A chemical substance is a form of
matter having constant
chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent
elements by physical separation methods, i.e., without breaking
chemical bond
A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
s. Chemical substances can be simple substances (substances consisting of a single
chemical element),
chemical compounds, or
alloys.
Chemical substances are often called 'pure' to set them apart from
mixtures. A common example of a chemical substance is pure
water; it has the same properties and the same
ratio of
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
to
oxygen whether it is isolated from a river or made in a
laboratory. Other chemical substances commonly encountered in pure form are
diamond (carbon),
gold,
table salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
(
sodium chloride) and refined
sugar (
sucrose). However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical.
Chemical substances exist as
solids,
liquids,
gas
Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma).
A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
es, or
plasma, and may change between these
phases of matter with changes in
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
or
pressure and
time. Chemical substances may be combined or converted to others by means of
chemical reactions.
Definition
A chemical substance may well be defined as "any
material with a definite chemical composition" in an introductory general chemistry textbook.
[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] According to this definition a chemical substance can either be a pure
chemical element or a pure chemical compound. But, 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. The chemical substance index published by
CAS also includes several
alloys of uncertain composition.
Non-stoichiometric compound
In chemistry, non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); m ...
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".
In
geology, substances of uniform composition are called
minerals, while physical mixtures (
aggregates) of several minerals (different substances) are defined as
rocks. Many minerals, however, mutually dissolve into
solid solution
A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...
s, 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, the
EU regulation
REACH 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 to 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 the
molar mass distribution. For example,
polyethylene is a mixture of very long chains of -CH
2- repeating units, and is generally sold in several molar mass distributions,
LDPE
Low-density polyethylene (LDPE) is a thermoplastic made from the monomer ethylene. It was the first grade of polyethylene, produced in 1933 by Imperial Chemical Industries (ICI) using a high pressure process via free radical polymerization. Its ...
,
MDPE,
HDPE
High-density polyethylene (HDPE) or polyethylene high-density (PEHD) is a thermoplastic polymer produced from the monomer ethylene. It is sometimes called "alkathene" or "polythene" when used for HDPE pipes. With a high strength-to-density ratio, ...
and
UHMWPE
Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, (HMPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. T ...
.
History
The
concept of a "chemical substance" became firmly established in the late eighteenth century after work by the chemist
Joseph Proust
Joseph Louis Proust (26 September 1754 – 5 July 1826) was a French chemist. He was best known for his discovery of the law of definite proportions in 1794, stating that chemical compounds always combine in constant proportions.
Life
Joseph L. ...
on the composition of some pure chemical compounds such as
basic copper carbonate
Basic copper carbonate is a chemical compound, more properly called copper(II) carbonate hydroxide. It is an ionic compound (a salt) consisting of the ions copper(II) , carbonate , and hydroxide .
The name most commonly refers to the compound w ...
.
[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.] 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
In chemistry, the law of definite proportions, sometimes called Proust's law, or law of constant composition states that a given
chemical compound always contains its component elements in fixed ratio (by mass) and does not depend on its source an ...
. 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
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
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
In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...
– 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
diastereomer
In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have di ...
s with one diastereomer forming two
enantiomers.
Chemical elements
An
element 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 a
nuclear reaction. This is 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 2019, 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
Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: th ...
). For instance, oxygen exists as both diatomic oxygen (O
2) and
ozone (O
3). The majority of elements are classified as
metals. These are elements with a characteristic
lustre such as
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
,
copper, and
gold. Metals typically conduct electricity and heat well, and they are
malleable
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
and
ductile
Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
.
[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.] Around 14 to 21 elements, 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
Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
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
A chemical compound is a chemical substance that is composed of a particular set of
atoms 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
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s 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 compound
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and s ...
s.
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 chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
.
Coordination complexes are compounds where a
dative 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
Tetraamminecopper(II) sulfate is the salt with the formula u(NH3)4O4·H2O. This dark blue to purple solid is a salt of the metal complex u(NH3)4(H2O)sup>2+. It is closely related to Schweizer's reagent, which is used for the production of cell ...
3)4">u(NH3)4O
4·H
2O. 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 by
boron trifluoride etherate BF
3OEt
2, where the highly
Lewis acidic, but nonmetallic boron center takes the role of the "metal". If the ligand bonds to the metal center with multiple atoms, the complex is called a
chelate
Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are ...
.
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, and often may be isolated without spontaneously interconverting. 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,
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 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
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
gram
The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.
Originally defined as of 1795 as "the absolute weight of a volume of pure water equal to th ...
s (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
Iron(II) sulfide or ferrous sulfide (Br.E. sulphide) is one of a family chemical compounds and minerals with the approximate formula . Iron sulfides are often iron-deficient non-stoichiometric. All are black, water-insoluble solids.
Preparatio ...
, 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 precise technical term that is synonymous with ''chemical'' for chemists, the word ''chemical'' is used in general usage to refer to both (pure) chemical substances and mixtures (often called ''compounds''), and especially when produced or purified in a laboratory or an industrial process.
[''chemical''](_blank)
in Oxford Online Dictionaries 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.
[Random House Unabridged Dictionary](_blank)
, 1997
Within the chemical industry, manufactured "chemicals" are chemical substances, which can be classified by production volume into bulk chemicals,
fine chemical
In chemistry, fine chemicals are complex, single, pure chemical substances, produced in limited quantities in multipurpose plants by multistep batch chemical or biotechnological processes. They are described by exacting specifications, used f ...
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.
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
The ''United States Pharmacopeia'' (''USP'') is a pharmacopeia (compendium of drug information) for the United States published annually by the United States Pharmacopeial Convention (usually also called the USP), a nonprofit organization that ...
). "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 and
octane rating.
Naming and indexing
Every chemical substance has one or more
systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature.
A semisystematic name or semitrivial ...
s, 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
A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical sy ...
and
pharmaceuticals are also given simpler names, for example the mild pain-killer
Naproxen
Naproxen is a nonsteroidal anti-inflammatory drug (NSAID) used to treat pain, menstrual cramps, inflammatory diseases such as rheumatoid arthritis, gout and fever. It is taken orally. It is available in immediate and delayed release formulat ...
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
: ''For a broader class of literature, see Academic publishing.''
Scientific literature comprises scholarly publications that report original empirical and theoretical work in the natural and social sciences. Within an academic field, scie ...
by professional chemists around the world. 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.
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
The International Chemical Identifier (InChI or ) is a textual identifier for chemical substances, designed to provide a standard way to encode molecular information and to facilitate the search for such information in databases and on the we ...
or InChI.
Isolation, purification, characterization, and identification
Often a pure substance needs to be isolated from a
mixture, for example from a
natural source
A natural product is a natural compound or substance produced by a living organism—that is, found in nature. In the broadest sense, natural products include any substance produced by life. Natural products can also be prepared by chemical sy ...
(where a sample often contains numerous chemical substances) or after a
chemical reaction (which often gives mixtures of chemical substances).
See also
*
Hazard symbol
*
Homogeneous and heterogeneous mixtures
*
Prices of chemical elements
*
Dedicated bio-based chemical
*
Fire diamond
"NFPA 704: Standard System for the Identification of the Hazards of Materials for Emergency Response" is a standard maintained by the U.S.-based National Fire Protection Association. First "tentatively adopted as a guide" in 1960, and revised se ...
*
Research chemical
Notes and references
External links
*
{{DEFAULTSORT:Chemical Substance
General chemistry
Artificial materials