Glossary of Chemistry Terms

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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](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#precision).

acid

A compound which, when dissolved in water, gives a pH of less than 7.0, or donates a hydrogen ion.

acid anhydride

A compound with two acyl groups bound to a single oxygen atom.

acid dissociation constant (Ka)

Also acid ionization constant or acidity constant.

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 often given as its base-10 cologarithm, pKa.

actinides

Also actinoids.

The periodic series of metallic elements with atomic numbers 89 to 103, from actinium through lawrencium.

activated complex

A structure that forms because of a collision between molecules while new bonds are formed.

activation energy

The minimum energy which must be available to a chemical system with potential reactants in order to result in a chemical reaction.

activity series

See [reactivity series](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#reactivity_series).

acyclic

Containing only linear structures of atoms (particularly in hydrocarbons).

addition reaction

In organic chemistry, when two or more molecules combine to make a larger one.

adhesion

The tendency of dissimilar particles or surfaces to cling to one another as a result of intermolecular forces. Contrast [cohesion](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#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.

aeration

The mixing of air into a liquid or a solid.

alcohol

Any organic compound consisting of a hydroxyl functional group attached to a saturated carbon atom.

aldehyde

Any organic compound consisting of a carbonyl group attached to a hydrogen atom and any other R-group.

alkali metal

Any of the metallic elements belonging to Group 1 of the periodic table: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).

alkaline earth metal

Any of the metallic elements belonging to Group 2 of the periodic table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

alkane

Any saturated acyclic hydrocarbon.

alkene

An unsaturated hydrocarbon containing at least one pair of double-bonded carbons.

alkyl group

A functional group consisting of an alkane missing a hydrogen atom.

alkyne

An unsaturated hydrocarbon containing at least one pair of triple-bonded carbons.

allomer

A substance that differs in chemical composition but has the same crystalline structure as another substance.

allotrope

Elements that can have different structures (and therefore different forms), such as carbon (diamonds, graphite, and fullerene).

alloy

A mixture of metals or of a metal and another element which in combination exhibit a metallic bonding character. Common examples include bronze, brass, and pewter.

amalgam

Any alloy of mercury with another metal.

amount of substance

Also enplethy, chemical amount, or simply amount.

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 base unit for amount of substance is the mole (mol).

analytical chemistry

The branch of chemistry which studies and makes use of instruments and methods to separate, quantify, and identify chemical substances, both by classical wet chemistry techniques such as precipitation, extraction, distillation, and observational analysis, and by modern instrumental techniques such as chromatography, spectroscopy, and electrochemistry.

anion

A negatively charged ion.

anode

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](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#cathode).

aqueous solution

A solution in which the solvent is water. It is denoted in chemical equations by appending (aq) to a chemical formula.

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.

aryl

Any functional group or substituent derived from an aromatic ring, such as phenyl or naphthyl. The symbol Ar is often used as a placeholder for a generic aryl group in structural diagrams.

atom

A chemical element in its smallest form, made up of protons and neutrons within the nucleus and electrons circling the nucleus.

An atom with protons, neutrons, and electrons labelled

atomic mass

The mass of an atom, typically expressed in unified atomic mass units and nearly equivalent to the mass number.

atomic mass unit

See [unified atomic mass unit](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#unified_atomic_mass_unit).

atomic number (Z)

Also proton number.

The number of protons found in the nucleus of an atom of a given chemical element. It is identical to the charge number of the nucleus and is used in the periodic table to uniquely identify each chemical element.

atomic orbital

Any region in which one or more electrons may be found in an individual atom (as opposed to that within a molecule).

Avogadro's number

Also Avogadro constant.

The number of discrete constituent particles (such as molecules, atoms, or ions) in one mole of a substance, defined as exactly 6.02214076×1023 such particles.

azeotrope

A mixture of liquids whose composition is unchanged by distillation.

barometer

A device used to measure atmospheric pressure.

base

A substance that accepts a proton and has a pH above 7.0. A common example is sodium hydroxide (NaOH).

base anhydride

Oxides of group I and II metal elements.

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.

boiling

See [vaporization](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#vaporization).

boiling point

Also vaporization point.

The temperature at which a substance changes state from a liquid to a gas (or vapor). It depends on pressure and is usually specified for a given substance under standard conditions.

boiling-point elevation

the process where the boiling point is elevated by adding a substance

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.

Brønsted–Lowry acid

Any chemical species that readily donates a proton.

Brønsted–Lowry base

Any chemical species that readily accepts a proton.

buffered solution

Also simply called a buffer.

An aqueous solution consisting of a weak acid and its conjugate base or a weak base and its conjugate acid that resists changes in pH when strong acids or bases are added.

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.

burette

Also spelled buret.

Glassware used to dispense specific amounts of liquid when precision is necessary (e.g. during titrations and resource-dependent reactions).

calorimeter

A device used to measure heat.

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](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#anode).

cation

A positively charged ion.

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.

cell potential

The force in a galvanic cell that pulls electrons through a reducing agent to an oxidizing agent.

charge number

A quantized value of electric charge calculated as the electric charge in coulombs divided by the elementary-charge constant, or z = q/e. Charge numbers for ions are denoted in superscript (e.g. Na+ indicates a sodium ion with a charge number of positive one). Atomic numbers are charge numbers of atomic nuclei.

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.

chelation

A type of bonding involving the formation of two separate coordinate covalent bonds between a polydentate ligand and a single central metal ion. The ligand is usually an organic compound called a chelant or [chelating agent](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#chelating_agent).

chemical composition

The identity and relative number of the elements that make up a chemical compound, which can often be expressed with a chemical formula.

chemical decomposition

The breakdown of a single particle or entity (such as a molecule or reactive intermediate) into two or more fragments, or a chemical reaction in which two or more products are formed from a single reactant. Contrast [chemical synthesis](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#chemical_synthesis).

chemical formula

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.

chemical law

A law of nature relevant to chemistry, such as the law of conservation of mass.

chemical process

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.

chemical reaction

The change of one or more substances into one or more different substances.

chemical species

Also simply called a chemical.

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.

chemical substance

Also pure substance or simply substance.

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.

chemical synthesis

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.

chemistry

The scientific discipline that studies chemical substances, compounds, and molecules composed of atoms of various chemical elements, as well as their compositions, structures, properties, behaviors, and the changes they undergo during reactions with other substances.

cohesion

The tendency of similar particles or surfaces to cling to one another as a result of intermolecular forces. Contrast [adhesion](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#adhesion).

colligative property

Any property of a solution 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.

colloid

A mixture of evenly dispersed substances, such as many milks.

combustion

An exothermic reaction between an oxidant and a fuel that produces large amounts of heat and often light.

An example of combustion

compression

An area in a longitudinal wave where the particles are closer and pushed in.

compound

A substance that is made up of two or more chemically bonded elements.

concentration

The abundance of a constituent of a mixture divided by the total volume of the mixture. Several different definitions of concentration are widely used in chemistry, including mass concentration, volume concentration, and molar concentration.

condensation

The phase transition of a substance from a gas to a liquid.

condosity

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).

conductor

Any object or material that allows the flow of an electric current in one or more directions. Contrast [insulator](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#insulator).

conformation

The spatial arrangement of atoms affording distinction between stereoisomers which can be interconverted by rotations about formally single bonds.

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.

coulomb

The SI unit of electric charge (symbol: C), defined as the charge transported by a constant current of one ampere in one second.

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.

crystallography

The branch of chemistry concerned with determining the arrangement of atoms within crystalline solids.

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.

deliquescence

A substance's affinity for water, often characterized as its tendency to absorb moisture from the atmosphere to form aqueous solutions. Most strongly deliquescent substances are salts, such as calcium chloride and potassium carbonate.

delocalized electron

Any electron in a molecule, ion, or solid metal that is not associated with an individual atom or covalent bond. The term may refer to electrons involved in resonance in conjugated systems or aromatic compounds; to free electrons which facilitate electrical conductivity; or to electrons within delocalized molecular orbitals encompassing several adjacent atoms.

density

An intensive property of a substance defined as mass per unit volume and expressed by the equation d = m/V.

denticity

The number of donor groups in a single ligand that bind to a central atom in a coordination complex.

deposition

The settling of particles within a solution or mixture.

Dewar flask

See [vacuum flask](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#vacuum_flask).

diatomic

Composed of two atoms, of the same or different elements. Contrast [monatomic](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#monatomic) and [polyatomic](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#polyatomic).

diatomic molecule

Any molecule composed of only two atoms, of the same or different elements.

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.

dipole moment

The polarity of a polar covalent bond.

dispersion

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.

dissolution

Also solvation.

The interaction of a solvent with the molecules or ions of a solute, involving bond formation, hydrogen bonding, and van der Waals forces.

A sodium ion (Na+) forms a solvation complex with water molecules when dissolved in an aqueous solution

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.

double bond

A bond involving the covalent sharing of two pairs of electrons.

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.

earth metal

See [alkaline earth metal](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#alkaline_earth_metal).

electric charge

A measured property (coulombs) that determines electromagnetic interaction.

electrolyte

A solution that conducts a certain amount of electric current and can be split categorically into weak and strong electrolytes.

electrochemical cell

Using a chemical reaction's current, electromotive force is made.

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.

electron

A type of subatomic particle with a net charge that is negative.

electron configuration

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.

electron pair

Two electrons which occupy the same molecular orbital but have opposite spins. Electron pairs form chemical bonds or occur as lone pairs of valence electrons; it is also possible for electrons to occur individually as unpaired electrons.

electron shell

An orbital around the nucleus of an atom which contains a fixed number of electrons (usually two or eight).

electronegativity (χ)

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.

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.

element

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.

elementary reaction

Any chemical reaction in which one or more chemical species react directly to form products in a single reaction step and with a single transition state, i.e. without any intermediates. Contrast [stepwise reaction](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#stepwise_reaction).

energy

A system's ability to do work.

enplethy

See [amount of substance](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#amount_of_substance).

enthalpy

A measure of the total internal energy of a thermodynamic system, usually symbolized by H.

entropy

The amount of energy not available for work in a closed thermodynamic system, usually symbolized by S.

enzyme

A biological protein catalyst that speeds up a chemical reaction.

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.

Eppendorf tube

A generalized and trademarked name used to refer to a microcentrifuge tube.

Erlenmeyer flask

A 500-milliliter Erlenmeyer flask

extensive property

A physical quantity whose value is proportional to the size of the system it describes or to the quantity of matter in the system. Examples include mass, volume, enthalpy, and entropy. Contrast [intensive property](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#intensive_property).

freezing

The phase transition of a substance from a liquid to a solid.

Faraday constant

A unit of electric charge widely used in electrochemistry which represents 1 mole of electrons: 6.022 × 1023 electrons. It is equal to approximately 96,500 coulombs (F = 96 485.339 9(24) C/mol).

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.

formal charge (FC)

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

F
C
=
V
−
N
−

B
2

{\displaystyle FC=V-N-{\frac {B}{2}}\ }

, where

V

{\displaystyle V}

is the number of valence electrons of the neutral atom in its ground state;

N

{\displaystyle N}

is the number of valence electrons of the atom which are not participating in bonds in the molecule; and

B

{\displaystyle B}

is the number of electrons shared in bonds with other atoms in the molecule.

free radical

See [radical](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#radical).

freezing point

Also crystallization point.

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.

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.

galvanic cell

A type of battery made up of electrochemicals with two different metals connected by a salt bridge.

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.

gas chromatography

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.

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.

geochemistry

The study of the chemistry and chemical composition of the Earth and geological processes.

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.

ground glass joint

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.

group

Also family.

A vertical column of the periodic table of the elements and the elements that share it. Contrast [period](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#period).

halogen

Any of the five non-metallic elements of Group 17 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).

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.

heat of fusion

See [enthalpy of fusion](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#enthalpy_of_fusion).

Hess' law of constant heat summation

Also simply called Hess' law.

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.

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.

hydrogen bond

A form of electrostatic interaction between an electronegative atom 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.

hydrolysis

The cleavage of a chemical bond by the addition of water.

ideal gas constant

Also universal gas constant.

The proportionality constant in the ideal gas law, defined as 0.08206 L·atm/(K·mol).

ideal gas law

A chemical law which states that the volume of a gas is proportional to the amount of gas and its Kelvin temperature and inversely proportional to its pressure.

ideal solution

A solution for which the gas phase exhibits thermodynamic properties analogous to those of a mixture of ideal gases.

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

inorganic compound

Any chemical compound that does not contain carbon, though there are exceptions. Contrast [organic compound](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#organic_compound).

inorganic chemistry

The branch of chemistry concerning the chemical properties and reactions of inorganic compounds. Contrast [organic chemistry](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#organic_chemistry).

insulator

Any material that resists the flow of an electric current. Contrast [conductor](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#conductor).

intensive property

A physical quantity whose value does not depend on the size of the system or the quantity of matter for which it is measured. Examples include density, temperature, and pressure. Contrast [extensive property](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#extensive_property).

International Union of Pure and Applied Chemistry (IUPAC)

An international federation of chemists that is recognized as the world authority in developing standards for chemical nomenclature and other methodologies in chemistry.

ion

A molecule that has gained or lost one or more electrons from its neutral state and therefore possesses a negative or positive electric charge.

ionic bond

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.

ionization

The breaking up of a chemical compound into separate ions.

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.

isomers

Ions or molecules with identical chemical formulas but distinct structures or spatial arrangements. Isomers do not necessarily share similar properties. The two main types of isomers are structural isomers and stereoisomers.

isotope

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

joule (J)

The SI unit of energy (symbol: J). One joule is defined as one Newton-meter.

kelvin (K)

The SI base unit of temperature (symbol: K). The Kelvin scale is an absolute thermodynamic temperature scale that uses absolute zero as its null point.

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

kinetics

A subfield of chemistry specializing in reaction rates.

kinetic energy

The energy of an object due to its motion.

lanthanides

Also lanthanoids.

The periodic series of metallic elements with atomic numbers 57 through 71, from lanthanum through lutetium.

lattice

The unique arrangement of atoms or molecules in a crystalline liquid or solid.

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.

ligand

An ion, functional group, or other molecule that binds to a central metal atom to form a coordination complex. Such bonding can range from covalent to ionic, but generally involves formal donation of one or more of the ligand's electron pairs to the metal.

light

Also referred to as visible light.

The portion of the electromagnetic spectrum which is visible to the unaided human eye.

liquid

One of the four fundamental states of matter, characterized by nearly incompressible fluid particles that retain a definite volume but no fixed shape.

London dispersion forces

A type of weak intermolecular force.

malleability

See [ductility](https://en.wikipedia.org/wiki/Glossary_of_chemistry_terms#ductility).

manometer

An instrument used to measure pressure invented by Evangelista Torricelli in 1643.

mass

A property of physical matter that is a measure of its resistance to acceleration when a net force is applied. The SI base unit for mass is the kilogram (kg).

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.

matter

Any substance that has mass and takes up space by having volume.

metal

Any chemical element which is a good conductor of both electricity and heat and which readily forms cations and ionic bonds with non-metals.

melting

The phase transition of a substance from a solid to a liquid.

melting point

Also liquefaction point.

The temperature at which a substance changes state from a solid to a liquid. It depends on pressure and is usually specified for a given substance under standard conditions. The melting point of a substance is identical to its freezing point.

metalloid

A chemical element or substance possessing properties of both metals and non-metals.

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