Steam and liquid water are two different forms of the same pure chemical substance, water.
Achemical substance is a unique form ofmatter with constantchemical composition and characteristicproperties.[1][2] Chemical substances may take the form of a singleelement orchemical compounds. If two or more chemical substances can be combined withoutreacting, they may form a chemicalmixture.[3] If a mixture is separated to isolate one chemical substance to a desired degree, the resulting substance is said to bechemically pure.[4]
Chemical substances can exist in several different physicalstates orphases (e.g.solids,liquids,gases, orplasma) without changing their chemical composition. Substancestransition between thesephases of matter in response to changes intemperature orpressure. Some chemical substances can be combined or converted into new substances by means ofchemical reactions. Chemicals that do not possess this ability are said to beinert.
Purewater is an example of a chemical substance, with a constant composition of two hydrogenatomsbonded to a single oxygen atom (i.e. H2O). Theatomic ratio of hydrogen to oxygen is always 2:1 in everymolecule of water. Pure water will tend toboil near 100 °C (212 °F), an example of one of the characteristic properties that define it. Other notable chemical substances includediamond (a form of the elementcarbon),table salt (NaCl; anionic compound), and refinedsugar (C12H22O11; anorganic compound).
Colors of a single chemical (Nile red) in different solvents, under visible and UV light, showing how the chemical interacts dynamically with its solvent environment
In addition to the generic definition offered above, there are several niche fields where the term "chemical substance" may take alternate usages that are widely accepted, some of which are outlined in the sections below.
Chemical Abstracts Service (CAS) lists severalalloys of uncertain composition within their chemical substance index.[5] While analloy could be more closely defined as amixture, referencing them in the chemical substances index allows CAS to offer specific guidance onstandard naming of alloy compositions.Non-stoichiometric compounds are another special case frominorganic chemistry, which violate the requirement for constant composition. For these substances, it may be difficult to draw the line between a mixture and a compound, as in the case ofpalladium hydride. Broader definitions of chemicals or chemical substances can be found, for example: "the term 'chemical substance' means any organic or inorganic substance of a particular molecular identity, including – (i) any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature".[6]
In the field ofgeology, inorganic solid substances of uniform composition are known asminerals.[7] When two or more minerals are combined to formmixtures (oraggregates), they are defined asrocks.[8] Many minerals, however, mutually dissolve intosolid solutions, such that a single rock is a uniform substance despite being a mixture in stoichiometric terms.Feldspars are a common example:anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium.
In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. For example, theEU regulationREACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition". The latter two consist of multiple chemical substances; however, their identity can be established either by direct chemical analysis or reference to a single manufacturing process. For example,charcoal is an extremely complex, partially polymeric mixture that can be defined by its manufacturing process. Therefore, although the exact chemical identity is unknown, identification can be made with a sufficient accuracy. The CAS index also includes mixtures.
Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance. However, the polymer may be defined by a known precursor or reaction(s) and themolar mass distribution. For example,polyethylene is a mixture of very long chains of -CH2- repeating units, and is generally sold in several molar mass distributions,LDPE,MDPE,HDPE andUHMWPE.
Theconcept of a "chemical substance" became firmly established in the late eighteenth century after work by thechemistJoseph Proust on the composition of some pure chemical compounds such asbasic copper carbonate.[9] He deduced that, "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound." This is now known as thelaw of constant composition.[10] Later with the advancement of methods forchemical synthesis particularly in the realm oforganic chemistry; the discovery of many more chemical elements and new techniques in the realm ofanalytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modernchemistry, the concept was defined as is found in most chemistry textbooks. However, there are some controversies regarding this definition mainly because the large number of chemical substances reported in chemistry literature need to be indexed.
Isomerism caused much consternation to early researchers, sinceisomers have exactly the same composition, but differ in configuration (arrangement) of the atoms. For example, there was much speculation about the chemical identity ofbenzene, until the correct structure was described byFriedrich August Kekulé. Likewise, the idea ofstereoisomerism – thatatoms have rigid three-dimensional structure and can thus form isomers that differ only in their three-dimensional arrangement – was another crucial step in understanding the concept of distinct chemical substances. For example,tartaric acid has three distinct isomers, a pair ofdiastereomers with one diastereomer forming twoenantiomers.
Anelement is a chemical substance made up of a particular kind of atom and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmuted into another element through anuclear reaction. This is because all of the atoms in a sample of an element have the same number ofprotons, though they may be differentisotopes, with differing numbers ofneutrons.
As of 2019, there are 118 known elements, about 80 of which are stable – that is, they do not change byradioactive decay into other elements. Some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O2) andozone (O3). The majority of elements are classified asmetals. These are elements with a characteristiclustre such asiron,copper, andgold. Metals typically conduct electricity and heat well, and they aremalleable andductile.[11] Around 14 to 21 elements,[12] such ascarbon,nitrogen, andoxygen, are classified asnon-metals. Non-metals lack the metallic properties described above, they also have a highelectronegativity and a tendency to formnegative ions. Certain elements such assilicon sometimes resemble metals and sometimes resemble non-metals, and are known asmetalloids.
Potassium ferricyanide is a compound of potassium, iron, carbon and nitrogen; although it contains cyanide anions, it does not release them and is nontoxic.
A chemical compound is a chemical substance that is composed of a particular set ofatoms orions. Two or more elements combined into one substance through achemical reaction form achemical compound. All compounds are substances, but not all substances are compounds.
Compounds in which components share electrons are known ascovalent compounds. Compounds consisting of oppositely chargedions are known asionic compounds, orsalts.
Coordination complexes are compounds where adative bond keeps the substance together without a covalent or ionic bond. Coordination complexes are distinct substances with distinct properties different from a simple mixture. Typically these have a metal, such as a copper ion, in the center and a nonmetals atom, such as the nitrogen in an ammonia molecule or oxygen in water in a water molecule, forms a dative bond to the metal center, e.g.tetraamminecopper(II) sulfate [Cu(NH3)4]SO4·H2O. The metal is known as a "metal center" and the substance that coordinates to the center is called a "ligand". However, the center does not need to be a metal, as exemplified byboron trifluoride etherate BF3OEt2, where the highlyLewis acidic, but non-metallic boron center takes the role of the "metal". If the ligand bonds to the metal center with multiple atoms, the complex is called achelate.
In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are calledisomers. Isomers usually have substantially different chemical properties, and often may be isolated without spontaneously interconverting. A common example isglucose vs.fructose. The former is analdehyde, the latter is aketone. Their interconversion requires eitherenzymatic oracid-base catalysis.
However,tautomers are an exception: the isomerization occurs spontaneously in ordinary conditions, such that a pure substance cannot be isolated into its tautomers, even if these can be identified spectroscopically or even isolated in special conditions. A common example isglucose, which has open-chain and ring forms. One cannot manufacture pure open-chain glucose because glucose spontaneously cyclizes to thehemiacetal form.
Cranberry glass, while appearing homogeneous, is amixture consisting ofglass and colloidalgold particles of about 40nm in diameter, giving it a red color.
All matter consists of various elements and chemical compounds, but these are often intimately mixed together. Mixtures contain more than one chemical substance, and they do not have a fixed composition.Butter,soil andwood are common examples of mixtures. Sometimes, mixtures can be separated into their component substances bymechanical processes, such aschromatography,distillation, orevaporation.[13]
Grey iron metal and yellowsulfur are both chemical elements, and they can be mixed together in any ratio to form a yellow-grey mixture. No chemical process occurs, and the material can be identified as a mixture by the fact that the sulfur and the iron can be separated by a mechanical process, such as using amagnet to attract the iron away from the sulfur.
In contrast, if iron and sulfur are heated together in a certain ratio (1 atom of iron for each atom of sulfur, or by weight, 56grams (1mol) of iron to 32 grams (1 mol) of sulfur), a chemical reaction takes place and a new substance is formed, the compoundiron(II) sulfide, with chemical formula FeS. The resulting compound has all the properties of a chemical substance and is not a mixture. Iron(II) sulfide has its own distinct properties such asmelting point andsolubility, and the two elements cannot be separated using normal mechanical processes; a magnet will be unable to recover the iron, since there is no metallic iron present in the compound.
While the termchemical substance is a precise technical term that is synonymous withchemical for chemists, the wordchemical is used in general usage to refer to both (pure) chemical substances and mixtures (often calledcompounds),[14] and especially when produced or purified in a laboratory or an industrial process.[15][16][17] In other words, the chemical substances of which fruits and vegetables, for example, are naturally composed even when growing wild are not called "chemicals" in general usage. In countries that require a list of ingredients in products, the "chemicals" listed are industrially produced "chemical substances". The word "chemical" is also often used to refer to addictive, narcotic, or mind-altering drugs.[15][16]
Within the chemical industry, manufactured "chemicals" are chemical substances, which can be classified by production volume into bulk chemicals,fine chemicals and chemicals found in research only:
Bulk chemicals are produced in very large quantities, usually with highly optimized continuous processes and to a relatively low price.
Fine chemicals are produced at a high cost in small quantities for special low-volume applications such as biocides, pharmaceuticals andspeciality chemicals for technical applications.
Research chemicals are produced individually for research, such as when searching for synthetic routes or screening substances for pharmaceutical activity. In effect, their price per gram is very high, although they are not sold.
The cause of the difference in production volume is the complexity of the molecular structure of the chemical. Bulk chemicals are usually much less complex. While fine chemicals may be more complex, many of them are simple enough to be sold as "building blocks" in the synthesis of more complex molecules targeted for single use, as named above. Theproduction of a chemical includes not only its synthesis but also its purification to eliminate by-products and impurities involved in the synthesis. The last step in production should be the analysis of batch lots of chemicals in order to identify and quantify the percentages of impurities for the buyer of the chemicals. The required purity and analysis depends on the application, but higher tolerance of impurities is usually expected in the production of bulk chemicals. Thus, the user of the chemical in the US might choose between the bulk or "technical grade" with higher amounts of impurities or a much purer "pharmaceutical grade" (labeled "USP",United States Pharmacopeia). "Chemicals" in the commercial and legal sense may also include mixtures of highly variable composition, as they are products made to a technical specification instead of particular chemical substances. For example,gasoline is not a single chemical compound or even a particular mixture: different gasolines can have very different chemical compositions, as "gasoline" is primarily defined through source, properties andoctane rating.
Many compounds are also known by their more common, simpler names, many of which predate the systematic name. For example, the long-knownsugarglucose is now systematically named 6-(hydroxymethyl)oxane-2,3,4,5-tetrol.Natural products andpharmaceuticals are also given simpler names, for example the mild pain-killerNaproxen is the more common name for the chemical compound (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid.
Chemists frequently refer tochemical compounds usingchemical formulae ormolecular structure of the compound. There has been a phenomenal growth in the number of chemical compounds being synthesized (or isolated), and then reported in thescientific literature by professional chemists around the world.[18] An enormous number of chemical compounds are possible through the chemical combination of the known chemical elements. As of Feb 2021, about "177 million organic and inorganic substances" (including 68 million defined-sequence biopolymers) are in the scientific literature and registered in public databases.[19] The names of many of these compounds are often nontrivial and hence not very easy to remember or cite accurately. Also, it is difficult to keep track of them in the literature. Several international organizations likeIUPAC and CAS have initiated steps to make such tasks easier. CAS provides the abstracting services of the chemical literature, and provides a numerical identifier, known asCAS registry number to each chemical substance that has been reported in the chemical literature (such aschemistry journals andpatents). This information is compiled as adatabase and is popularly known as the Chemical substances index. Other computer-friendly systems that have been developed for substance information are:SMILES and theInternational Chemical Identifier or InChI.
Often a pure substance needs to be isolated from amixture, for example from anatural source (where a sample often contains numerous chemical substances) or after achemical reaction (which often gives mixtures of chemical substances).
Stoichiometry is founded on thelaw of conservation of mass where the total mass of the reactants equals the total mass of the products, leading to the insight that the relations between quantities of reactants and products typically form a ratio of positive integers. This means that if the amounts of the separate reactants are known, then the amount of the product can be calculated. Conversely, if one reactant has a known quantity and the quantity of the products can be empirically determined, then the amount of the other reactants can also be calculated.
This is illustrated in the image here, where the balanced equation is:
CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l)
Here, onemolecule ofmethane reacts with two molecules ofoxygen gas to yield one molecule ofcarbon dioxide and two molecules of liquidwater. This particular chemical equation is an example ofcomplete combustion. The numbers in front of each quantity are a set of stoichiometric coefficients which directly reflect themolar ratios between the products and reactants. Stoichiometry measures these quantitative relationships, and is used to determine the amount of products and reactants that are produced or needed in a given reaction. Describing the quantitative relationships among substances as they participate in chemical reactions is known asreaction stoichiometry. In the example above, reaction stoichiometry measures the relationship between the quantities of methane and oxygen that react to form carbon dioxide and water: for every mole of methane combusted, two moles of oxygen are consumed, one mole of carbon dioxide is produced, and two moles of water are produced.
Because of the well known relationship of moles toatomic weights, the ratios that are arrived at by stoichiometry can be used to determine quantities by weight in a reaction described by a balanced equation. This is calledcomposition stoichiometry.
Gas stoichiometry deals with reactions solely involving gases, where the gases are at a known temperature, pressure, and volume and can be assumed to beideal gases. For gases, the volume ratio is ideally the same by theideal gas law, but the mass ratio of a single reaction has to be calculated from themolecular masses of the reactants and products. In practice, because of the existence ofisotopes,molar masses are used instead in calculating the mass ratio.
^Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S.General Chemistry, 4th ed., p37, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005.
^Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S.General Chemistry, 4th ed., pp 45–46, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005.
^The boundary between metalloids and non-metals is imprecise, as explained in the previous reference.
