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[[Image:Kochendes wasser02.jpg|thumb|right|Water and steam are two different forms of the same chemical substance.]]

In [[chemistry]], a '''chemical substance''' is  a [[material]] with a specific [[Chemical compound|chemical composition]].<ref>{{GoldBookRef |title=Chemical Substance |file=C01039}}</ref>

A common example of a chemical substance is pure [[Water (molecule)|water]]; it has the same properties and the same [[ratio]] of [[hydrogen]] to [[oxygen]] whether it is isolated from a river or made in a [[laboratory]]. Some typical chemical substances are [[diamond]], [[gold]], [[Edible salt|salt]] ([[sodium chloride]]) and [[sugar]] ([[sucrose]]). Generally, chemical substances exist as a [[solid]], [[liquid]], [[gas]], or [[plasma]] and may change between these [[Phase (matter)|phases of matter]] with changes in [[temperature]] or [[pressure]]. [[Chemical reaction]]s convert one chemical substance into another.

Forms of [[energy]], such as [[light]] and [[heat]], are not considered to be matter, and thus they are not "substances" in this regard.

==Definition==
Chemical substances (also sometimes referred to as a pure substance) are often defined as "any material with a definite chemical composition" in most introductory general chemistry textbooks.<ref name="HillPetrucciP5"> Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. ''General Chemistry'', 4th ed., p5, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005 </ref> According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound.  However, there are exceptions to this definition; a pure substance can also be defined as a form of [[matter]] that has both definite composition and distinct properties.<ref>[http://www.diracdelta.co.uk/science/source/p/u/pure%20substance/source.html Pure Substance - DiracDelta Science & Engineering Encyclopedia]</ref>  The chemical substance index published by [[Chemical Abstracts Service|CAS]] also includes several [[alloys]] of uncertain composition.<ref>[http://www.cas.org/ASSETS/58D34DD3892142D18F5C3B0A004D3A0C/indexguideapp.pdf Appendix IV: Chemical Substance Index Names<!-- Bot generated title -->]</ref> [[Non-stoichiometric compound]]s are a special case (in inorganic chemistry) that violates the law of constant composition, and for them, it is sometimes difficult to draw the line between a mixture and a compound, as in the case of [[palladium 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"<ref>{{cite web|url=http://www.epa.gov/oppt/newchems/pubs/invntory.htm|title=What is the TSCA Chemical Substance Inventory?|publisher=US Environmental Protection Agency|accessdate=2009-10-19}}</ref>

==History==
The [[concept]] of a "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]].<ref name='Hill_et_al_p37'>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.</ref> 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]].<ref>[http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/LawofDefiniteProportion.html Law of Definite Proportions]</ref> 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.

==Chemical elements==
{{Main|Chemical element}}
{{See also|List of elements by name}}
An [[chemical element|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 [[isotope]]s, with differing numbers of neutrons.

There are about 120 known elements, about 80 of which are stable - that is, they do not change by [[radioactive decay]] into other elements.  However, the number of chemical substances that are elements can be more than 120, because some elements can occur as more than a single chemical substance ([[allotropes]]).  For instance, oxygen exists as both diatomic oxygen (O<sub>2</sub>) and [[ozone]] (O<sub>3</sub>).  The majority of elements are classified as [[metal]]s.  These are elements with a characteristic [[Lustre (mineralogy)|lustre]] such as [[iron]], [[copper]], and [[gold]].  Metals typically conduct electricity and heat well, and they are [[Malleability|malleable]] and [[Ductility|ductile]].<ref name='Hill_et_al_pp44-45'>Hill, J. W.; Petrucci, R. H.; McCreary, T. W.; Perry, S. S. ''General Chemistry'', 4th ed., pp45-46, Pearson Prentice Hall, Upper Saddle River, New Jersey, 2005.</ref>  Around a dozen elements,<ref>The boundary between metalloids and non-metals is imprecise, as explained in the previous reference.</ref> such as [[carbon]], [[nitrogen]], and [[oxygen]], are classified as [[non-metal]]s.  Non-metals lack the metallic properties described above, they also have a high [[electronegativity]] and a tendency to form [[Ion|negative ions]].  Certain elements such as [[silicon]] sometimes resemble metals and sometimes resemble non-metals, and are known as [[metalloid]]s.

==Chemical compounds==
{{Main|Chemical compound}}
{{See also|List of organic compounds|List of inorganic compounds}}
A pure chemical compound is a chemical substance that is composed of a particular set of [[molecule]]s or [[ion]]s. 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 [[Chemical bond|bonded]] together in [[molecule]]s or [[crystal]]s in which atoms, molecules or ions form a [[crystal structure|crystalline lattice]].  Compounds based primarily on carbon and hydrogen atoms are called [[organic compound]]s, and all others are called [[inorganic compound]]s. Compounds containing bonds between carbon and a metal are called [[organometallic compound]]s.

Compounds in which components share electrons are known as [[covalent bond|covalent]] compounds. Compounds consisting of oppositely charged [[ion]]s are known as [[ionic bond|ionic]] compounds, or [[salt]]s.

In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called [[isomer]]s. 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 [[Glucose isomerase|enzymatic]] or [[Lobry-de Bruyn-van Ekenstein transformation|acid-base catalysis]]. However, there are also [[tautomer]]s, 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.

==Substances versus mixtures==
{{Main|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 [[Mechanics|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 (56 [[gram]]s (1 [[mole (unit)|mol]]) of iron to 32&nbsp;grams (1&nbsp;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==
While the term ''chemical substance'' is a somewhat technical term used most often by professional chemists, the word ''chemical''<ref>[http://www.nicnas.gov.au/Industry/New_Chemicals/Does_My_Chemical_Require_Notification/What_Is_A_Chemical.asp What is a chemical]</ref> is more widely used in the [[pharmaceutical]] industry, government and society in general. Thus the word ''chemical'' includes a much wider class of substances that includes many mixtures of chemical substances that often find application in many vocations;<ref>[http://www.bfr.bund.de/cd/569 BfR - Chemicals]</ref> and is most commonly used only for artificial or processed substances, such as the products of the [[chemical industry]].

Artificial chemicals can be classified by production volume into [[bulk chemical]]s, [[fine chemical]]s 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.

==Naming and indexing==
Every chemical substance has one or more [[systematic name]]s, usually named according to the [[IUPAC nomenclature|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.  [[Secondary metabolite|Natural products]] and [[pharmaceutical]]s 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.

[[Chemist]]s frequently refer to [[chemical compound]]s using [[chemical formula]]e 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.<ref>[http://www.rz.uni-karlsruhe.de/~ed01/Jslit/eduquim.htm Coping with the Growth of Chemical Knowledge: Challenges for Chemistry Documentation, Education, and Working Chemists]</ref> An enormous number of chemical compounds are possible through the chemical combination of the known chemical elements. At the last count, about thirty million chemical compounds are known.<ref>[http://www.cas.org/cgi-bin/regreport.pl Chemical Abstracts substance count]</ref> 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 journal]]s and [[patent]]s). 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: [[Simplified molecular input line entry specification|SMILES]] and the [[International Chemical Identifier]] or InChI.

{| border="1" cellpadding="2" style="border-collapse: collapse;"
|+Identification of a typical chemical substance
|-
! Common name || [[IUPAC nomenclature|Systematic name]] || [[Chemical formula]] || [[Chemical structure]] || [[CAS registry number]] || [[International Chemical Identifier|InChI]]
|-
| alcohol, or<br/>ethyl alcohol
| ethanol
| C<sub>2</sub>H<sub>5</sub>OH
| [[Image:Ethanol-skeletal.png|60px|center]]
| [64-17-5]
| 1/C2H6O/c1-2-3/h3H,2H2,1H3
|-
|}

==Isolation, purification, characterization, and identification==
Often a pure substance needs to be isolated from a [[mixture]], for example from a [[nature|natural source]] (where a sample often contains numerous chemical substances<!--HELP, I need a nice example, I think the extraction of cocaine from the coca-plant is stigmatising-->) or after a [[chemical reaction]] (which often give mixtures of chemical substances).
==See also==
{{Portal|Chemistry}}
* [[IUPAC nomenclature]]

==Notes and references==
{{Reflist}}

{{DEFAULTSORT:Chemical Substance}}
[[Category:Chemistry]]
[[Category:Chemical substances| ]]

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