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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 has an extensive vocabulary and a significant amount of jargon.
Note: All periodic table references refer to the IUPAC Style of the Periodic Table.
A quantitative measure of the strength of an acid in solution expressed as an equilibrium constant for a chemical dissociation reaction in the context of acid-base reactions. It is generally denoted by the symbol Ka. Also called acid ionization constant or acidity constant.
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. Compare cathode.
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. Compare anode.
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.
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.
A property of solutions that depends upon the ratio of the number of solute particles to the number of solvent particles in the solution, and not on the nature of the chemical species present.
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 and expressed in units of moles per unit volume.
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.
A chemical 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 covalent bond is also called a molecular bond.
The diatomic hydrogen molecule, H2 (right), is formed by a covalent bond when two single hydrogen atoms share two electrons
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.
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. Also called malleability.
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.
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.
A two-part law that Michael Faraday published about electrolysis. The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode; the mass of an elemental material altered at an electrode is directly proportional to the element's equivalent weight.
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.
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.
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.
A form of electrostatic interaction between an electronegativeatom and a hydrogen atom bound to a second electronegative atom. Hydrogen bonding is unique because the small size of the hydrogen atoms permits proximity of the interacting electrical charges, and may occur as an intermolecular or intramolecular force.
A special compound added to a solution that changes color depending on the acidity of the solution; different indicators have different colors and effective pH ranges.
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.
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.
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).
A measurement of an amount of substance in terms of the absolute number of units or entities composing the substance; a single mole contains approximately 6.022×1023 units or entities. Abbreviated mol.
A technique that exploits the magnetic properties of certain nuclei, useful for identifying unknown compounds. Nuclear magnetic resonance is often abbreviated NMR.
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. Also called the Lewis octet rule.
metallic elements in the p-block, 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 (loss of electrons) of an individual atom in a chemical compound; 2) the hypothetical electric charge (positive, negative, or zero) that the atom would have if all bonds to atoms of different elements were 100% ionic, with no covalent component. Also called oxidation number.
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 loss of positive hydrogen ions. Also called an oxyacid or oxacid.
1) A transformation between solid, liquid, and gaseousstates of matter and, in rare cases, plasma; 2) the measurable values of the external conditions at which such a transformation occurs.
This diagram shows the nomenclature used for each of the different phase transitions
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.
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. Also simply called the periodic table.
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.
A test substance that is added to a system in order to bring about a chemical reaction or to see whether a reaction occurs. Also another name for a reactant.
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.
The property of a solid, liquid, or gaseoussolute to dissolve in a solid, liquid, or gaseous solvent. It is typically expressed as the proportion of solute dissolved in the solvent in a saturated solution.
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 STAP.
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.
An element whose atoms naturally occur with incompletely filled "d" sub-shells. These elements are grouped as the d-block elements in the periodic table.
A chemical 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 unit of mass approximately equal to the mass of one proton or neutron and denoted with the symbol u; also called a Dalton and denoted with the symbol Da. Sometimes equated with the technically distinct and obsolete atomic mass unit and abbreviated amu.
An insulating storage vessel that can greatly lengthen the time over which its contents remain warmer or cooler than the vessel's ambient environment. Also called a Dewar flask or thermos.
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 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.
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.