A laboratory apparatus used for the distillation or dry distillation of chemical substances, traditionally consisting of a spherical vessel with a long, downward-pointing neck that conducts the condensed vapors produced by distillation into a separate collection vessel.  
  • Schrödinger equation A quantum state equation which represents the behaviour of an electron around an atom.  
  • solid One of the four fundamental states of matter, characterized by relatively low-energy particles packed closely together in rigid structures with definite shape and volume. See Young's modulus.  
  • solvent The part of a solution that dissolves the solute. For example, water (H2O) is the solvent in a solution of saline water.  
  • stoichiometry The calculation of quantities of reactants and products in chemical reactions. Stoichiometry is based on the law of conservation of mass and the observation that quantities of reactants and products typically exist in ratios of positive integers, implying that if the amounts of the separate reactants are known, then the amounts of the products can be calculated.  
  • suspension A heterogeneous mixture that contains solid particles which are sufficiently large for sedimentation to occur, by which such particles separate from and settle out of the fluid over time if left undisturbed. In a suspension, the solute does not dissolve but remains dispersed or suspended throughout the fluid solvent only transiently and with mechanical agitation. Contrast colloid and solution.  
  • temperature A proportional measure of the average kinetic energy of the random motions of the constituent microscopic particles of a system. The SI base unit for temperature is the kelvin.  
  • thermal conductivity The property of a material that allows it to conduct thermal energy or heat (a quantity often denoted by k).  
  • thermodynamics The study of the effects of changing temperature, volume or pressure (or work, heat, and energy) on a macroscopic scale.  
  • thermodynamic stability The condition of a system being in its lowest energy state with its environment (equilibrium).  
  • titration Also titrimetry or volumetric analysis. A laboratory method of quantitative chemical analysis that is used to determine the concentration of an identified analyte. The procedure involves preparing a particular reagent as a standard solution of known concentration and volume (called the titrant or titrator) and allowing it to react with a solution of the analyte (called the titrand) to determine the latter's concentration.  
  • torr A unit for measuring pressure, equivalent to 133.322 Pa or 1.3158×10−3 atm.  
  • UN number A four-digit code used to note hazardous and flammable substances.  
  • uncertainty The notion that any measurement that involves estimation of any amount cannot be exactly reproducible.  
  • uncertainty principle Knowing the location of a particle makes the momentum uncertain, while knowing the momentum of a particle makes the location uncertain.  
  • unified atomic mass unit (u) Also Dalton (Da). A unit of mass approximately equal to the mass of one proton or neutron. It is sometimes equated with the technically distinct and obsolete atomic mass unit and abbreviated amu.  
  • unit factor Statements used in converting between units.  
  • vacuum flask Also Dewar flask or thermos. A storage vessel consisting of two flasks or other containers, placed one within the other and joined at the neck, and a space in between that is partially evacuated of air, creating a near-vacuum that significantly reduces the transfer of heat between the vessel's interior and its ambient environment. Vacuum flasks can greatly lengthen the time over which their contents remain warmer or cooler than the ambient environment.  
  • valence bond theory A theory explaining the chemical bonding within molecules by discussing valencies, the number of chemical bonds formed by an atom.  
  • valency The combining capacity of an element.  
  • vapor When a substance is below the critical temperature while in the gas phase.  
  • vapor pressure Also equilibrium vapor pressure. The pressure exerted by a vapor which is in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is commonly described as the tendency of particles to spontaneously escape from the liquid or solid state into the gaseous state and is used as an indication of a liquid's evaporation rate.  
  • volatility A material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a gas, while a substance with low volatility is more likely to exist as a liquid or solid; equivalently, less volatile substances will more readily condense from a gaseous state than highly volatile ones.  
  • watch glass A circular, concave piece of glass commonly used in chemistry laboratories as a working surface for various purposes, such as evaporating liquids, holding solids while they are being weighed, heating small amounts of a substance, or as a cover for a beaker.  
  • water A polar inorganic compound with the chemical formula H2O that is a tasteless, odorless, and generally colorless liquid at standard temperature and pressure, though it also occurs naturally as a solid and a gas at the Earth's surface. It is the most abundant substance on Earth and therefore an integral component of virtually all chemical and biological systems. Water is often described as the "universal solvent" for its inherent ability to dissolve many substances.  
  • wave function A mathematical function describing the position of an electron in a three-dimensional space.  
  • wet chemistry Also bench chemistry or classical chemistry. A form of analytical chemistry which uses classical laboratory methods such as simple observation and elementary chemical tests to study chemicals and chemical reactions, i.e. without the use of sophisticated instruments or automated or computerized analysis. It is often used in schools to teach the principles of chemistry to students.  
  • X-ray diffraction a method for establishing structures of crystalline solids using single wavelength X-rays and looking at diffraction pattern.  
  • zone melting A way to remove impurities from an element by melting it and slowly travel down an ingot (cast).  
  • zwitterion A chemical compound whose net charge is zero and hence is electrically neutral. But there are some positive and negative charges in it, due to the formal charge, owing to the partial charges of its constituent atoms.  

Table of contents:

.A | .B | .C | .D | .E | .F | .G | .H | .I | .J | .K | .L | .M

.N | .O | .P | .Q | .R | .S | .T | .U | .V | .W | .X | .Y | .Z

External links

IUPAC Compendium of Chemical Terminology

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