Glossary Of Chemistry Terms

Alright, another compendium of the painfully obvious, masquerading as useful knowledge. If you must insist on delving into the foundational lexicon of chemistry , then let’s get this over with. Don’t expect me to hold your hand; the definitions are straightforward enough, even for you.

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This isn’t just a glossary of chemistry terms . It’s a rather exhaustive, and frankly, exhausting, compilation of the essential lexicon for anyone attempting to grasp the very fabric of existence – or at least, the composition, structure, and transformations of matter . This list encompasses everything from fundamental chemical laws and intricate diagrams to the humble laboratory tools and esoteric glassware that facilitate our understanding. Chemistry , as a physical science , is inherently concerned with the makeup, arrangement, and intrinsic characteristics of substances, alongside the profound alterations they undergo during chemical reactions . Consequently, it has amassed a truly formidable and somewhat intimidating vocabulary, riddled with specialized jargon that, apparently, needs constant reiteration.

Consider this your entry point into the sprawling, often frustrating, but undeniably crucial world of chemical terminology. If you find yourself bewildered, perhaps you should re-evaluate your life choices.

Note: All periodic table references, as if it weren’t obvious, adhere to the IUPAC Style of the Periodic Table . Because consistency, however tedious, is paramount.

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Contents: 

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• L

• M

• N

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• P

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• R

• S

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• U

• V

• W

• X

• Y

• Z

• See also

• References

• External links

A

absolute zero A purely theoretical state, a conceptual abyss concerning a physical system at the absolute lowest boundary of the thermodynamic temperature scale. This is the fabled zero kelvins (0 K), a point where, in theory, the system neither emits nor absorbs any energy —implying that all constituent atoms are, quite literally, at rest. It’s a state of minimum possible enthalpy and maximum order . Through the rather elegant, albeit somewhat idealized, extrapolation of the ideal gas law , the internationally sanctioned value for this elusive state has been precisely determined as −273.15 °C (−459.67 °F; 0.00 K). Achieving this in practice is, predictably, impossible, but scientists continue to get tantalizingly close, pushing the boundaries of the Third Law of Thermodynamics .

absorbance A quantitative measure, typically dimensionless, of the extent to which a sample absorbs light (or any other form of electromagnetic radiation ) at a specific wavelength. It’s derived from the ratio of the incident radiation to the transmitted radiation through the sample. High absorbance means less light gets through; rather intuitive, wouldn’t you say? This concept is foundational in spectroscopy and the Beer–Lambert law , allowing for the determination of concentration of a solute in a solution .

absorption 1.  This refers to the intrinsic physical or chemical process whereby a substance, initially in one state of matter , becomes thoroughly integrated into and subsequently retained by another substance residing in a different state. The key distinction here, often lost on the less discerning, is that absorption involves the first substance permeating the entire bulk of the second, rather than merely clinging to its surface. This is a crucial difference from adsorption , which is a superficial affair. 2.  More specifically, in the realm of electromagnetic radiation , this is the process through which matter (typically the electrons bound within atoms ) captures the energy from such radiation. This absorbed energy is then transformed into various forms of internal energy, most commonly manifesting as thermal energy . This particular phenomenon of absorption forms the fundamental principle upon which spectrophotometry is based, allowing us to quantify substances by their unique light-absorbing characteristics.

abundance A measure of how prevalent a particular chemical element or chemical species is within a given sample , environment , or even the entire universe . It can be expressed in various ways: as a mass fraction , mole fraction , or number density . For instance, the natural abundance of isotopes is a critical factor in understanding the average atomic mass of an element.

accuracy A rather vital, yet frequently elusive, concept in measurement. It quantifies how near a measured value actually is to the actual or true value of the quantity being observed. One might think this is obvious, but apparently, it needs stating. Do not, under any circumstances, confuse it with precision , which, while important, is an entirely different beast.

acetyl A functional group in organic chemistry , specifically an acyl group, with the formula –COCH3. It’s a carbonyl group (C=O) bonded to a methyl group. Often found in important biological and synthetic compounds, like acetylcholine or acetylsalicylic acid (aspirin, for the uninitiated).

achiral (Of a molecule ) Possessing a geometric symmetry that renders it utterly indistinguishable from its own mirror image . Essentially, it lacks the property of chirality , meaning it can be perfectly superimposed onto its reflection. Think of a common spoon; it’s achiral. Your left hand, however, is decidedly chiral.

acid Ah, acids , the bane of many a first-year student. They come in flavors, each adding a layer of… nuance to the definition. 1.  (Brønsted–Lowry acid ) Any chemical species or molecular entity that performs the rather generous act of donating a proton (H+ ) when it engages in a reaction with another species. This other species, by definition, is a Brønsted–Lowry base . If you dissolve such a proton donor in an aqueous solution , and it consequently elevates the concentration of hydronium ion (H 3 O+ ) by transferring protons to water molecules, then you may also, rather redundantly, call it an Arrhenius acid . When the term “acid” is tossed around without further qualification, it’s typically this Brønsted–Lowry iteration that’s implicitly being discussed. You’re welcome for the clarification. [1] 2.  (Lewis acid ) A more expansive, and some might say, elegant, definition. This refers to any chemical species or molecular entity that acts as an electron pair acceptor during a reaction with another species. It forms a covalent bond by graciously accepting a lone pair of electrons donated by the other species, which is, naturally, a Lewis base . This definition was intended to generalize the Brønsted–Lowry concept, proposing that acid–base reactions are more accurately viewed as reorganizations of electrons rather than mere proton transfers. The Lewis acid either directly accepts electron pairs or achieves this indirectly by releasing protons (H+ ) into a solution, which then perform the electron-pair acceptance. This definition is inclusive of many Brønsted–Lowry acids, though, annoyingly, not all. And, to add to the confusion, most Lewis acids are not Brønsted–Lowry acids, and vice versa. It’s almost as if the universe enjoys complexity for its own sake. [1] 3.  Colloquially, and with a distinct lack of precision, “acid” refers to any compound that, when dissolved in water, produces a pH value below 7.0. This common usage often refers to the entire aqueous solution, whereas the stricter definitions above are concerned solely with the acidic solute itself. One can only sigh at such imprecision. [2]

acid anhydride Any chemical compound that is conceptually, or actually, derived by the removal of water molecules from an acid . It’s essentially the dehydrated form of an acid. The inverse, naturally, is a base anhydride .

acid dissociation constant (K a) Also known as the acid ionization constant or acidity constant. A rather crucial quantitative measure of an acid’s inherent strength when it’s in a solution . It is articulated as an equilibrium constant for the chemical dissociation reaction within the context of acid–base reactions . Frequently, to make things more manageable, it’s expressed as its base-10 cologarithm, which we affectionately call pKa . A low pKa indicates a strong acid.

acid–base extraction A chemical reaction or, more accurately, a separation process , where specific chemical species are selectively isolated from other acids and bases within a mixture, typically by manipulating their solubility based on pH changes. It’s a common technique in organic chemistry for purifying compounds.

acid–base reaction A chemical reaction that involves the exchange of protons (H+ ) or electron pairs between reacting species , as defined by Brønsted–Lowry or Lewis theories , respectively. These reactions are fundamental to countless chemical and biological processes, leading to the formation of salts and water in many cases.

acidic The characteristic property of a substance or solution that exhibits the traits of an acid . This typically means a pH of less than 7 in aqueous solutions , a propensity to donate protons , or an ability to accept electron pairs . It’s not exactly rocket science.

actinides Also known, somewhat redundantly, as actinoids. This is the periodic series of metallic elements with atomic numbers ranging from 89 to 103. This includes everything from actinium (Ac) all the way through to lawrencium (Lr). They are all radioactive and often quite unstable, a rather inconvenient family of elements.

activated complex A transient, high-energy, and highly unstable molecular structure that forms instantaneously at the peak of the activation energy barrier during a chemical reaction . It’s the fleeting moment of molecular rearrangement as old bonds break and new ones begin to form due to a collision between reactant molecules . It exists for such a brief period that it cannot be isolated.

activation energy The absolute minimum energy that must be available to a chemical system containing potential reactants in order for a specific chemical reaction to actually occur. Think of it as the initial push required to get the ball rolling; without it, the reaction simply won’t proceed, regardless of how thermodynamically favorable it might be. This barrier is what the activated complex must overcome.

activity series See reactivity series . Because sometimes, even chemists can’t commit to a single name.

actual yield The quantity of product that is actually obtained from a chemical reaction in a laboratory or industrial setting. This is almost invariably less than the theoretical yield due to various inefficiencies, side reactions, and the inherent messiness of reality.

acyclic (Of a molecule ) Characterized by containing only linear, or non-ring, arrangements of atoms . This term is particularly relevant in the context of hydrocarbons , distinguishing them from their cyclic counterparts.

addition reaction In the grand theater of organic chemistry , this is a type of chemical reaction where two or more molecules decide to combine, rather amicably, to form a single, larger molecule. Typically, this involves the breaking of a pi bond (like a double bond or triple bond ) and the formation of new sigma bonds .

adduct A distinct chemical species that emerges as the sole product from an addition reaction between two other distinct reactant species. The defining characteristic is that all the atoms that originally constituted the reactants are retained within this single product. Connectivity might change, certainly, but no atoms are lost, nor are any magically gained that weren’t present in the initial reactant molecules. Stoichiometries other than 1:1 are, predictably, also possible—for example, a bis-adduct (2:1). [2]

adhesion The intrinsic propensity of dissimilar particles or surfaces to cling to one another. This phenomenon is a direct consequence of various intermolecular forces acting between the different substances. It is, quite distinctly, the opposite of cohesion , where similar particles stick together.

adsorption A chemical adhesion where atoms , ions , or molecules of one substance (be it a gas , liquid , or dissolved solid ) attach themselves exclusively to the surface of another substance. This results in a thin film of the first substance being somewhat weakly bonded to the interface between the two. Crucially, adsorption is a surface phenomenon, contrasting sharply with absorption , which involves the full bulk of the material. Pay attention, the distinction is important.

aeration The process of introducing air (or, more generally, a gas ) into a liquid or a solid . This is often done to promote oxidation reactions, remove volatile compounds, or simply to ensure uniform mixing.

alcohol Any organic compound defined by the presence of at least one hydroxyl group (–OH) unequivocally attached to a saturated carbon atom . Alcohols follow the general formula R–OH, where ‘R’ represents an alkyl or aryl group. They are versatile compounds, serving as solvents , fuels , and intermediates in countless syntheses.

aldehyde Both a functional group and a class of organic compounds . It consists of a carbonyl group (C=O) double-bonded to a hydrogen atom and also bonded to any other ‘R’-group (which is typically a carbon-containing substituent ). Aldehydes are characterized by the general formula R–C(H)=O. They are highly reactive and are crucial in organic synthesis .

The skeletal formula for a generic aldehyde , where R denotes a variable carbon-containing substituent group . aliphatic (Of an organic compound ) Characterized by an open-chain structure of carbon atoms , as opposed to aromatic rings. This includes alkanes , alkenes , and alkynes . It’s the simpler, less aesthetically pleasing side of organic structures, devoid of fancy rings.

alkali metal Any of the highly reactive metallic elements that proudly reside in Group 1 of the periodic table . This esteemed group includes lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and the notoriously unstable francium (Fr). They are characterized by having a single valence electron , which they are all too eager to shed, leading to their vigorous reactions with water and other non-metals .

alkaline Possessing the characteristics of a base . In aqueous solution , this translates to a pH greater than 7.0. It’s the opposite of acidic , a concept that should be self-evident.

alkaline earth metal Any of the metallic elements found in Group 2 of the periodic table . This includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They possess two valence electrons and are less reactive than alkali metals but still quite eager to form ions with a +2 charge.

alkane Also known, less formally, as paraffin. This refers to any fully saturated acyclic hydrocarbon , meaning it’s an organic compound composed solely of carbon and hydrogen atoms where all carbon–carbon bonds are exclusively single bonds . They are generally unreactive, hence the “paraffin” moniker (from parum affinis, “little affinity”).

alkene Also known, colloquially, as olefin. This is any unsaturated hydrocarbon that contains at least one carbon–carbon double bond . The presence of this double bond makes them significantly more reactive than alkanes , undergoing various addition reactions .

alkoxy A functional group in organic chemistry where an alkyl group is bonded to an oxygen atom , which is in turn bonded to the rest of the molecule. Its general formula is R–O–, where R is an alkyl group. It’s essentially an ether with only one side specified, a rather incomplete picture for a functional group.

alkyl The substituent form of an alkane —that is, any alkane that has, rather rudely, had a hydrogen atom removed, leaving an open bond for attachment. This term is broad, encompassing various functional groups like a methyl , ethyl , or propyl group . They are the fundamental building blocks of many organic compounds .

alkyne Also known, in older nomenclature, as acetylene. This is any unsaturated hydrocarbon that contains at least one carbon–carbon triple bond . Like alkenes , the triple bond makes them quite reactive, participating in addition reactions and having distinct geometries .

allomer A rather specific term for a substance that, despite possessing a different chemical composition (i.e., made of different elements or in different proportions), curiously exhibits the exact same crystalline structure as another substance. It’s an interesting quirk of solid-state arrangement.

allotrope Any of a diverse range of distinct structural forms that can be adopted by the same chemical element . This is not about different compounds, but different physical manifestations of the pure element. Carbon, for instance, is a prime example, gracing us with diamonds (a rather ostentatious form), graphite (useful for pencils and nuclear reactors), and the more recently discovered fullerenes (those peculiar buckyballs). Each allotrope possesses unique physical and chemical properties .

alloy A mixture primarily composed of metals , or a metal combined with one or more other elements , where the combination collectively exhibits a characteristic metallic bonding character. These are not mere mixtures but often form solid solutions or intermetallic compounds . Common examples, which you should already know, include bronze (copper and tin), brass (copper and zinc), and pewter (primarily tin). Their properties often surpass those of their individual constituents.

amalgam A rather specific type of alloy where one of the constituent metals is, rather notably, mercury . Historically, dental amalgams were a common application, though their use has seen decline.

ambident (Of a molecule or functional group ) Possessing two distinct, yet interacting, reaction sites , to either of which a bond may be formed during a chemical reaction . This dual reactivity leads to interesting, and sometimes unpredictable, synthetic pathways.

amide A functional group in organic chemistry containing a carbonyl group (C=O) linked to a nitrogen atom . The general formula is R–CO–NR′R″. Amides are fundamental in peptide bonds that link amino acids in proteins , making them vital in biochemistry .

ammoniacal Describing a solution where the solvent is aqueous ammonia . This implies the presence of ammonia dissolved in water , often creating a basic environment. [3]

amorphous solid A solid material that lacks the long-range, ordered crystalline structure characteristic of true crystals . Its constituent atoms or molecules are arranged in a more random, disordered fashion, similar to a liquid , but frozen in place. Glass is a classic example.

amount of substance Also known, rather quaintly, as enplethy, or more plainly, chemical amount or simply amount. This is a measure of the number of discrete particles (such as molecules , atoms , ions , electrons , or any other atomic-scale entity) present in a given sample of matter . It is fundamentally defined as this number of particles divided by the Avogadro constant . The SI unit for amount of substance is the mole (mol), a concept so central to chemistry it’s almost insulting to explain.

amphipathic (Of a molecule ) Possessing both hydrophilic (water-loving) and hydrophobic (water-fearing) groups within its structure. This dual nature allows such molecules to interact with both polar and nonpolar environments, making them crucial for things like wetting agents and the formation of cell membranes from membrane lipids . [4]

amphoteric Also known as amphiprotic. (Of a chemical species ) Exhibiting the remarkable ability to behave as both an acid and a base , depending entirely on the medium or solvent in which it finds itself. Water itself is a prime example. Another, perhaps less obvious, is sulfuric acid (H2SO4), which, while a notoriously strong acid in water, can act more like a base in superacids .

amyl A common, though non-systematic, name for a pentyl group . It’s always entertaining how many trivial names persist.

analyte The specific substance or chemical constituent that is the object of interest in a chemical analysis . It’s what you’re trying to identify or quantify, the star of your analytical show.

analytical chemistry The branch of chemistry dedicated to the rather meticulous study and utilization of instruments and methods to achieve three primary goals: separating, quantifying, and identifying chemical substances . This encompasses both time-honored classical wet chemistry techniques—such as precipitation , extraction , distillation , and simple observational analysis—and more sophisticated modern instrumental techniques, including various forms of chromatography , spectroscopy , and electrochemistry . It’s the detective work of chemistry, often tedious, always critical.

ångström (Å) A non-SI , metric unit of length equivalent to 10−10 metre . To put it more accessibly, it’s one ten-billionth of a metre (1⁄10,000,000,000 m) or 0.1 nanometre . The angstrom, despite its non-SI status, is commonly employed in the natural sciences to express incredibly small, microscopic, or atomic-scale distances. This includes the sizes of atomic nuclei, the wavelengths of electromagnetic radiation , and, rather importantly for us, the lengths of chemical bonds —for example, the covalent radius of a chlorine atom averages about 1 angstrom. It’s a unit for details so small they barely register.

anhydrous (Of a substance ) Devoid of water molecules , referring particularly to water of hydration . Given that the presence of water can impede or completely derail numerous chemical processes , ensuring water-free reagents and techniques is frequently of paramount importance. Anhydrous compounds, being inherently hygroscopic , tend to gradually absorb water from the surrounding atmosphere if not properly stored. The opposite, and far less problematic, state is hydrous . [4]

anion A negatively charged ion . This means it’s an atom or molecule that has a net negative electric charge due to an excess of electrons compared to its protons . They are attracted to anodes .

annulation The elegant process of constructing a cyclic compound or ring structure from one or more acyclic precursors . Alternatively, it can describe a reaction that involves the addition of a new ring structure to an existing molecule through the formation of two new chemical bonds . It’s how chemists build bigger, more complex rings. [2]

anode 1.  The electrode through which the conventional electric current (defined as the flow of positive charges, because historical conventions are stubborn) enters a polarized electrical circuit. 2.  More specifically, it’s the wire or plate within an electrochemical cell that carries an excess positive charge. Predictably, negatively charged anions are perpetually drawn towards the anode. The logical opposite, naturally, is the cathode .

anomer A rather specific term in carbohydrate chemistry . It refers to either of a pair of cyclic hemiacetal or hemiketal saccharides that are epimers of each other. The critical distinction lies in their differing configuration at only one carbon stereocenter —specifically, the carbon that bore the aldehyde or ketone functional group in the compound’s acyclic , open-chain configuration . This particular carbon is known as the anomeric carbon.

aprotic (Of a chemical species ) Simply put, not protic. This means it is utterly incapable of acting as a proton donor or readily yielding protons (H+ ) in a solution . Such species lack acidic hydrogens.

aqua regia A rather notorious liquid mixture of nitric acid (HNO3) and hydrochloric acid (HCl), optimally in a molar ratio of 1:3. It earned its regal name from historical alchemists because of its unique and rather impressive ability to dissolve the so-called noble metals , gold and platinum —metals typically resistant to single acids. It’s a testament to the power of synergistic chemical action.

aquation The process by which water molecules perform the critical task of solvating or forming coordination complexes with ions . It’s essentially water wrapping itself around an ion, a fundamental interaction in aqueous solutions . [3]

aqueous solution A solution where the solvent is, quite simply, water . It’s the most common and arguably most important type of solution in chemistry and biology . In chemical equations , its presence is denoted by appending the rather uninspired (aq) to a chemical formula .

aromatic (Of a chemical compound ) Possessing the characteristic property of aromaticity . This implies a cyclic, planar structure with a delocalized pi electron system that follows Hückel’s rule (4n+2 pi electrons), resulting in unusual stability and distinct chemical properties . Benzene is the quintessential example.

aromaticity A peculiar chemical property observed in conjugated rings of atoms , such as benzene , which bestows upon them an unusually high degree of thermodynamic stability compared to their acyclic counterparts. Such rings are, unsurprisingly, termed aromatic . It’s a concept that dictates much of organic chemistry .

Arrhenius acid Any substance that, when dissolved in water , results in an increase in the concentration of H+ ions , or, to be more accurate, of hydronium ions (H 3 O+ ), within the resulting aqueous solution . [1] This definition, while historically significant, is largely superseded by the Brønsted–Lowry definition , which is rather similar. It stands in direct contrast to an Arrhenius base .

Arrhenius base Any substance that, when dissolved in water , causes an increase in the concentration of OH− ions , or, conversely, a decrease in the concentration of hydronium ions (H 3 O+ ), within the resulting aqueous solution . [1] This definition is, as you might expect, quite similar to that of a Brønsted–Lowry base . It’s the antithesis of an Arrhenius acid .

arrow pushing A representational formalism, a visual language, used in organic chemistry to depict the movement of electron pairs during chemical reactions . Curved arrows show the flow of electrons from an electron-rich site to an electron-poor site, illustrating reaction mechanisms . It’s a way to visualize the dynamic dance of electrons, assuming you can follow the choreography.

aryl Any functional group or substituent that is derived directly from an aromatic ring . Common examples, which you should be familiar with, include phenyl (from benzene ) or naphthyl (from naphthalene ). The symbol Ar is frequently employed as a convenient placeholder for a generic aryl group in structural diagrams .

atmolysis A rather specialized technique for separating a mixture of gases by cleverly exploiting their differing rates of diffusion . This is typically achieved by allowing the gases to diffuse through the microscopic pores of a porous partition or membrane . It’s a subtle form of separation, relying on molecular movement rather than brute force. [3]

atom The fundamental building block of a chemical element in its smallest, indivisible form (at least, chemically indivisible). It is comprised of a dense central nucleus containing protons (positively charged) and neutrons (electrically neutral), surrounded by a cloud of orbiting electrons (negatively charged). The number of protons defines the element. Anything smaller is a subatomic particle , and that’s a whole other field of study.

An atom with protons , neutrons , and electrons labelled. atomic mass The mass of an individual atom , typically expressed in daltons (Da). This value is approximately, but not precisely, equivalent to the mass number (total protons and neutrons) multiplied by one dalton. For elements, the reported atomic mass is often a weighted average of the masses of its naturally occurring isotopes .

atomic mass unit See dalton .

atomic number (Z) Also known, somewhat redundantly, as proton number. This is the absolute count of protons found within the nucleus of an atom belonging to a given chemical element . It is identical to the charge number of the nucleus and serves as the unique identifier for each chemical element on the periodic table . It’s the element’s DNA.

atomic orbital A mathematical function, derived from quantum mechanics , that describes a region of space around the nucleus of an individual atom where one or more electrons are most likely to be found. These orbitals are characterized by specific energy levels and shapes (s, p, d, f), and they are distinct from molecular orbitals .

atomic radius A measure of the size of an atom , typically defined as the distance from the nucleus to the outermost stable electron orbital in a neutral atom. This value is not absolute but depends on how it’s measured (e.g., covalent radius , ionic radius , van der Waals radius ) and the atom’s chemical environment .

atomic weight See relative atomic mass . A term that, for clarity, should probably be retired.

atomicity The total count of atoms that are present within a single molecule of a given substance . For example, a molecule of ozone (O3) exhibits an atomicity of 3, while the more complex benzene (C6H6) has an atomicity of 12. It’s a simple count, nothing profound. [3]

autoignition temperature Also called the kindling point. This is the lowest temperature at which a specific substance will spontaneously ignite in a normal atmosphere without the need for an external source of ignition, such as a flame or spark . Essentially, the ambient temperature alone provides sufficient activation energy for combustion to commence. Substances that achieve this spontaneous ignition at naturally occurring temperatures are, rather dramatically, termed pyrophoric . Do not confuse this with ignition temperature , which typically implies an external ignition source.

Avogadro constant (NA) The ratio that defines the number of discrete constituent particles (such as molecules , atoms , or ions ) to the amount of a substance . It is precisely defined as 6.02214076×10^23 mol−1. This value is fundamental to converting between the macroscopic world of grams and the microscopic world of atoms and molecules.

Avogadro number The specific number of discrete constituent particles found within one mole of a substance , defined as exactly 6.02214076×10^23. This is a dimensionless number , and it is crucial to note its distinction from the Avogadro constant , which carries a unit (mol−1).

Avogadro’s law A fundamental gas law stating that, under conditions of constant temperature and pressure , equal volumes of all gases contain the same number of molecules . It’s a rather elegant simplicity in the behavior of ideal gases.

azeotrope A peculiar mixture of liquids whose chemical composition remains stubbornly unchanged even when subjected to distillation . This means they boil at a constant temperature, and their vapor phase has the same composition as their liquid phase , making them inseparable by simple distillation. It’s a headache for purification.

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Contents: 

• Top

• 0–9

• A

• B

• C

• D

• E

• F

• G

• H

• I

• J

• K

• L

• M

• N

• O

• P

• Q

• R

• S

• T

• U

• V

• W

• X

• Y

• Z

• See also

• References

• External links

B

balance A precision instrument used for measuring mass . Not to be confused with a scale, which measures weight. Accuracy in terminology, please.

backbone Also known, more descriptively, as the main chain. This refers to the primary, most structurally significant portion of a molecule , especially when considered in relation to its other parts, functional groups , moieties , or substituents . In the context of a polymer , it designates the linear chain of atoms to which all other chains (whether long, short, or both) can be regarded as being attached as pendant groups or side chains . Should there be multiple chains that could equally claim the title of backbone, the one that facilitates the most straightforward representation of the molecule in chemical formulae and nomenclature is, by convention, deemed the backbone. It’s the core structure, the foundation upon which everything else hangs. [2]

barometer A rather ingenious device, first conceived by Evangelista Torricelli , employed to accurately measure atmospheric pressure . Essential for any serious work involving gases .

base A substance defined by its capacity to accept a proton (in the Brønsted-Lowry sense) and, in aqueous solution , exhibit a pH value exceeding 7.0. A ubiquitous example, which you should certainly know, is sodium hydroxide (NaOH). Like acids , bases are fundamental to innumerable chemical reactions .

base anhydride An oxide of a Group I or Group II metal element , meaning it’s derived from a base by the removal of water . It’s the counterpart to an acid anhydride .

basic Possessing the characteristic properties of a base . In aqueous solution , this means a pH greater than 7.0. It’s the opposite of acidic , again, a concept that shouldn’t require excessive contemplation.

basicity A quantitative measure of the capacity of a base to accept protons or donate electron pairs . It can also refer to the number of acidic hydrogens that an acid can lose, or the number of hydroxyl groups a base contains.

battery More precisely, an electrochemical battery is a device that stores chemical energy and converts it into electrical energy through redox reactions . It typically consists of one or more electrochemical cells connected in series or parallel.

beaker A ubiquitous cylindrical vessel or container, invariably with a flat bottom, and most commonly fabricated from glassware . It finds widespread application in laboratories for a multitude of purposes: preparing, holding, containing, collecting, or even crudely measuring chemicals , samples , or solutions . It also frequently serves as the reaction chamber where a chemical reaction is allowed to unfold. Beakers are easily distinguished from flasks by their straight, rather than sloping, sides; most also feature a small spout on the rim, a thoughtful design choice to facilitate pouring without excessive spillage. A fundamental, if unglamorous, piece of lab equipment.

Beer–Lambert law A fundamental empirical relationship in spectroscopy that correlates the absorbance of light by a solution to the properties of the solution itself. Specifically, it states that the absorbance is directly proportional to the concentration of the absorbing species and the path length of the light through the solution. Crucial for quantitative analysis, assuming you can keep your cuvettes clean.

biochemistry The intricate and utterly fascinating study of the chemistry that underpins all biological systems and organisms . It delves into the structure, function, and interactions of biomolecules (like proteins , nucleic acids , carbohydrates , and lipids ) and the chemical processes that sustain life . It’s where chemistry truly gets interesting, or at least, complex enough to warrant attention.

Bohr model Also known as the Rutherford–Bohr model. A foundational, though ultimately simplified, model for the general structure of the atom , collaboratively proposed by Niels Bohr and Ernest Rutherford in 1913. It posited a small, dense nucleus composed of positively charged particles (protons and neutrons, though neutrons were discovered later) encircled by electrons orbiting in discrete, quantized energy levels . These electrons are, predictably, attracted to the nucleus by electrostatic forces . This interpretation revolutionized atomic theory, replacing several earlier, less accurate hypotheses and swiftly becoming the prevailing standard model for depicting atomic structure, even if it has since been refined by quantum mechanics .

boiling Also known, somewhat dramatically, as ebullition. This is a more rapid, intensely energetic form of vaporization , a stark contrast to the much slower, quiescent process of simple evaporation. During boiling, a substance undergoes a distinct phase transition from a liquid to a gas . It occurs specifically when a liquid is heated to its boiling point , at which temperature the liquid’s internal vapor pressure surpasses the external pressure exerted upon it by the surrounding atmosphere . This pressure differential causes the gaseous phase to separate from the liquid phase rapidly, often with considerable, and sometimes violent, effervescence.

boiling flask Also known, more charmingly, as a Florence flask. This is a specific type of laboratory flask , typically crafted from glass , characterized by a large, spherical body, a relatively long, narrow neck, and a flat bottom. Its design is particularly optimized for heating, boiling, and distilling liquids , and its rounded shape makes swirling the contents remarkably easy without spillage. See also round-bottom flask , which is similar but lacks the flat bottom.

boiling point Also known as the vaporization point. This is the precise temperature at which a substance undergoes a phase transition from a liquid state to a gas (or vapor ). It is inherently dependent on external pressure and is therefore usually specified for a given substance under standard conditions . It’s a critical physical property for identifying and separating compounds.

boiling-point elevation The phenomenon by which the boiling point of a solvent is observed to increase when a non-volatile solute is dissolved within it. This is a colligative property , meaning it depends on the number of solute particles, not their identity. It’s a consequence of the solute reducing the solvent’s vapor pressure .

bond Any persistent, enduring attraction that exists between atoms , ions , or molecules , which ultimately enables the formation of chemical compounds . These bonds arise from a vast array of electrochemical forces , whose strengths can vary immensely. They are broken only when these attractive forces are overcome by other, more potent forces (e.g., thermal energy). The specific types, intrinsic strengths, and sheer quantities of bonds holding together chemical substances are the fundamental determinants of the overall structure and macroscopic bulk properties of matter . Without bonds, nothing would exist as we know it.

bond angle The geometric angle formed between two adjacent covalent bonds originating from the same atom within a molecule . This angle is a crucial determinant of molecular geometry and is influenced by electron-pair repulsion .

Boyle’s law A classical gas law which rather elegantly states that, for a given mass of gas maintained at a constant temperature , its volume is inversely proportional to its pressure . Reduce the volume, pressure goes up; increase the volume, pressure goes down. Simple, yet profoundly important.

Bragg’s law A fundamental principle in X-ray crystallography that describes the conditions for constructive interference of X-rays diffracted by a crystal lattice . It relates the angle of incidence, the wavelength of the X-rays, and the interplanar spacing of the crystal to produce a measurable diffraction pattern . Essential for determining crystal structures .

bridge A chemical bond (or, less commonly, an atom or an unbranched chain of atoms) that serves to connect two distinct parts of the same molecule . Essentially, it’s an intramolecular bond that links different moieties or functional groups within a larger structure. [2]

bridgehead Either of the two tertiary atoms that, by forming bonds with each other, create an intramolecular bridge within a molecule . These atoms are often part of fused or bridged ring systems and have specific stereochemical constraints . [2]

Brønsted–Lowry acid Any chemical species that, with remarkable generosity, readily donates a proton (H+ ). This is one of the more widely used definitions of an acid .

Brønsted–Lowry acid–base reaction A chemical reaction characterized by the transfer of a proton from a Brønsted–Lowry acid to a Brønsted–Lowry base . It’s a fundamental concept in understanding acid–base chemistry .

Brønsted–Lowry base Any chemical species that, with equal readiness, accepts a proton (H+ ). This is the complementary definition to a Brønsted–Lowry acid .

Brownian motion The erratic, seemingly random movement of particles suspended in a fluid (a liquid or a gas ) resulting from their collision with the fast-moving atoms or molecules in the fluid. First observed by Robert Brown , it provides compelling evidence for the existence of atoms and molecules and is a manifestation of thermal energy .

Büchner flask A type of heavy-walled laboratory flask , typically made of glass , with a side arm for connection to a vacuum source . It is specifically designed for vacuum filtration in conjunction with a Büchner funnel , allowing for rapid separation of solids from liquids.

buffered solution Also known, more simply, as a buffer. This is an aqueous solution meticulously prepared to consist of a weak acid and its conjugate base , or, conversely, a weak base and its conjugate acid . Its defining characteristic is its remarkable ability to resist significant changes in pH when small amounts of strong acids or bases are added. Essential for maintaining stable pH environments in biological systems and many chemical processes.

bumping A rather annoying and potentially dangerous phenomenon in which a homogeneous liquid , when heated to its boiling point , becomes superheated (i.e., its temperature rises above its boiling point without actually boiling). Upon the sudden formation of a nucleation site (like a tiny scratch or trapped air bubble), the superheated liquid rapidly and violently boils to the gas phase , resulting in an abrupt and often forceful expulsion of the liquid from its container. In extreme, and rather inconvenient, cases, the container itself may shatter. This can be mitigated by frequent stirring, selecting an appropriate container, and, most effectively, by adding boiling chips to provide continuous nucleation sites.

bung Also known as a stopper or cork. This is a cylindrical or conical plug or closure, typically made of rubber or cork, used to create a seal for a container such as a bottle, test tube, flask , or barrel. Its purpose is, predictably, to prevent spillage or evaporation.

burette Also spelled buret. A precision piece of glassware equipped with a stopcock at the bottom and precise volumetric markings, specifically designed to dispense highly accurate and specific amounts of liquid . Its primary use is in procedures where exceptional volumetric precision is paramount, such as during titrations or other resource-dependent reactions .

butyl A common, non-systematic name for an alkyl functional group derived from either of the two isomers of butane . It carries the generic chemical formula –C4H9. This group can exist as a substituent within organic compounds or, less commonly, as an independent ion or radical . In IUPAC nomenclature , the presence of a butyl substituent is indicated by the prefix “butyl” in the compound’s name, or by the abbreviation “Bu” in chemical formulae . For example, butyl alcohol (butanol), which can exist in five different isomeric forms depending on the arrangement of its four carbon atoms, is often written with the generic formula CH4CH9OH or BuOH. It’s a slightly more complex alkyl group, but still rather straightforward.

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• References

• External links

C

calorific value A quantitative measure of the amount of heat energy released per unit mass when a given substance undergoes complete combustion . It is typically expressed in units of megajoules per kilogram (MJ/kg) or kilojoules per gram (kJ/g). This value is crucial for assessing the energy content of fuels . [3]

calorimeter Any of various devices meticulously designed and employed to measure thermal properties (i.e., heat ). This includes determining calorific values or the heats of chemical reactions . It’s essentially a thermally isolated system, allowing for the precise measurement of heat exchange, assuming you’ve assembled it correctly. [3]

calx A rather archaic term for a metal oxide that is formed when a metal ore is heated in the presence of air . It’s the product of early metallurgy and alchemy . [3]

carbanion Any organic ion that bears a net negative electric charge specifically on a carbon atom . Its general formula is typically R3C− . Carbanions are frequently encountered as highly reactive intermediate species in certain organic reactions , playing a crucial role in forming new carbon-carbon bonds. It’s the negatively charged carbon’s moment in the spotlight. [3] Contrast carbocation .

carbide A distinct class of interstitial compounds composed of carbon bonded to a particular metal (typically a large-radius transition metal ) within a densely packed crystal lattice . In this arrangement, the carbon atoms occupy the small spaces (interstices) between the larger metal atoms. Tungsten carbide (WC) is a well-known example, prized for its extreme hardness.

carbocation Any organic ion that bears a net positive electric charge specifically on a carbon atom . Its general formula is typically R3C+ . Carbocations are highly reactive intermediate species in many organic reactions , acting as electrophiles due to their electron deficiency. They are the positively charged carbon’s fleeting claim to fame. Contrast carbanion .

carbon The sixth chemical element on the periodic table , with the symbol C. It is a non-metal and is absolutely fundamental to all known life , forming the backbone of organic compounds through its unique ability to form stable covalent bonds with itself and many other elements. Its allotropes include diamond and graphite . Without it, you wouldn’t be here, so perhaps show some respect.

carbonic acid A weak inorganic acid with the chemical formula H2CO3. It forms when carbon dioxide dissolves in water . It’s responsible for the fizz in carbonated drinks and plays a role in ocean acidification .

carbonization 1.  The transformative process by which organic compounds , such as those found within biological organisms , are converted into other forms of carbon or carbonic residues . This can occur through heating, burning, or over geological timescales during fossilization . 2.  More broadly, it refers to the process of coating a substance with carbon residues (like charcoal ) or causing a substance to become scorched, blackened, or charred. It’s essentially the art of turning things to carbon.

carbonyl 1.  A ubiquitous functional group composed of a carbon atom double-bonded to an oxygen atom , represented by the formula C=O. Carbonyl groups are central to many classes of organic compounds , including aldehydes , ketones , carboxylic acids , and esters , and they often form part of larger functional groups. Its reactivity is pivotal in organic synthesis. 2.  In a different context, it can refer to an inorganic or organometallic coordination complex where carbon monoxide acts as a ligand (e.g., a metal carbonyl ). Context is, as always, everything.

carboxyl A functional group consisting of a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom . Its formula is –COOH. It is the defining characteristic of carboxylic acids and is responsible for their acidic properties.

carboxylic acid A prominent class of organic acids and a defining functional group characterized by a carboxyl group (–COOH) attached to a substituent group . Carboxylic acids adhere to the general formula R–COOH (also conveniently written as R–CO2H), where R can be an alkyl , alkenyl , aryl , or any other carbon-containing substituent. They are typically weak acids but play crucial roles in biochemistry and industrial processes.

The skeletal formula for a generic carboxylic acid , with R denoting a variable carbon-containing substituent group . CAS Registry Number (CAS RN) Also known, more simply, as a CAS Number. This is a unique numerical identifier, rather diligently assigned by the Chemical Abstracts Service (CAS), to virtually every chemical substance that has ever been described in the open scientific literature. This includes an astonishingly vast collection of over 182 million organic and inorganic compounds , minerals , isotopes , alloys , polymers , and mixtures , as well as the somewhat vaguely defined “UVCBs” (substances of unknown or variable composition, complex reaction products, or biological origin). CAS numbers are an internationally recognized standard, providing an unambiguous identification system used by scientists, industries, and regulatory bodies alike. It’s the chemical world’s equivalent of a social security number, only far more useful. [5]

catalyst Any element or compound that, with remarkable efficiency, facilitates an increase in the rate of a chemical reaction without itself being consumed or destroyed during the reaction. It participates, but it doesn’t get used up. It’s therefore considered both a reactant and a product of the reaction, a rather unique duality. Enzymes are biological catalysts.

cathode An electrode from which the conventional electric current (the flow of positive charges, again, stubborn conventions) exits a polarized electrical circuit. Predictably, positively charged cations are always drawn towards the cathode. However, the cathode’s actual polarity (positive or negative) can vary, depending on the specific type of electrical device and how it is being operated. It’s the counterpart to the anode .

cation A positively charged ion . This means it’s an atom or molecule that has lost one or more electrons from its neutral state, resulting in a net positive electric charge . They are attracted to cathodes .

cell potential More accurately, electromotive force (emf) or electrode potential . This is the driving force, measured in volts , within a galvanic cell (a type of electrochemical cell ) that effectively “pulls” electrons from a reducing agent (where oxidation occurs) towards an oxidizing agent (where reduction occurs). It’s the measure of the cell’s ability to do electrical work.

centrifugation A common laboratory technique that involves the application of centrifugal force to separate particles from a solution . This separation occurs based on differences in their size, shape, and, crucially, their density . Larger and/or denser substances will migrate away from the axis of the centrifuge , while smaller and/or less dense substances will migrate towards the axis. It’s a powerful method for isolating components in a mixture.

centrifuge A mechanical device specifically designed to perform centrifugation . It achieves separation of substances based on their size, shape, and density by rapidly rotating vessels containing the substances around a central axis at extremely high velocities. It’s a rather effective, if sometimes noisy, way to separate things.

chain reaction A chemical reaction (or nuclear reaction ) where the products of one step in the reaction become the reactants for the next step, leading to a self-propagating series of events that can rapidly accelerate. Nuclear fission and the combustion of hydrocarbons are classic examples.

charge number A quantized value representing electric charge . It is calculated by dividing the electric charge (measured in coulombs ) by the elementary-charge constant (e), or z = q / e. For ions , charge numbers are denoted in superscript (e.g., Na+ indicates a sodium ion with a charge number of positive one). Atomic numbers are, in essence, the charge numbers of atomic nuclei .

Charles’s law A classical gas law which states that when the pressure on a sample of a dry gas is held absolutely constant, its Kelvin temperature is directly proportional to its volume . Heat it up, it expands; cool it down, it contracts (assuming constant pressure). Another fundamental, and rather obvious, observation about gases.

chelating agent An organic compound , often referred to as a chelant or chelator , that possesses multiple donor atoms capable of forming two or more separate coordinate covalent bonds with a single central metal ion . This multi-point attachment results in a stable ring-like structure, known as a chelate complex .

chelation A specific type of bonding interaction that involves the formation of two or more separate coordinate covalent bonds between a polydentate ligand (a molecule with multiple sites capable of donating electron pairs ) and a single central metal ion . The ligand is typically an organic compound referred to as a chelant or chelating agent . This results in a more stable complex compared to monodentate ligands.

chemical See chemical species and chemical compound . A term often used loosely, to my perpetual irritation.

chemical bond See bond .

chemical composition The precise identity and the relative number of the elements that constitute a chemical compound or mixture . This information is frequently, and conveniently, expressed using a chemical formula . It tells you what something is made of.

chemical compound See compound .

chemical decomposition The process by which a single particle or molecular entity (such as a molecule or a reactive intermediate ) breaks down or separates into two or more distinct fragments. Alternatively, it refers to a chemical reaction where two or more products are formed from a single reactant . It’s the opposite of chemical synthesis .

chemical element See element .

chemical formula Any of various standardized methods for concisely displaying information about the chemical composition of a compound or molecule , utilizing a combination of letters (element symbols), numbers (stoichiometric ratios), and/or typographical symbols. While chemical formulas, such as empirical and molecular formulas , can only convey the identities and numerical proportions of the atoms within a compound, they are thus more limited in their descriptive power than full chemical names or detailed structural formulas . They are a shorthand, useful but incomplete.

chemical law A law of nature that governs the behavior and interactions of matter at the atomic and molecular level, providing fundamental principles for chemistry . A classic example is the law of conservation of mass . These are not suggestions; they are inviolable rules.

chemical nomenclature The systematic process of naming chemical compounds and chemical species according to established rules, primarily those set by the IUPAC . Its purpose is to provide unambiguous and universally understood names for the vast array of chemical entities. It’s tedious but necessary to avoid utter chaos.

chemical physics A distinct branch of science that sits at the intersection of chemistry and physics . It applies the principles and techniques of physics to understand chemical phenomena at the atomic and molecular level. Unlike physical chemistry , it often focuses more on fundamental physical processes that govern chemical systems.

chemical process 1.  Any method or means by which one or more chemicals or chemical compounds undergo a change in any way, whether occurring naturally or artificially induced, spontaneously or through the application of external forces. It’s a broad term for any transformation. 2.  In the specialized field of chemical engineering , this term specifically refers to any method employed on an industrial scale (particularly within manufacturing ) to alter the composition of one or more chemicals or materials .

chemical reaction The fundamental process involving the rearrangement of atoms and molecules , resulting in the transformation of one or more substances (the reactants ) into one or more different substances (the products ). This always involves the breaking and/or formation of chemical bonds . It’s how everything changes.

chemical species Also known, somewhat generically, as simply a chemical. This refers to a chemical substance or an ensemble of substances composed of chemically identical molecular entities . These entities are capable of exploring the same set of molecular energy levels within a characteristic or delineated time scale . It’s a precise term for a distinct chemical entity, a concept often misused by the uninitiated.

chemical substance Also known as a pure substance or, rather lazily, simply a substance. This is a form of matter that possesses a constant chemical composition and a unique set of characteristic properties . Crucially, it cannot be separated into simpler components by purely physical methods (i.e., without disrupting chemical bonds ). It is frequently referred to as a “pure substance” to clearly distinguish it from a mixture .

chemical synthesis The deliberate, artificial execution of one or more chemical reactions with the explicit goal of obtaining one or more desired products . In contemporary laboratory contexts , well-designed chemical syntheses are expected to be both reliable and reproducible. It’s the art and science of making new things.

chemistry The scientific discipline that, for some inexplicable reason, dedicates itself to the study of chemical substances , compounds , and molecules —all of which are composed of atoms from various chemical elements . It meticulously examines their compositions , structures , properties , observed behaviors , and, most importantly, the profound changes they undergo during reactions with other substances. It’s the study of everything around you, and within you.

chirality A fundamental property of asymmetry inherent in a molecule or ion that renders it distinguishable from its mirror image . This distinction means that the molecule cannot be superimposed upon its reflection by any combination of simple geometric rotations , translations , or even some conformational changes . [6] [7] Such a molecule or ion is deemed chiral, and it invariably exists in two distinct forms, known as enantiomers , which are stereoisomers of each other. These forms are differentiated as either “right-handed” or “left-handed” based on their absolute configuration or some other established criterion. Various forms 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 intrinsic curvature of a molecule can also impart inherent chirality. It’s a property that profoundly impacts biological activity, proving that sometimes, being a mirror image isn’t enough.

Structural diagrams of two chiral molecules , the non-superposable mirror-image enantiomers (S)-alanine (left) and (R)-alanine (right). Despite having identical structural formulas and the same physical properties (except for optical activity ), they are nonetheless distinct chemicals and exhibit differential reactivity with other chiral compounds.

chromatography A powerful and versatile family of laboratory techniques used for the separation of mixtures . It operates on the principle that components of a mixture will move at different rates through a stationary phase when carried by a mobile phase, based on differences in their adsorption , solubility , or other physical or chemical properties . Examples include gas chromatography and liquid chromatography . It’s how we untangle chemical messes.

chromometer See colorimeter .

cis–trans isomerism A specific type of stereoisomerism , also known as geometric isomerism, that arises in molecules where there is restricted rotation around a chemical bond (most commonly a double bond or within a ring structure ). It describes the different spatial arrangements of substituents relative to each other on either side of this restricted bond. Cis isomers have substituents on the same side, while trans isomers have them on opposite sides. This seemingly minor difference can lead to vastly different physical and chemical properties .

closed system A thermodynamic system that can exchange energy (e.g., heat and work ) with its surroundings but cannot exchange matter . Its mass remains constant. It’s a useful idealization for studying energy transformations without the added complication of mass transfer.

cluster In chemistry , a cluster refers to a group of two or more atoms or molecules that are bound together, often by intermolecular forces or covalent bonds , forming a discrete entity with properties intermediate between those of individual atoms/molecules and bulk matter. Metal clusters , for instance, are important in catalysis .

cohesion The intrinsic tendency of similar particles or surfaces to cling to one another. This attractive force is a direct result of intermolecular forces acting between identical substances. It is, quite simply, the opposite of adhesion .

colligative property Any property of a solution that depends solely upon the ratio of the number of solute particles to the number of solvent particles within that solution, and not on the specific chemical nature of the species present. Examples, which demonstrate this rather elegant universality, include osmotic pressure , freezing-point depression , and boiling-point elevation . They’re a testament to the power of counting, rather than identifying.

colloid A mixture in which microscopic, insoluble particles are stably suspended within and evenly dispersed throughout another substance . This other substance is typically a liquid , but the term also encompasses aerosols (solids/liquids in a gas) and gels (solids in a liquid). Thus, a colloid inherently comprises a dispersed phase and a continuous phase. Many milks are classic examples of colloids, demonstrating their commonality in everyday life.

color standard A meticulously prepared liquid solution of precisely known chemical composition and concentration , and consequently, of a known and standardized color . This standard is utilized as a reference point in the optical analysis of samples of unknown strength. It provides a baseline for comparison, assuming your eyes are calibrated. [4]

color test A quantitative analysis of a substance performed by comparing the intensity of the color produced when that substance is exposed to a reagent with a standard color. This standard color is, naturally, produced similarly in a solution of known strength. It’s a visual, albeit sometimes subjective, method of quantification. [4]

colorimeter Also known as a chromometer. Any instrument specifically designed and employed for the measurement of color based on optical comparison with established standard colors . [2] More specifically in chemistry , it’s a device used in colorimetry that measures the absorbance of specific wavelengths of light by a given solution . This measurement allows for the determination of the concentration of a known solute in the solution, by applying the principle that solute concentration is directly proportional to absorbance (the Beer–Lambert law ).

combustion An exothermic reaction (one that releases heat ) occurring rapidly between an oxidant (typically oxygen ) and a fuel . This reaction produces substantial amounts of heat and, quite often, visible light . It’s a vigorous process, often associated with flames and rapid energy release.

An example of large-scale, rapid combustion . Commission on Isotopic Abundances and Atomic Weights (CIAAW) A rather important, though obscure, international scientific body responsible for periodically reviewing and recommending standard atomic weights and isotopic abundances of the elements . They ensure that the fundamental data chemists rely on is accurate and up-to-date.

complex A molecular entity that forms through a rather loose association between two or more component molecular entities, which can be either ionic or uncharged. The corresponding chemical species also fall under this umbrella. The bonding between these components is typically weaker than a full covalent bond . [2] See also coordination complex for a more specific type.

compound A substance that is meticulously formed from two or more distinct chemical elements that are chemically bonded together in a fixed stoichiometric ratio . Unlike a mixture , a compound has properties entirely different from its constituent elements. Water (H2O) is a compound, not just a mixture of hydrogen and oxygen.

Compton rule An empirical law of physical chemistry which rather curiously states that the heat of fusion of a given element , when multiplied by its atomic weight and then divided by its melting point in kelvin , consistently approximates a value of 2. It’s an interesting, if niche, observation about the relationship between these properties. [4]

concatemer A long continuous molecule that contains multiple copies of the same DNA or RNA sequence linked in series. More broadly, it can refer to any polymer composed of multiple identical monomer units linked end-to-end.

concentration The precise quantity or abundance of a particular constituent within a mixture , expressed per unit quantity of that mixture. For instance, the amount, in moles , of a dissolved solute per unit volume of a solution is a common measure known as molar concentration or molarity . Various definitions of concentration are employed in chemistry , including mass concentration and volume concentration . Choose your poison, but be precise.

condensation The phase transition of a substance from a gas to a liquid . It’s the reverse of vaporization , often occurring when a gas is cooled or compressed. Think of dew forming on grass.

condosity A rather specialized comparative measurement of the electrical conductivity of a solution . It is defined as the molar concentration of a sodium chloride (NaCl) solution that exhibits the same specific electrical conductance as the solution under examination. It is typically quantified in units of moles per litre (or per some other unit of volume). It’s a way to standardize conductivity measurements.

conduction The process of heat transfer (or electrical charge transfer) through direct contact between particles, without any net movement of the material itself. In thermal conduction , kinetic energy is passed from more energetic particles to less energetic ones. In electrical conduction , electrons move through the material.

conductivity See electrical conductivity and thermal conductivity . Because specifying which type of conductivity is crucial.

conductor Any object or material that readily permits the flow of an electric current in one or more directions. These materials typically have delocalized electrons that can move freely. The opposite, naturally, is an insulator .

conformation The specific spatial arrangement of atoms within a molecule that allows for distinction between stereoisomers which can be interconverted solely by rotations around formally single bonds . These are not different molecules, but different shapes of the same molecule, interconverting rapidly at room temperature. Chairs and boats in cyclohexane are classic examples.

conjugate acid In a Brønsted–Lowry acid–base reaction , the species formed when a base accepts a proton . It’s essentially the base with an extra proton.

conjugate base In a Brønsted–Lowry acid–base reaction , the species formed when an acid donates a proton . It’s essentially the acid minus a proton.

conjugated system A molecule that contains double or triple bonds that are separated by one single bond . For instance, the compound buta-1,3-diene , with the chemical structure H2C=CH−CH=CH2, exemplifies conjugated double bonds. In such molecules, there is a degree of delocalization of electrons within the pi orbitals that extends across the carbon atoms linked by the single bond. This delocalization imparts enhanced stability and unique spectroscopic properties . [3]

constitutional isomer See structural isomer .

constitutional unit An atom or a group of atoms (including any pendant atoms or groups, should they be present) that collectively comprise a repeating part of the structure of a macromolecule , oligomer , polymer , block copolymer , or chain . It’s the fundamental repeating module from which larger structures are built. [2]

convection A mechanism of heat transfer (or mass transfer ) that occurs through the movement of fluids (liquids or gases). In convection, warmer, less dense fluid rises, while cooler, denser fluid sinks, creating a circulation current that transfers heat or mass.

cooling curve A graphical representation, typically a line graph , that illustrates the change between different phases of matter , commonly from a gas to a solid or a liquid to a solid, as a function of time and temperature . For example, it can show how the temperature of a liquid substance decreases over time as it condenses and eventually freezes below its freezing point . Plateaus on the curve indicate phase transitions .

coordinate chemistry A rather vague term. Presumably, this refers to coordination chemistry , the study of coordination complexes . Precision, please.

coordinate covalent bond See dipolar bond .

coordination complex A chemical compound distinguished by a central atom or ion , which is typically metallic and referred to as the coordination center. This central entity is bonded to a surrounding array of other groups of atoms, such as molecules or ions , which are known as ligands or complexing agents. Many metal-containing compounds , particularly those involving transition metals , fall under this category of coordination complexes. They are ubiquitous in biology , catalysis , and materials science. See also complex .

corrosion An irreversible interfacial chemical reaction of a material , most notably a metal , with its surrounding environment . This destructive process results in either the consumption of the material itself or its dissolution into an external component of the environment. It’s the slow, inevitable decay of matter, a rather depressing but unavoidable reality.

coulomb (C) The SI unit of electric charge . It is precisely defined as the amount of charge transported by a constant electric current of one ampere flowing for one second . It’s a fundamental unit for quantifying electrical phenomena.

counterion The ion that serves as the necessary counterpart to an oppositely charged ion within a dissociated ionic species . Essentially, it’s the cation that pairs with a given anion , or vice versa, ensuring overall charge neutrality . For instance, Na+ is the counterion to Cl− , and vice versa, in solutions of sodium chloride (NaCl). It’s the balancing act of charged particles.

covalent bond Also known as a molecular bond. This is a type of chemical bond that involves the mutual sharing of electron pairs between atoms . The stable balance of attractive and repulsive forces that arises between atoms when they engage in this sharing of electrons is precisely what constitutes covalent bonding . It’s a strong, directional bond, the workhorse of organic chemistry .

A diatomic hydrogen molecule , H2 (right), is formed by a covalent bond when two single hydrogen atoms share two electrons between them. critical point The definitive end point of a phase equilibrium curve or a pressure-temperature curve . At this specific set of conditions, the distinctions between different phases of a substance , such as its liquid and vapor forms, vanish, and they can coexist indistinguishably. The critical point is precisely defined by the intersection of a critical temperature (Tc) and a critical pressure (pc); above these values, all distinction between phases disappears, and the substance transforms into a supercritical fluid , possessing properties of both a liquid and a gas.

crucible A specialized ceramic or metal dish or other vessel, designed to withstand extremely high temperatures . Its purpose is to allow substances to be melted, calcined , or otherwise subjected to intense heat without degrading the container itself. Essential for pyrochemistry . [3]

crystal A solid material whose constituent particles (such as atoms , ions , or molecules ) are arranged in an exceptionally orderly, periodic microscopic structure . This arrangement forms a repeating lattice with a definite geometry that extends throughout all directions. Such materials are, unsurprisingly, described as crystalline. Their ordered structure gives rise to many unique physical properties .

crystallization The physical process by which a solid forms, where the atoms or molecules are highly organized into a repeating, three-dimensional crystal lattice . It can occur from a solution , a melt , or even directly from a gas (deposition). It’s the ultimate act of molecular self-assembly, assuming conditions are favorable.

crystallization point See freezing point .

crystallography The branch of chemistry and physics specifically concerned with the meticulous study of crystalline solids . This includes the precise determination of their internal structure (e.g., using X-ray diffraction ) and the comprehensive understanding of their resulting properties .

cuvette A small, transparent container, typically rectangular, used in spectroscopy experiments. Usually constructed from plastic , glass , or quartz , it is specifically designed to hold a sample (most commonly a liquid ) for measurement inside a spectrometer . Cuvettes must be impeccably clean and as optically transparent as possible to minimize any interference with the beams of light upon which spectroscopic techniques rely. Their purity directly impacts measurement accuracy.

cyclic (Of a molecule ) Containing one or more ring structures formed by the bonding of atoms . This contrasts with acyclic compounds, and the presence of rings often imparts unique stereochemical and reactivity properties.

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• References

• External links

D

dalton (Da) Also known as the unified atomic mass unit (u). This is a unit of mass defined with remarkable precision as exactly 1/12 of the mass of a free, unexcited atom of carbon-12 at rest. It is, rather conveniently, approximately equal to the mass of one nucleon (proton or neutron ). It is the standard unit for expressing atomic and molecular masses .

Dalton’s law of partial pressures An empirical law concerning gases which states that, in a mixture of non-reacting gases , the total pressure exerted by all of the combined gases is precisely equal to the sum of the partial pressures exerted by each individual gas. It’s a simple additive principle for gas mixtures, assuming they behave themselves.

d-block The colloquial name given to the section of the periodic table that encompasses Groups 3 through 12. The elements residing here are known as the transition metals , characterized by their d-orbitals being partially filled with electrons . They exhibit variable oxidation states and often form colorful coordination complexes .

dative bond See dipolar bond .

debye (D) A non-SI unit of measurement specifically for electric dipole moment , defined as 10−18 statcoulomb-centimetres. While not SI, it’s widely used in chemistry and atomic physics due to its convenient scale for molecular dipoles. See also electric dipole moment .

deionization The process of removing ions from a solution by any suitable method. In the context of water , this typically refers to the removal of mineral ions such as sodium , iron , and calcium , resulting in “deionized water” (DI water), which is less conductive.

deliquescence A substance’s inherent affinity for water , often characterized by its pronounced tendency to absorb moisture directly from the atmosphere to the point where it forms an aqueous solution . Most strongly deliquescent substances are salts , such as calcium chloride and potassium carbonate . They essentially dissolve themselves in the air’s humidity.

delocalized electron Any electron within a molecule , ion , or solid metal that is not exclusively associated with an individual atom or a single covalent bond . This term can refer to electrons involved in resonance within conjugated systems or aromatic compounds ; to free electrons that enable electrical conductivity in metals ; or to electrons residing within delocalized molecular orbitals that span multiple adjacent atoms. They are, in essence, shared by many.

density An intensive property of a substance , defined as its mass per unit volume . It is elegantly expressed by the equation d = m/V. It tells you how much “stuff” is packed into a given space.

denticity A specific measure in coordination chemistry that quantifies the number of donor groups within a single ligand that are simultaneously bonded to a central atom (typically a metal ion ) in a coordination complex . A monodentate ligand has one donor group, a bidentate ligand has two, and so on.

deposition The process by which particles (typically solid) settle out from within a solution or mixture . More broadly, in phase transitions , it refers to the direct change of a substance from a gas to a solid without passing through a liquid phase (reverse of sublimation ).

depression of freezing point See freezing-point depression .

desiccant Also known as a drying agent. This is a hygroscopic substance employed to induce or maintain a state of dryness or desiccation (i.e., the absence of moisture ) in its immediate vicinity. It achieves this by abstracting water molecules from other substances. Desiccants manifest in a variety of forms and operate via diverse principles, ranging from simple absorption to the direct chemical bonding of water molecules. They are the silent guardians against unwanted humidity.

desiccation The process of extreme drying, or the complete removal of moisture from a substance . Often achieved using a desiccant or through methods like freeze-drying .

deuterium Also known as hydrogen-2 or heavy hydrogen, and symbolized as 2H or D. It is one of the two stable isotopes of a hydrogen atom , distinguished by its nucleus containing one proton and one neutron . Deuterium is both noticeably heavier and considerably less abundant in nature than its more common counterpart, protium (1H). Its presence can significantly alter reaction rates (kinetic isotope effect).

deuteron The nucleus of a deuterium atom (an isotope of hydrogen ), containing precisely one proton and one neutron . It’s the core of heavy hydrogen.

Dewar flask See vacuum flask .

dianion A compound or molecular entity that bears precisely two negative electric charges . These charges may be localized on a single atom , distributed across different atoms, or even delocalized across a larger structure. [2]

diastereomer A type of stereoisomer that is not a mirror image of another stereoisomer. Diastereomers have the same chemical formula and connectivity but differ in the spatial arrangement of their atoms and are not enantiomers . They have different physical and chemical properties .

diatomic Composed of exactly two atoms , which may be of the same or different elements . Oxygen (O2) and carbon monoxide (CO) are examples. Contrast monatomic and polyatomic .

diatomic molecule Any molecule that consists of only two atoms , regardless of whether those atoms are of the same element (e.g., O2, N2) or different elements (e.g., HCl, CO).

diffusion The spontaneous net movement of atoms or molecules from a region of higher concentration to a region of lower concentration. This process is driven by a gradient in the chemical potential of the diffusing species and fundamentally relies on the random walk of particles. Consequently, it results in the mixing or mass transport of substances without requiring any directed bulk motion. It’s how things spread out.

dilatant (Of a substance ) Possessing the rather counter-intuitive ability to increase in volume when its shape is forcibly changed. These are sometimes called shear-thickening fluids. [4]

dilution The process of reducing the concentration of a solute in a solution by adding more solvent . It’s a common laboratory procedure for preparing solutions of desired concentrations.

dimer An oligomer consisting of precisely two monomers that are joined together by chemical bonds . These bonds can be quite variable in strength (strong or weak) and nature (covalent or intermolecular ). A homodimer is composed of two identical molecules, while a heterodimer consists of two different molecules.

dipolar bond Also known as a coordinate covalent bond, coordinate bond, dative bond, and semipolar bond. This is a specific type of covalent bond formed through the coordination of two or more electrically neutral moieties . The combination results in a charge-separated molecule or coordination complex , where two electrons originating from the same atom are shared between the donor atom and an acceptor atom , thereby creating an internal, two-center molecular dipole moment . [2]

dipole The electric or magnetic separation of electric charge into a pair of charges that possess equal magnitude but opposite sign (one positively charged, one negatively charged), separated by some typically small distance. It’s a fundamental concept in understanding molecular polarity .

dipole moment A general term. See electric dipole moment , magnetic dipole moment , molecular dipole moment , bond dipole moment , electron electric dipole moment , electron magnetic dipole moment , and nuclear magnetic moment . Be specific, please.

dispersion A system in which particles of one material are distributed throughout a continuous phase of another material. The two phases may exist in the same or different states of matter . Dispersions where the particles are sufficiently large to undergo sedimentation are termed suspensions , while those with smaller particles are classified as colloids or, in the case of true molecular-level mixing, solutions .

dissociation Any process by which a polyatomic molecule or molecular entity (e.g., an ionic compound or coordination complex ), or an aggregate of molecular entities, separates or splits into two or more smaller molecules , atoms , ions , radicals , or other constituent parts. This process is usually, though not always, reversible . Common examples include unimolecular heterolysis and homolysis , the dissolution of salts , and acid dissociation . It’s the act of breaking apart. Contrast association .

dissolution Also known as solvation. This is the intimate interaction of a solvent with the molecules or ions of a solute . This interaction involves various forces, including bond formation , hydrogen bonding , and van der Waals forces . It’s the process by which a solute disperses evenly throughout a solvent to form a solution .

A sodium ion (Na+ ) forms a solvation complex with water molecules when dissolved in an aqueous solution . distillation A fundamental separation process employed to isolate the component substances of a liquid mixture . It cleverly exploits differences in the relative volatility of the mixture’s components through a sequence of selective boiling and subsequent condensation . The apparatus specifically used to distill a substance is called a still, and the re-condensed substance yielded by the process is, rather predictably, called the distillate.

double bond A type of covalent bond that involves the sharing of two pairs of electrons between two atoms . This makes it stronger and shorter than a single bond but also more reactive, due to the presence of a pi bond .

double decomposition An older, less precise term for a double displacement reaction , also known as a metathesis reaction . It implies that both reactants “decompose” and exchange parts.

double displacement Also known as a salt metathesis reaction . A type of chemical reaction in which the cations and anions of two different ionic compounds switch partners, forming two new compounds. This often leads to the formation of a precipitate , a gas , or water .

double salt 1.  A salt that is composed of more than one distinct cation or anion , or one which, upon hydrolysis , yields two different cations and anions. 2.  Alternatively, it can refer to a salt that is a molecular combination of two other salts, forming a single crystalline structure with distinct properties. [4]

double-replacement reaction See double displacement .

dropping point A specific temperature at which a grease undergoes a phase transition from a semi-solid to a liquid state under standardized conditions. [4] It represents the upper limit at which the grease retains its structural integrity, though it doesn’t necessarily indicate the maximum temperature at which it can still be effectively used. It’s a critical parameter for lubricant selection.

dry box A specialized, sealed chamber or container meticulously designed to maintain an interior environment of extremely low humidity . This is often achieved by continuously flushing it with an inert gas like argon or with air rigorously scrubbed of carbon dioxide and water vapor . Its purpose is to provide an inert atmosphere where highly reactive chemicals or moisture-sensitive procedures can be safely manipulated in the laboratory . Essential for handling pyrophoric or hygroscopic substances. [4]

drying agent See desiccant .

ductility Also known as malleability. This is a crucial measure of a material’s inherent ability to undergo significant plastic deformation (i.e., change shape permanently) before ultimately rupturing. It is typically expressed as a percentage of elongation or percentage of area reduction observed in a tensile test and is popularly characterized by the material’s capacity to be stretched into a thin wire . Metals like copper and gold are excellent examples.

dystectic mixture A rather specific mixture of two or more substances which, rather remarkably, possesses the highest melting point of all possible mixtures of those particular substances. It’s the peak of the melting point curve. [4] Contrast eutectic mixture .

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E

earth metal See alkaline earth metal . A rather imprecise term.

ebullition See boiling .

effective molecular diameter The conceptual physical extent of the electron cloud enveloping a molecule of a particular gas . This value is calculated through various methods and is usually expressed in nanometres or ångströms . It’s a way to estimate how much space a molecule effectively occupies, which is important for gas kinetics. [4]

effervescence The spontaneous escape of gas from an aqueous solution without the application of external heat . This process is accompanied by characteristic bubbling, foaming, or fizzing. A common example is the release of carbon dioxide from carbonated water . It’s a visual indicator of a gas coming out of solution.

electric charge A fundamental, intrinsic property of matter , measured in coulombs , that dictates how a particle will experience and generate electromagnetic interaction . It can be positive or negative, and like charges repel, while opposite charges attract. It’s the basis of all electrical phenomena.

electric dipole moment A quantitative measure of the separation of positive and negative electric charges within an electrical system. Essentially, it quantifies 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 due to its convenient scale for molecular dipoles. It tells you how unevenly charge is distributed.

electrical conductivity The measure of a material’s ability to conduct an electric current . Materials with high electrical conductivity (e.g., metals ) allow charge to flow easily, while those with low conductivity (insulators ) resist it. It’s the inverse of electrical resistivity .

electrical resistivity A fundamental property of a material that quantifies how strongly it resists the flow of electric current . It’s the inverse of electrical conductivity .

electricity The set of physical phenomena associated with the presence and flow of electric charge . This includes things like currents , fields , and potentials . It’s a fundamental force of nature, harnessed for myriad purposes.

electride A rather unusual ionic compound for which the anion is, remarkably, a mere electron . In these compounds, electrons are trapped within cavities in the crystal lattice and act as anionic species. They are highly reactive.

electrochemical cell A specialized device capable of performing one of two functions: either generating electrical energy from chemical reactions (in which case it is known as a galvanic or voltaic cell ), or utilizing electrical energy to drive chemical reactions that would otherwise not occur spontaneously (in which case it is known as an electrolytic cell ). For instance, a battery fundamentally contains one or more galvanic cells, each meticulously constructed from two electrodes arranged to facilitate an oxidation–reduction reaction that produces an electromotive force .

electrochemistry A distinct branch of physical chemistry that meticulously investigates the intrinsic relationship between electrical potential difference and identifiable chemical change . This understanding is achieved either through analyzing the chemical reactions that accompany the passage of an electric current or by studying the potential difference that arises from a particular chemical reaction. It’s where electrons truly show their power.

electrolyte A solution that possesses the ability to conduct a certain amount of electric current . Electrolytes can be broadly categorized into weak and strong electrolytes , depending on the extent to which their constituent ions dissociate in solution. Crucial for electrochemical cells and biological processes.

electromagnetic radiation A type of wave that, unlike sound waves, requires no medium and can propagate through a vacuum as well as through material . It is fundamentally classified as a self-propagating wave, composed of oscillating electric and magnetic fields . This vast spectrum includes everything from radio waves to gamma rays , with visible light being a tiny, familiar portion.

electromagnetic spectrum The full range of all possible frequencies of electromagnetic radiation , extending from very low-frequency radio waves to extremely high-frequency gamma rays , and encompassing microwaves , infrared , visible light , ultraviolet , and X-rays in between. It’s a continuous spectrum, a testament to the versatility of light.

electromagnetism The fundamental physical interaction that occurs between electrically charged particles . It is responsible for all electric and magnetic phenomena , and it dictates how particles move and interact due to their electric charge and electrical properties. It’s one of the four fundamental forces of the universe, and arguably the most relevant to chemistry.

electromotive force (emf) The electrical potential difference generated by an electrochemical cell or other energy source that drives an electric current . It’s the “push” that makes electrons flow, measured in volts .

electron A fundamental type of subatomic particle that carries an elementary, immutable negative electric charge . Electrons orbit the nucleus of an atom in specific electron shells and are primarily responsible for forming chemical bonds and mediating chemical reactions . They are the currency of chemistry. Contrast positron .

electron acceptor A chemical species that is capable of receiving electrons from another species in a redox reaction . By accepting electrons, it becomes reduced and acts as an oxidizing agent .

electron capture A specific type of nuclear transformation whereby the proton-rich nucleus of an electrically neutral atom absorbs, or “captures,” an electron from one of its own inner electron shells (frequently those closest to the nucleus). This event triggers a reaction that results in a nuclear proton being converted into a neutron , accompanied by the simultaneous emission of an electron neutrino . [2]

electron configuration The precise distribution of the electrons of an atom or molecule within their respective atomic or molecular orbitals . An elaborate system of notation is employed to concisely and uniquely represent this information for each atomic species . Knowledge of the specific arrangements of electrons in different atoms is indispensable for comprehending the formation of chemical bonds and the underlying organization of the periodic table of the elements . It’s the blueprint for an atom’s reactivity.

electron deficiency A state in a molecule or ion where there are not enough valence electrons to form a complete octet around a central atom or to fully satisfy all bonding requirements. Electrophiles are typically electron deficient.

electron donor A chemical species that is capable of providing electrons to another species in a redox reaction . By donating electrons, it becomes oxidized and acts as a reducing agent .

electron electric dipole moment (d e) An intrinsic property of an electron such that its potential energy is linearly related to the strength of its electric field . It’s a measure of the spatial distribution of an electron’s negative charge within the electric field it generates. While theoretically predicted to be zero in the Standard Model , experimental searches for a non-zero electron EDM are crucial for probing new physics . See also electric dipole moment .

electron magnetic dipole moment Also known as the electron magnetic moment. This refers to the intrinsic magnetic moment of an electron , which arises from the inherent properties of its spin and electric charge . It is approximately equal to −9.284764×10−24 joules per tesla . It’s why electrons behave like tiny magnets.

electron neutrino A lepton , a fundamental subatomic particle with no electric charge and very little mass , that is produced during certain radioactive decays , such as beta decay and electron capture .

electron pair Two electrons that occupy the same molecular orbital but possess opposite spins . Electron pairs are the fundamental units that form chemical bonds (as in a covalent bond ) or exist as lone pairs of valence electrons . It is also possible, though less common in stable molecules, for electrons to exist individually as unpaired electrons (as in radicals ).

electron shell A conceptual orbital or region around the nucleus of an atom that contains a fixed number of electrons (typically two in the innermost shell, and often eight in subsequent shells, as described by the octet rule ). Electrons within the same shell have similar energy levels .

electronegativity (χ) A crucial chemical property that quantifies the intrinsic tendency of an atom within a molecule to attract a shared pair of electrons (or, more broadly, electron density ) towards itself. An atom’s electronegativity is influenced by both its nuclear charge (which is proportional to the number of protons in its nucleus) and the number and spatial location of the electrons present in its [atomic shells](/Electron_