This glossary of chemistry terms is a list of terms and definitions relevant to chemistry, including chemical laws, diagrams and formulae, laboratory tools, glassware, and equipment. Chemistry is a physical science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions; it features an extensive vocabulary and a significant amount of jargon.
Note: All periodic table references refer to the IUPAC Style of the Periodic Table.
A 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).
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.
1. (Brønsted–Lowry acid) Any chemical species or molecular entity that acts as a proton donor when reacting with another species, because it loses at least one proton (H+ ) which is then transferred or 'donated' to the other species, which by definition is a Brønsted–Lowry base. When dissolved in an aqueous solution, a proton donor which increases the concentration of hydronium ion (H 3O+ ) by transferring protons to water molecules may also be called an Arrhenius acid. The term "acid", when not otherwise qualified, often refers implicitly to a Brønsted–Lowry acid.[1]
2. (Lewis acid) Any chemical species or molecular entity that acts as an electron pair acceptor when reacting with another species, forming a covalent bond by accepting a lone pair of electrons donated by the other species, which is known as a Lewis base. This definition was intended as a generalization of the Brønsted–Lowry definition by proposing that acid-base reactions are best viewed as reorganizations of electrons rather than transfers of protons, with the acid being a species that accepts electron pairs from another species either directly or by releasing protons (H+ ) into the solution, which then accept electron pairs from the other species. The Lewis definition is inclusive of many Brønsted–Lowry acids, though not all: most Lewis acids are not Brønsted–Lowry acids, and most Brønsted–Lowry acids are not Lewis acids.[1]
3. Colloquially, any compound which, when dissolved in water, yields a pH of less than 7.0. The term "acid" is commonly used to refer to the entire aqueous solution, whereas stricter definitions refer only to the acidic solute.[2]
A distinct chemical species that is the sole product of an addition reaction between two other distinct reactant species, in which all of the atoms comprising the reactants are retained in the single product. Changes in connectivity may occur, but there is no loss of any of the original atoms and no gain of atoms that are not present in the reactant molecules. Stoichiometries other than 1:1 are also possible, e.g. a bis-adduct (2:1).[2]
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.
A functional group and a class of organic compounds consisting of a carbonyl group attached to a hydrogen atom and any other R-group. Aldehydes have the general formula R–C(H)=O.
The skeletal formula for a generic aldehyde, where R denotes a variable carbon-containing substituent group
The substituent form of an alkane, i.e. any alkane missing a hydrogen atom. The term may be used to refer to many different functional groups, e.g. a methyl or ethyl group.
The number of discrete particles (such as molecules, atoms, ions, electrons, or any other atomic-scale entity) in a given sample of matter, divided by the Avogadro constant. The SI unit for amount of substance is the mole (mol).
A non-SI, metric unit of length equal to 10−10metre, i.e. 1⁄10000000000 of a metre or 0.1 nanometre. The angstrom is commonly used in the natural sciences to express microscopic or atomic-scale distances, including the sizes of atomic nuclei, wavelengths of electromagnetic radiation, and lengths of chemical bonds (e.g. the covalent radius of a chlorine atom averages about 1 angstrom).
Having or containing no water molecules, referring especially to water of hydration.[4] Because many processes in chemistry are impeded in the presence of water, it is often of critical importance that water-free reagents and techniques are used. Anhydrous compounds tend to gradually absorb water from the atmosphere. Contrast hydrous.
1. An electrode through which the conventional electric current (the flow of positive charges) enters into a polarized electrical circuit.
2. The wire or plate of an electrochemical cell having an excess positive charge. Negatively charged anions always move toward the anode. Contrast cathode.
Either of a pair of cyclichemiacetal or hemiketalsaccharides that are epimers of each other, differing at only one carbon stereocenter, specifically the carbon that bears the aldehyde or ketone functional group in the compound's acyclic, open-chain configuration, known as the anomeric carbon.
The separation of a mixture of gases by exploiting their different rates of diffusion, usually by allowing the gases to diffuse through the walls of a porous partition or membrane.[3]
The total number of atoms present in a single molecule of a given substance; e.g. ozone (O3) has an atomicity of 3, while benzene (C6H6) has an atomicity of 12.[3]
The lowest temperature at which a given substance will spontaneously ignite in a normal atmosphere without an external source of ignition such as a flame or spark, i.e. when the ambient temperature is sufficiently high to provide the activation energy needed for combustion. Substances which spontaneously ignite at naturally occurring temperatures are termed pyrophoric. Compare ignition temperature.
The ratio of the number of discrete constituent particles (such as molecules, atoms, or ions) to the amount of a substance, defined as exactly 6.02214076×1023 mol−1.
The number of discrete constituent particles in one mole of a substance, defined as exactly 6.02214076×1023. This dimensionless number differs from the Avogadro constant in that it has no unit.
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.
A model of the general structure of the atom proposed by Niels Bohr and Ernest Rutherford in 1913, featuring a small, dense nucleus of positively charged particles surrounded by orbitingelectrons, which are attracted to the nucleus by electrostatic forces. This interpretation replaced several earlier hypotheses and quickly became the prevailing standard model for depicting atomic structure.
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.
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.
A measure of the heat per unit mass produced by complete combustion of a given substance, usually expressed in megajoules per kilogram (MJ/kg) or in kilojoules per gram (kJ/g).[3]
1. A functional group composed of a carbon atom double-bonded to an oxygen atom, with the formula . Carbonyl groups are common to many classes of organic compounds and are also a part of many larger functional groups.
A unique numerical identifier assigned by the Chemical Abstracts Service (CAS) to every chemical substance described in the open scientific literature, including more than 182 million organic and inorganic compounds, minerals, isotopes, alloys, polymers, and mixtures, as well as so-called "UVCBs" (substances of unknown or variable composition, complex reaction products, or biological origin). CAS numbers are an internationally recognized standard used by scientists, industries, and regulatory bodies.[5]
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.
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.
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.
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.
Any of various means of concisely displaying information about the chemical composition of a compound or molecule using letters, numbers, and/or typographical symbols. Chemical formulas, such as empirical and molecular formulas, can only indicate the identities and numerical proportions of the atoms in a compound and are therefore more limited in descriptive power than chemical names and structural formulas.
1. Any method or means of changing one or more chemicals or chemical compounds in any way, either naturally or artificially, spontaneously or by the actions of external forces.
2. In chemical engineering, any method used on an industrial scale (especially in manufacturing) to change the composition of one or more chemicals or materials.
A chemical substance or ensemble of substances composed of chemically identical molecular entities which can explore the same set of molecular energy levels on a characteristic or delineated time scale.
A form of matter that has constant chemical composition and characteristic properties and which cannot be separated into simpler components by purely physical methods (i.e. without breaking chemical bonds). It is often called a pure substance to distinguish it from a mixture.
The artificial execution of one or more chemical reactions in order to obtain one or more products. In modern laboratory contexts, specific chemical syntheses are both reliable and reproducible.
A property of asymmetry in which a molecule or ion is distinguishable from its mirror image such that it cannot be superposed upon it by any combination of geometric rotations, translations, or some conformational changes.[6][7] Such a molecule or ion is said to be chiral, and exists in two forms, known as enantiomers, which are stereoisomers of each other; these forms are distinguished as either "right-handed" or "left-handed" by their absolute configuration or some other criterion. Several different types of asymmetry can give rise to chirality, most commonly when molecules possess stereogenic elements such as one or more stereocenters (central chirality), a stereogenic axis (axial chirality), or a stereogenic plane (planar chirality); additionally, the inherent curvature of a molecule can cause it to possess inherent chirality.
Structural diagrams of two chiral molecules, the non-superposable mirror-image enantiomers (S)-alanine (left) and (R)-alanine (right). Though they have identical structural formulas and the same physical properties, they are nevertheless different chemicals, and react differently with other chiral compounds.
A liquid solution of known chemical composition and concentration, and hence of known and standardized color, used as a reference in the optical analysis of samples of unknown strength.[4]
color test
The quantitative analysis of a substance by comparing the intensity of the color produced when the substance is exposed to a reagent with a standard color produced similarly in a solution of known strength.[4]
The quantity or abundance of a constituent of a mixture per unit quantity of the mixture; e.g. the amount of a dissolved solute per unit volume of the solution, a measure known as molarity. Several different definitions of concentration are widely used in chemistry, including mass concentration, volume concentration, and molar concentration.
A comparative measurement of the electrical conductivity of a solution defined as the molar concentration of a sodium chloride (NaCl) solution that has the same specific electrical conductance as the solution under test. It is typically expressed in units of moles per litre (or per some other unit of volume).
A chemical compound consisting of a central atom or ion, usually metallic and known as the coordination center, bonded to a surrounding array of other groups of atoms, e.g. molecules or ions, which are known as ligands or complexing agents. Many metal-containing compounds, especially those of the transition metals, are coordination complexes. See also complex.
An irreversible interfacial chemical reaction of a material, especially a metal, with its environment, which results in consumption of the material or dissolution into the material of an external component of the environment.
The ion that is the counterpart to an oppositely charged ion in a dissociated ionic species; the cation that pairs with a given anion, or vice versa. For example, Na+ is the counterion to Cl− , and vice versa, in solutions of sodium chloride (NaCl).
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.
A diatomic hydrogen molecule, H 2 (right), is formed by a covalent bond when two single hydrogen atoms share two electrons between them.
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.
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.
An empirical law which states that in a mixture of non-reacting gases, the total pressure exerted by all of the gases combined is equal to the sum of the partial pressures exerted by each gas individually.
A hygroscopic substance used to induce or sustain a state of dryness or desiccation (i.e. the absence of moisture) in its vicinity by abstracting water molecules from other substances. Desiccants come in many different forms and work by many different principles, ranging from simple absorption to the chemical bonding of water molecules.
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.
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.
The electric or magnetic separation of electric charge into a pair of charges of equal magnitude but opposite sign, one positively charged and one negatively charged, separated by some typically small distance.
A system in which particles of one material are distributed within a continuous phase of another material; the two phases may be in the same or different states of matter. Dispersions of particles sufficiently large for sedimentation are called suspensions, while those of smaller particles are called colloids or solutions.
The process of separating the component substances of a liquidmixture 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.
The temperature at which a grease changes from a semi-solid to a liquid state under standardized conditions,[4] i.e. the upper limit at which the grease retains its structure, though not necessarily the maximum temperature at which it can be used.
A chamber or container in which the interior is maintained at very low humidity, often by filling it with argon or with air lacking carbon dioxide, in order to provide an inert atmosphere in which manipulation of very reactive chemicals or moisture-sensitive procedures can be carried out in the laboratory.[4]
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.
The escape of gas from an aqueoussolution without the application of heat, and the bubbling, foaming, or fizzing that results; e.g. the release of carbon dioxide from carbonated water.
A measure of the separation of positive and negative electric charges within an electrical system, i.e. a measure of the system's overall electrical polarity. The SI unit for measuring electric dipole moment is the coulomb-metre (C⋅m), but the debye (D), a non-SI unit, is also widely used in chemistry and atomic physics.
A device capable of either generating electrical energy from chemical reactions, in which case it is known as a galvanic or voltaic cell, or using electrical energy to cause chemical reactions, in which case it is known as an electrolytic cell. For example, a battery contains one or more galvanic cells, each of which consists of two electrodes arranged such that an oxidation–reduction reaction produces an electromotive force.
A branch of physical chemistry concerned with the relationship between electrical potential difference and identifiable chemical change, as understood through either the chemical reactions accompanying the passage of an electric current or the potential difference that results from a particular chemical reaction.
A type of nuclear transformation by which the proton-rich nucleus of an electrically neutral atom absorbs or 'captures' an electron from one of its own inner shells, often those closest to the nucleus, which provokes a reaction that results in a nuclear proton changing into a neutron accompanied by the simultaneous emission of an electron neutrino.[2]
The distribution of the electrons of an atom or molecule within atomic or molecular orbitals. An extensive system of notation is used to concisely and uniquely display information about the electron configuration of each atomic species. Knowledge of the specific arrangements of electrons in different atoms is useful for understanding chemical bonds and the organization of the periodic table of the elements.
An intrinsic property of an electron such that its potential energy is linearly related to the strength of its electric field; a measure of the distribution of an electron's negative charge within the electric field it creates. See also electric dipole moment.
A chemical property that describes the tendency of an atom to attract a shared pair of electrons (or electron density) towards itself. An atom's electronegativity is affected both by its nuclear charge (which is proportional to the number of protons in its nucleus) and the number and location of the electrons present in its atomic shells (which influences the distance of the nucleus from the valence electrons). The higher an atom or substituent's electronegativity, the more it attracts electrons towards itself. As it is usually calculated, electronegativity is not a property of an atom alone but rather of an atom within a molecule; it therefore varies with an element's chemical environment, though it is generally considered a transferable property.
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.
A species of atoms having the same number of protons in their atomic nuclei and hence the same atomic number. Chemical elements constitute all of the ordinary matter in the universe; 118 elements have been identified and are organized by their various chemical properties in the periodic table of the elements.
The process of extracting one material from another by washing with a solvent. Elution works by running a solution containing an analyte past an adsorbent matrix designed to selectively bind the analyte molecules, and subsequently washing the adsorbent/analyte complex with a solvent, known as an eluent. The solvent molecules displace the analyte by binding to the adsorbent in its place, allowing the analyte, now part of the eluate, to be carried out of the complex and into a collector for analysis.
A type of colloid in which small particles of one liquid are dispersed in another liquid; e.g. a dispersion of water in an oil, or of an oil in water. Emulsions are often stabilized by the addition of a substance, known as an emulsifier, that has both lyophilic and lyophobic parts in its molecules.[3]
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 in a reacting system have stopped changing in time.
A class of organic and inorganic compounds derived from the reaction of an acid with an alcohol, in which at least one hydroxyl group (–OH) is replaced by an alkoxy group (–O–). Esters have the general formula RCO2R′, where R and R' represent any alkyl or aryl group.
The skeletal formula for a generic ester, with R and R′ denoting variable carbon-containing substituent groups
A class of organic compounds and a functional group containing an oxygen atom connected to two alkyl or aryl groups, which may be the same or different. Ethers have the general formula R–O–R′, where R and R′ represent the alkyl or aryl groups.
The skeletal formula for a generic ether, with R and R′ denoting variable carbon-containing substituent groups
A unit of electric charge widely used in electrochemistry equal to the negative of the molar charge (electric charge per mole) of electrons. It is equal to approximately 96,500 coulombs per mole (F = 96485.33212... C/mol).
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.
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.
The lowest temperature at which the vapors above a volatile material will continue to burn for at least five seconds after ignition by an open flame of standard dimension.[8] The fire point should not be confused with the flash point, a slightly lower temperature at which a substance will ignite briefly but at which vapor is not produced at a rate sufficient for sustained combustion.
The lowest temperature at which the vapors above a volatile material will ignite if given an ignition source. At the flash point, the application of an open flame causes only a momentary "flash" rather than sustained combustion, for which the ambient temperature is still too low. The flash point should not be confused with the fire point, which occurs at a slightly higher temperature, nor with the kindling point, which is higher still.
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.
The process by which the dispersed particles in a colloid come out of suspension to aggregate into larger clumps known as floc or flake, either spontaneously or due to the addition of a clarifying agent. The term is often used to refer to a reversible aggregation in which the forces holding the particles together are weak and the colloid can be re-dispersed by agitation.[3]
The electric charge assigned to an atom in a molecule, assuming that all electrons in all bonds are shared equally between atoms, regardless of each atom's relative electronegativity. The formal charge of any atom that is part of a molecule can be calculated by the equation , where is the number of valence electrons of the neutral atom in its ground state; is the number of valence electrons of the atom which are not participating in bonds in the molecule; and is the number of electrons shared in bonds with other atoms in the molecule.
The fractionation of a mixture of liquids into its component parts, or fractions, by the process of distillation, typically by using a long vertical column attached to the distillation vessel and filled with glass beads. The mixture is heated to a temperature at which one or more of the component compounds will vaporize; the vapor rises up the column until it condenses and runs back into the vessel, creating a temperature and volatility gradient and permitting various fractions to be drawn off at different points along the length of the column.[3] Common in industrial chemistry, the technique is sensitive enough to separate compounds which have boiling points that differ by less than 25 °C (45 °F) from each other at standard pressure.
A separation process in which a particular quantity of a mixture is divided during a phase transition into a number of smaller quantities, known as fractions, for which the chemical composition varies according to a gradient. Fractionation exploits subtle differences in some specific property (e.g. mass, boiling point, solubility, etc.) between the mixture's component compounds, making it possible to isolate more than two components of a mixture at the same time. There are many varieties of fractionation employed in many branches of science and technology.
The temperature at which a substance changes state from a liquid to a solid. Because freezing is the reverse of melting, the freezing point of a substance is identical to its melting point, but by convention only the melting point is referred to as a characteristic property of a substance.
A type of chromatography commonly used in analytical chemistry to isolate and analyze chemical compounds that can be vaporized without decomposition. Gas chromatography is often used to test the purity of substances, to identify unknown substances, and to measure the relative amounts of the different components of mixtures.
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.
Any of a class of aliphaticdihydric alcohols in which the two hydroxy groups are bonded to two different carbon atoms, which are usually but not necessarily adjacent to each other; e.g. ethylene glycol (HOCH 2CH 2OH).[2]
An apparatus designed to quickly and easily fit two pieces of leak-tight glassware together, featuring ground glass surfaces and typically a custom-made conical taper.
The lowest possible energy state for a given quantum mechanical system, at which the Gibbs energy is actually or theoretically minimized. Whatever energy remains in the system in its ground state is called the zero-point energy.[2] Contrast excited state.
A Lewis acid with an electron-accepting centre that is only weakly polarizable. Hard acid species also tend to have high charge states and relatively small atomic nuclei, in contrast to soft acids.[2]
A law of physical chemistry which states that the total enthalpy change during the course of a chemical reaction is the same whether the reaction is completed in one step or in multiple steps.
Any chemical reaction between molecular hydrogen (H 2) and another chemical species, typically resulting in the reduction or saturation of the other species by the addition of one or more pairs of hydrogen atoms to a compound or element. The presence of a catalyst is usually required for hydrogenation reactions to occur; non-catalytic hydrogenation takes place only at extreme temperatures.
A diatomicanion consisting of a hydrogen atom covalently bonded to an oxygen atom, having an overall negative charge, with the chemical formula OH− ; or any member of a class of organic and inorganic compounds containing a hydroxy group, e.g. sodium hydroxide (NaOH).[4]
A hypothetical gas composed of many randomly moving point particles that do not participate in any interparticle interactions, thereby making it mathematically convenient to describe and predict their behavior as state variables change. The ideal gas concept is useful because it obeys the ideal gas law and can be analyzed within the framework of statistical mechanics.
The equation of state of a hypothetical ideal gas, which states that the volume of such a gas is proportional to the amount of gas and its Kelvintemperature, and inversely proportional to its pressure. The ideal gas law combines Boyle's law, Charles's law, Gay-Lussac's law, and Avogadro's law into a single equation, conventionally formulated as , where is the ideal gas constant. The relationships between the state variables described in this equation are a good approximation of the behavior of many gases under a wide range of conditions, though there are some limitations.
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.
The boundary between two spatial regions occupied by different matter, especially by matter in different phases or physical states. See also surface and phase boundary.
A type of alloy that forms an ordered solid-state compound between two or more metallic elements. Intermetallics are generally hard and brittle, and have useful mechanical properties at high temperatures.
Any force that mediates interaction between molecules, e.g. electromagnetic forces of attraction or repulsion, hydrogen bonding, and the van der Waals force, all of which act between the atoms of one molecule and the atoms or ions of nearby molecules. Intermolecular forces are weak compared to intramolecular forces such as covalent bonds, which hold individual molecules together.
An international federation of chemists that is recognized as the world authority in developing standards for chemical nomenclature and other methodologies in chemistry.
An electrostatic attraction between oppositely charged ions.
An ionic bond between a sodium atom (Na) and a fluorine atom (F). The sodium atom loses its sole valence electron (leaving the atom with a positive electrical charge), and the fluorine atom gains this same electron via an exothermic process (giving the atom a negative electrical charge). The oppositely charged ions are then attracted to each other to form a new compound called sodium fluoride.
The phenomenon of two or more chemical species (atoms, molecules, ions, etc.) being composed of different elements but having the same number of valenceelectrons 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.
A variant of a particular chemical element which differs in the number of neutrons present in the nucleus. All isotopes of a given element have the same number of protons in each atom.
The energy released upon the formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to exist initially in the gaseous state. Lattice energy can be viewed as a measure of the cohesive forces that bind ionic solids; it is therefore directly related to many other physical properties of the solid, including solubility, hardness, and volatility.
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.
A very large molecule comprising many atoms and bonds, or any molecule with a high relative molecular mass, especially one whose structure is formed by the multiple repetition of discrete subunits derived, actually or conceptually, from molecules with low relative molecular mass (e.g. monomers, substituents, and functional groups). The term is often used interchangeably with polymer.[2]
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.
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.
The alkyl group derived from methane, consisting of one carbon atom bonded to three hydrogen atoms, with the chemical formula CH 3. It is the simplest hydrocarbonfunctional group and occurs as a substituent in numerous organic compounds, though it may also exist independently as an ion or radical. The presence of a methyl substituent may be indicated with the prefix methyl in the name of the compound, or with the abbreviation Me in chemical formulae; e.g. methyl alcohol (methanol), which is often written with the formula CH 3OH or MeOH.
Various ways of depicting a methyl group in structural formulae
The tendency or capability of two or more substances (most commonly liquids, but also applicable to solids and gases) to blend uniformly when combined, i.e. to dissolve in each other, forming a homogeneous mixture that exists in a single phase, without separation of phases, regardless of the proportions of each substance. Substances that do not mix uniformly in all proportions are said to be immiscible.[4][2]
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).
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.
A measure of the concentration of a solute in a solution in terms of the amount of the solute per unit mass of the solvent. Molality is typically expressed in units of moles per kilogram (mol/kg); a solution with a concentration of exactly 1 mol/kg is sometimes said to be 1 molal. Contrast molarity.
Also molarity, amount concentration, or substance concentration.
A measure of the concentration of a chemical species, especially of a solute in a solution, in terms of the amount of the species per unit volume of solution. Molarity is typically expressed in units of moles per litre (mol/L); a solution with a concentration of exactly 1 mol/L is commonly said to be 1 molar, abbreviated 1 M. Contrast molality.
For a given chemical compound, the mass of a sample of that compound divided by the amount of compound in the sample, usually expressed in grams per mole (g/mol). As a bulk property, molar mass is an average of the masses of many instances of the compound, each of which may vary slightly due to the presence of isotopes of the compound's constituent atoms; it is commonly derived from the compound's molecular weight, which itself is a sum of the standard atomic weights of the constituent atoms, and is therefore a function of the relative abundance of the isotopes as they occur naturally on Earth. Molar mass allows easy conversion between mass and number of moles when considering bulk quantities of a substance.
A unit (symbol: mol) used to measure the amount of a substance in terms of the absolute number of particles or entities composing the substance. By definition, one mole of any substance contains exactly the Avogadro number (i.e. 6.022×1023) of particles or entities.
Having only one atom, as opposed to a molecule composed of more than one. Virtually all elements are monatomic in the gas phase at sufficiently high temperatures. Contrast diatomic and polyatomic.
A technique that exploits the magnetic properties of certain atomic nuclei, useful for identifying unknown compounds. Nuclear magnetic resonance is often abbreviated NMR.
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.
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.
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 valenceshell 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.
The spontaneous net movement or diffusion of molecules of a solvent (e.g. water) through a selectively permeable membrane separating two solutions with different concentrations of dissolved solutes, in the direction that tends to equalize the solute concentrations on the two sides, i.e. from the more dilute solution to the more concentrated solution, or, equivalently, from a region of high water potential to a region of low water potential. Because the solute is unable to cross the membrane, the tendency towards equilibration compels the solvent to cross the membrane instead. This continues until an equilibrium is reached, where neither side of the membrane is more or less concentrated than the other.
During osmosis, the transfer of solvent molecules out of the more dilute solution (in the left beaker, on the left side of the membrane) increases that solution's solute concentration, while the simultaneous addition of solvent to the more concentrated solution on the other side of the membrane decreases its own concentration. The eventual result is an equilibrium of the solute concentrations on both sides of the membrane, though the volumes on each side are no longer equal (right beaker).
Any of the metallic elements in the p-block, which are characterized by having a combination of relatively low melting points (all less than 950 K) and relatively high electronegativity values (all more than 1.6, revised Pauling).
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.
1. A chemical species that gains or accepts one or more electrons from another species, called the reducing agent, in a redox reaction, thereby causing the oxidation of the other species and in turn being itself reduced. The oxidizing agent's oxidation state decreases, while the reducing agent's increases.[3]
2. A chemical species that transfers strongly electronegativeatoms, usually oxygen, to a substrate.
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.
The process of coating a substance with a thin layer of a protective material, often a metal oxide, to create a shield against corrosion or other chemical reactions with the environment, thereby rendering the coated substance "passive" or less susceptible to undesirable reactions.
passivity
A state of chemical inactivity, especially of a metal that is relatively resistant to corrosion due to natural or induced loss of chemical reactivity (as with passivation).[4]
pentabasic
(of a chemical compound) Having five hydrogen atoms which may be replaced by metals or bases.[4]
A prefix in IUPAC chemical nomenclature meaning complete, exhaustive, or extreme, as in a completely substitutedhydrocarbon; or indicating the presence of a peroxy group.[4]
A tabular arrangement of the chemical elements organized by their atomic number, electron configuration, and other chemical properties, whose adopted structure shows periodic trends and is used by chemists to derive relationships between various elements as well as predict the properties and behaviors of undiscovered or newly synthesized elements. The first periodic table of the elements was published by Russian chemist Dmitri Mendeleev in 1869.
A functional group consisting of two oxygen atoms directly connected to each other by a single bond and each also connected to one other atom. Peroxides have the general structural formula –O–O–.
A logarithmic scale used to specify the acidity or basicity of an aqueous solution. The pH scale approximates the negative of the base-10 logarithm of the molar concentration of hydrogenions in a solution. At room temperature, pure water is neutral (pH = 7); solutions with a pH less than 7 are acidic and those with a pH greater than 7 are basic.
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.
A graphical representation of the equilibrium relationships between thermodynamically distinct phases of a chemical compound, mixture, or solution, indicating the physical conditions (e.g. temperature and pressure) under which various phases (e.g. solid, liquid, and vapor) occur or coexist.[4]
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.
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.
The process of producing a separable solidphase within a liquid medium, e.g. by transforming the dissolved solute of a supersaturated solution into an insoluble solid; or the diffusion of a distinct solid phase out of a solid alloy. A reagent that causes such a reaction is called the precipitant, and the separable solid itself is the precipitate.[4] More generally, the term may refer to the formation of any new condensed phase by changing the physical properties of a system (e.g. water vapor condensing into liquid water droplets).
precision
How close the results of multiple experimental trials or observations are to each other. Compare accuracy.
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.
The simplest, most commonly known, or canonical form of a chemical compound with multiple similar or isomeric forms. For example, in a primary alcohol, the carbon is bonded to a single substituent group (R 1CH 2OH), whereas a secondary alcohol is doubly substituted (R 1R 2CHOH) and a tertiary alcohol is triply substituted (R 1R 2R 3COH).[4]
An equimolarmixture 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.
Any atom, molecule, or ion that has at least one unpairedvalence electron. With few exceptions, such unpaired electrons make radicals highly chemically reactive, and therefore organic radicals are usually short-lived.
The branch of chemistry involving the study of radioactive substances and radioactivity, including the use of radioactive isotopes to study non-radioactive isotopes and ordinary chemical reactions.
A radioactivenuclide of a specified element, especially a particular isotope of that element which characteristically undergoes spontaneous decay into one or more stable nuclides by emitting excess energy from the nucleus.[2]
Also called rare-earth metals or used interchangeably with lanthanides.
Any of the 17 nearly indistinguishable, silvery-white, soft, heavymetallicelements belonging to a set including the lanthanide series (atomic numbers 57 through 71) as well as scandium and yttrium.
The step-by-step sequence of elementary reactions by which a larger chemical reaction or overall change occurs. A complete mechanism must describe and explain which bonds are broken and which are formed (and in what order), as well as all reactants, products, and catalysts involved; the amounts of each; all intermediates, activated complexes, and transition states; and the stereochemistry of each chemical species. Because the detailed processes of a complex reaction are not observable in most cases, a reaction mechanism is often a theoretical conjecture based on thermodynamic feasibility and what little support can be gained from experiment.
The tendency of a particular chemical substance to undergo a chemical reaction, either by itself or with other substances, generally referring to either or both of two distinct observations: whether or not a substance reacts under a specific set of circumstances, and how quickly it reacts (i.e. the reaction rate). Thermodynamically, a chemical reaction occurs because the products (taken as a group) exist at a lower free energy than the reactants, and hence are more energetically "stable", but the concept of reactivity may also embody kinetic factors, depending on the usage. Chemical stability and chemical compatibility are related but distinct concepts.
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.
A chemical species that loses or donates one or more electrons to another species, called the oxidizing agent, in a redox reaction, thereby causing the reduction of the other species and in turn being itself oxidized. The reducing agent's oxidation state increases, while the oxidizing agent's decreases.[3]
2. A material that is resistant to decomposition by heat, pressure, or chemical attack, and retains its strength and form at high temperatures, making it suitable for applications in environments exposed to such conditions. Refractories are usually polycrystalline, polyphase, inorganic, non-metallic, porous, and heterogeneous compounds.
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.
A chemical reaction that can proceed in either direction depending on the reaction conditions, i.e. from reactants to products or from products to reactants, especially implying one in which both conversions occur simultaneously. Contrast irreversible reaction.
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.
A measure of the solubility of an ionic solute, expressed as the arithmetic product of the concentrations of its ions in a fully saturated solution, with respect to the solute's particular dissociationequilibria and the particular ions present. For a dissociation equilibrium , the solubility product of the ionic solute is given by , where and are the concentrations of the solute's ionic constituents in a saturated solution. The solubility product is derived from and functions like the equilibrium constant of dissociation, though unlike an equilibrium constant it is not dimensionless. If the product of ionic concentrations in a solution exceeds the solubility product, then precipitation occurs.[3]
Any stabilizing interaction of a solute with a solvent, or a similar interaction between a solvent and groups of an insoluble material (e.g. the ionic groups of an ion-exchange resin). Such interactions generally involve electrostatic forces and van der Waals forces, as well as compound-specific effects such as hydrogen bonding.[2] See also dissolution.
The process of irradiating a substance with sound energy, usually at ultrasound (>20 kHz) frequencies, in order to agitate the particles in a sample for various purposes, such as increasing the rate of a chemical reaction or preparing vesicles in mixtures of surfactants and water.[2]
The heat capacity of a sample of a substance divided by the mass of the sample. Informally, it is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. The SI unit of specific heat capacity is joule per kelvin per kilogram (J/K/kg). Specific heat capacity often varies with temperature and with each state of matter.
A standardisation of ambient temperature and pressure used in order to easily compare experimental results. Standard temperature is 25 degrees Celsius (°C) and standard pressure is 100.000 kilopascals (kPa). Standard conditions are often denoted with the abbreviation STP or SATP.
The condition of matter existing in a distinct, homogeneous, macroscopic form. Solid, liquid, gas, and plasma are the four traditional states of matter and the most well-known. See also phase.
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, and vice versa.
An acid that completely dissociates in solution according to the reaction , or to such an extent that the concentration of the undissociated species is too low to be measured. Any acid with a pKa of less than approximately -2 is generally considered a strong acid; an example is hydrochloric acid (HCl). Contrast weak acid.
A graphical representation of the molecular structure and geometry of a particular chemical compound, showing how the atoms are arranged in real, three-dimensional space. Chemical bonding within the molecule is also shown, either implicitly or explicitly. When known with certainty, structural formulas are very useful because they allow chemists to visualize the molecules and the structural changes that occur in them during chemical reactions.
An atom or a group of atoms which replaces another atom or group of atoms within a larger molecule as the product of a chemical reaction, thereby becoming a moiety of the newly formed compound, generally without causing any significant change to other parts of the same molecule. For example, a hydroxyl group may be substituted for any of the hydrogen atoms in benzene to form phenol. See also side chain and functional group.
A substance which lowers the surface tension of the medium in which it is dissolved, and/or the interfacial tension with other phases, and, accordingly, is positively adsorbed at the liquid–vapor and/or other interfaces.[2]
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.
A thin layer of corrosion that forms on the surface of copper, brass, aluminum, magnesium, and other soft metals or alloys as their outermost layer undergoes a chemical reaction with the surrounding air, often but not necessarily involving atmospheric oxygen. Tarnish usually appears as a dull grey, black, or sometimes iridescent film or coating on the metal. It is a self-limiting surface phenomenon, as the tarnished top layers of the metal protect underlying layers from reacting.
A proportional measure of the average kinetic energy of the random motions of the constituent microscopic particles of a system. The SI unit for temperature is the kelvin.
A class of naturally occurring unsaturatedhydrocarbons with carbon skeletons derived from one or more units of isoprene (C 5H 8). Terpenes are often subclassified according to the total number of carbon atoms they contain, e.g. the C 5 hemiterpenes, C 10monoterpenes, C 20diterpenes, etc.
1. Any of a class of organosulfur compounds consisting of a sulfur atom attached to a hydrogen atom and any other organic substituent, with the general formula R–SH. Thiols are the sulfur analogues of alcohols. Also thiol derivative and mercaptan.
2. The –SH functional group itself. Also sulfhydryl, sulfanyl, and mercapto.
The skeletal formula for a generic thiol, where R denotes a variable carbon-containing substituent group
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.
An element whose atoms naturally occur with incompletely filled "d" sub-shells. These elements are grouped as the so-called d-block elements in the periodic table.
A bond that involves the covalent sharing of three pairs of electrons (for example, the diatomic nitrogen molecule, N2, is composed of two nitrogen atoms linked by a triple bond).
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.
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.
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.
The quantity of three-dimensional space enclosed by a closed surface, or the space that a substance (solid, liquid, gas, or plasma) or shape occupies or contains. The SI unit for volume is the cubic metre (m3).
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.
A polarinorganic 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.
Water molecules that are present inside crystals. Upon crystallization from water or aqueous solutions, many compounds incorporate water in their crystalline frameworks; the water molecules are typically present in a stoichiometric ratio and may interact to varying degrees with the atoms of the crystal.
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.
Any of several methods of purifying crystalline solids which involve applying heat to a small region of a larger solid (particularly a metal ingot) until localized melting occurs, creating a molten zone which is then slowly moved along the surface to other parts of the solid by moving the target of the heating element. As it moves, the forward edge of the molten zone continuously melts new areas of impure solid, while leaving a path of purer solid behind it as previously melted areas are cooled and resolidified; because the molten liquid phase can hold a higher concentration of impurities than the solid phase, the impurities of melted areas tend to concentrate in the molten zone and be carried along as it moves, leaving behind regions with fewer impurities. The process is commonly used in the refinement of high-purity metalloids for use in semiconductors.
^ abcdOtoxby, D. W.; Gillis, H. P.; Butler, L. J. (2015). Principles of Modern Chemistry (8th ed.). Brooks Cole. p. 617. ISBN978-1305079113.
^ abcdefghijklmnopqrIUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook.
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