Chemical substance

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The term chemical substance refers either to a technical meaning used by specialists (generally chemists, e.g., from the IUPAC Gold Book), or to a broader meaning that encompasses regulatory and common uses that expands the former. As a technical term, chemical substance refers to a form of matter—solid, liquid, or gas—that has constant chemical composition of its component atoms, molecules, or other entities, that results in physical properties (e.g., melting point, refractive index, density, etc.) that can be measured to characterize it.

In this narrow technical definition, chemical substances fall into several clear subcategories: they can be elemental materials in which all atoms have the same atomic number[1] (e.g. metallic gold, the material diamond, the diatomic molecule H2, or the polyatomic molecule S8). Alternatively they can be non-stoichiometric compounds composed of non-integer proportions of different atoms (e.g., palladium hydride, PdHx, x ≠ 1,2,3...); or they can be chemical compounds, pure matter consisting of two or more different chemical elements in fixed proportion, however simple or complex—e.g. ammonia gas from a cylinder, pure water from a still, paracetemol before going into a tablet, sodium chloride and sucrose as a components in table salt and sugar, lead(II) sulfate before it goes into batteries, drugs whether man-made (e.g. aripiprazole) or isolates from natural sources (e.g., THC), or a humanized antibody (e.g., adalimumab) or even a BRCA1 gene or protein (if its structure is fully determined, including sequence, covalent modifications, and counter ions).

In addition, when the type of chemical entity composing the substance is a molecule rather than an atom, the definition can include mixtures of defined, constant composition—composed to specification, with precise proportions of chemical ingredients—if such a mixture presents consistent, measurable physical properties (e.g., formulated motor oils, of given viscosity, flash point, etc.). Hence, these four subcategories are formal subsets of the category of chemical substances (by this technical definition), rather than being interchangeable as synonyms, as sometimes might appear to be the case in non-technical (and even occasional technical) writing venues.

However, chemical substance can also more broadly connote—in venues ranging from EPA documents, to databases exhibiting flexibility for sake of expedience, to common parlance—any substance produced by, used for, or related to chemistry or chemical operations or production. For instance, the EPA definition of this term is based on text from a legislative action, and includes any particular molecular identity, organic or inorganic, alone or in combination, from nature or artificial chemical reaction; this definition, therefore, falls outside the IUPAC definition, and within the general meaning of a "substance" that is "chemical."

As noted, chemical substances exist as solids, liquids, or gases, and may change between these phases of matter with changes in temperature or pressure.[relevant? ] As the title term always refers to matter, all forms of energy (heat, light, etc.) are not chemical substances.[not verified in body] Some types of chemical substances can generally be thought of as pure (as defined operationally, for the substance, in a particular use); chemical substance classes that can be though of in terms of purity include chemical elements, chemical compounds, and non-stoichiometric compounds (examples listed above). Other types of chemical substances that are best understood as being well-defined in composition, without reference to purity, are mixtures of defined composition that present consistent, measurable physical properties. Finally, the most loose definition in use, of any material that is chemical in origin or association, makes no claim for purity or defined composition; hence, the title term also appears, e.g., to refer to spills of chemical compounds, preparations, or mixtures of unknown composition, when occurring in the field.


Definition[edit]

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.

Into the second decade of the new millennium, the combined title term is left undefined in major dictionaries such as Merriam-Webster and Oxford; however, chemical substance is both defined by the International Union of Pure and Applied Chemistry, as a technical chemistry term (see below), and it can be understood based on its more general definition as a material "substance", with the modifier "chemical". The former case, the technical, IUPAC use of chemical substance, is clearly a broadening of other more specific chemical categories (e.g., like chemical compound), but also a narrowing of the general meaning of any "substance" that can be identified as "chemical", e.g., "there was a chemical substance spilled on the floor."

The formal IUPAC definition is that a chemical substance is any material presenting itself that one can conclude is made up of "matter of constant composition," where the constant composition can be of any chemical component—entities such as "molecules, formula units, atoms," etc.—and where the result of the constancy is that the substance presents "[p]hysical properties such as density, refractive index, electric conductivity, melting point etc." that can be measured to characterize it.[2] This formal definition is the basis of other definitions used by particular groups or agencies—e.g., the EPA, see below,—or publications (e.g., encyclopedias[3]). Critically, by this definition, chemical substances include:

In the last case, formulated motor oils and other such complex mixtures qualify as they are composed to specification of precise proportions of chemical ingredients, and therefore exhibit a set of measurable physical properties (e.g., viscosity, flash point, etc.).[citation needed] A slight narrowing of the IUPAC definition often appears in introductory (general) chemistry textbooks, where a definition such as "any material with a definite chemical composition" is stated, omitting the consequent, derivative measurable physical properties (often appearing in later explanations).[4][full citation needed][better source needed] In such cases, a simple example is often chosen from among the compounds, such pure water, H2O, obscuring the more general meaning of substance and the more specific meaning of compound.[citation needed]

The most general meaning of chemical substance that appears is as a subcategory of all substances—any real physical matter with a tangible, presence, often composing something otherwise identified[5][6]—specifically, those substances that can, in some some respect, be identified as "chemical"—anything produced by, used for, or related to chemistry or chemical operations or production.[7][8] In this regard, the EPA offers a definition based in legislative action [Section 8(b) of the Toxic Substances Control Act] that includes any "particular molecular identity," organic or inorganic, alone or in combination, from nature or artificial chemical reaction, and therefore appears as this more general type of meaning.[9] Hence, while encompassing pure and well-defined chemical elements and chemical compounds, an appropriate use of the title term remains, "the spill was a chemical substance," even though the material does not necessary derive from anything pure or composed of fixed proportions (e.g., water or sulfuric acid), nor even from anything of constant composition and reproducible physical properties (IUPAC definition, e.g., new motor oil). In short, the title term has connotations that include impure and ill-defined chemical cases.[citation needed][9]

In specific non-chemistry disciplines, more nuanced versions of the foregoing definitions may apply. In geology, substances of uniform composition are called minerals, while physical mixtures (aggregates) of several minerals (different substances) are defined as rocks.[citation needed] Many minerals, however, mutually dissolve into solid solutions, such that a single rock is a uniform substance despite being a mixture in stoichiometric terms.[citation needed] Feldspars are a common example: anorthoclase is an alkali aluminium silicate, where the alkali metal is interchangeably either sodium or potassium.[clarification needed][citation needed] Finally, the term can even be used loosely within the chemical profession, when expedience dictates. For instance, the "chemical substance" index published by CAS also includes several alloys of uncertain composition.[10]

History[edit]

The term "chemical substance" became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate.[11] 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 the law of constant composition.[12] Later with the advancement of methods for chemical synthesis particularly in the realm of organic chemistry; the discovery of many more chemical elements and new techniques in the realm of analytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modern chemistry, 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, since isomers have exactly the same composition, but differ in configuration (arrangement) of the atoms. For example, there was much speculation for the chemical identity of benzene, until the correct structure was described by Friedrich August Kekulé. Likewise, the idea of stereoisomerism - that atoms 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 of diastereomers with one diastereomer forming two enantiomers.

Chemical elements[edit]

Native sulfur crystals. Sulfur occurs naturally as elemental sulfur, in sulfide and sulfate minerals and in hydrogen sulfide.
Main article: Chemical element

An element is a chemical substance that is made up of a particular kind of atoms and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmutated into another element through a nuclear reaction. This is so, because all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons.

As of 2012, there are 118 known elements, about 80 of which are stable – that is, they do not change by radioactive 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) and ozone (O3). The majority of elements are classified as metals. These are elements with a characteristic lustre such as iron, copper, and gold. Metals typically conduct electricity and heat well, and they are malleable and ductile.[13] Around a dozen elements,[14] such as carbon, nitrogen, and oxygen, are classified as non-metals. Non-metals lack the metallic properties described above, they also have a high electronegativity and a tendency to form negative ions. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids.

Chemical compounds[edit]

Potassium ferricyanide is a compound of potassium, iron, carbon and nitrogen; although it contains cyanide anions, it does not release them and is nontoxic.
Main article: Chemical compound

A pure chemical compound is a chemical substance that is composed of a particular set of molecules or ions. Two or more elements combined into one substance through a chemical reaction form a chemical compound. All compounds are substances, but not all substances are compounds.

A chemical compound can be either atoms bonded together in molecules or crystals in which atoms, molecules or ions form a crystalline lattice. Compounds based primarily on carbon and hydrogen atoms are called organic compounds, and all others are called inorganic compounds. Compounds containing bonds between carbon and a metal are called organometallic compounds.

Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds, or salts.

In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers. Isomers usually have substantially different chemical properties, may be isolated and do not spontaneously convert to each other. A common example is glucose vs. fructose. The former is an aldehyde, the latter is a ketone. Their interconversion requires either enzymatic or acid-base catalysis. However, there are also tautomers, where isomerization occurs spontaneously, such that a pure substance cannot be isolated into its tautomers. A common example is glucose, which has open-chain and ring forms. One cannot manufacture pure open-chain glucose because glucose spontaneously cyclizes to the hemiacetal form. Materials may also comprise other entities such as polymers. These may be inorganic or organic and sometimes a combination of inorganic and organic.

Substances versus mixtures[edit]

Cranberry glass, while it looks homogeneous, is a mixture consisting of glass and gold colloidal particles of ca. 40 nm diameter, which give it a red color.
Main article: Mixture

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. In principle, they can be separated into the component substances by purely mechanical processes. Butter, soil and wood are common examples of mixtures.

Grey iron metal and yellow sulfur 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 a magnet 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, 56 grams (1 mol) of iron to 32 grams (1 mol) of sulfur), a chemical reaction takes place and a new substance is formed, the compound iron(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 as melting point and solubility, 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.

Chemicals versus chemical substances[edit]

While the term chemical substance is a precise technical term that is synonymous with "chemical" for professional chemists, the meaning of the word chemical varies for non-chemists within the English speaking world or those using English. For industries, government and society in general in some countries,[15] the word chemical includes a wider class of substances that contain many mixtures of such chemical substances, often finding application in many vocations.[16] In countries that require a list of ingredients in products, the "chemicals" listed would be equated with "chemical substances".[17]

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 and speciality 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. The production 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).

Naming and indexing[edit]

Every chemical substance has one or more systematic names, usually named according to the IUPAC rules for naming. An alternative system is used by the Chemical Abstracts Service (CAS).

Many compounds are also known by their more common, simpler names, many of which predate the systematic name. For example, the long-known sugar glucose is now systematically named 6-(hydroxymethyl)oxane-2,3,4,5-tetrol. Natural products and pharmaceuticals are also given simpler names, for example the mild pain-killer Naproxen is the more common name for the chemical compound (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid.

Chemists frequently refer to chemical compounds using chemical formulae or molecular structure of the compound. There has been a phenomenal growth in the number of chemical compounds being synthesized (or isolated), and then reported in the scientific 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 May 2011, about sixty million chemical compounds are known.[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 the track of them in the literature. Several international organizations like IUPAC 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 as CAS registry number to each chemical substance that has been reported in the chemical literature (such as chemistry journals and patents). This information is compiled as a database and is popularly known as the Chemical substances index. Other computer-friendly systems that have been developed for substance information, are: SMILES and the International Chemical Identifier or InChI.

Identification of a typical chemical substance
Common name Systematic name Chemical formula Chemical structure CAS registry number InChI
alcohol, or
ethyl alcohol
ethanol C2H5OH
Ethanol-2D-skeletal.png
[64-17-5] 1/C2H6O/c1-2-3/h3H,2H2,1H3

Isolation, characterization, and identification[edit]

Chemical substances, depending on the whether the narrow IUPAC or broad definition is being discussed, may be pure (as in the case of chemical elements and chemical compounds), or may be mixtures, well- or ill-defined (see Definitions). If a further chemical substance is desired in pure form from any type of mixture—e.g., from a natural source or from a chemical reaction mixture—the aim is isolation (purification) if individual entities from a sample that often contains numerous component chemical entities, followed by characterization of properties and structure, and then identification (if already known) or description (if new) of the purified entity.

See also[edit]

Notes and references[edit]

  1. ^ IUPAC (ed.). "chemical element". http://iupac.org. doi:10.1351/goldbook.C01022. 
  2. ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "Chemical Substance".
  3. ^ E.g., for the comparable definition from Dirac Delta Consultants, see: "Pure Substance – DiracDelta Science & Engineering Encyclopedia". Diracdelta.co.uk. Retrieved 2013-06-06. 
  4. ^ See, for instance, Hill, J. W.; Petrucci, R. H.; McCreary, T. W. & Perry, S.S., 2005, General Chemistry, 4th edn., p. 5, Upper Saddle River, NJ, USA:Pearson/Prentice Hall.[full citation needed]
  5. ^ "Substance". Merriam-Webster's Collegiate® Dictionary, Eleventh Edition (online). Springfield, MA, USA: Merriam-Webster. 2015. Retrieved 2 July 2015. 
  6. ^ "Substance". Oxford Dictionaries (online). Oxford, OXF, GBR: Oxford University Press. 2015. Retrieved 2 July 2015. 
  7. ^ "Chemical". Merriam-Webster's Collegiate® Dictionary, Eleventh Edition (online). Springfield, MA, USA: Merriam-Webster. 2015. Retrieved 2 July 2015. 
  8. ^ "Chemical". Oxford Dictionaries (online). Oxford, OXF, GBR: Oxford University Press. 2015. Retrieved 2 July 2015. 
  9. ^ a b EPA, 2014, "Basic Information: Background" at TSCA Chemical Substance Inventory (online), per Section 8 (b) of the Toxic Substances Control Act, see [1], accessed 2 July 2015. The full definition there is:"'chemical substance' means any organic or inorganic substance of a particular molecular identity, including any combination of these substances occurring in whole or in part as a result of a chemical reaction or occurring in nature, and any element or uncombined radical. Chemicals substances on the Inventory include: organics, inorganics, polymers, and UVCBs (chemical substances of unknown or variable composition, complex reaction products, and biological materials)."
  10. ^ Appendix IV: Chemical Substance Index Names
  11. ^ 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.
  12. ^ Law of Definite Proportions[dead link]
  13. ^ 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.
  14. ^ The boundary between metalloids and non-metals is imprecise, as explained in the previous reference.
  15. ^ "What is a chemical". Nicnas.gov.au. 2005-06-01. Retrieved 2013-06-06. 
  16. ^ "BfR – Chemicals". Bfr.bund.de. 1980-09-18. Retrieved 2013-06-06. 
  17. ^ There is only one definition for "chemical", that of a substance, in the US Unabridged Edition of the Random House Dictionary of the English Language, New York, 1966.
  18. ^ Joachim Schummer. "Coping with the Growth of Chemical Knowledge: Challenges for Chemistry Documentation, Education, and Working Chemists". Rz.uni-karlsruhe.de. Retrieved 2013-06-06. 
  19. ^ "Chemical Abstracts substance count". Cas.org. Retrieved 2013-06-06. 

Further reading[edit]

External links[edit]