A chemical substance, also known as a pure substance, is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e., without breaking chemical bonds. Chemical substances can be chemical elements, chemical compounds, ions 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 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 (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, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure. Chemical substances may be combined or converted to others by means of chemical reactions. Forms of energy, such as light and heat, are not matter, and are thus not "substances" in this regard.
1 Definition 2 History 3 Chemical elements 4 Chemical compounds 5 Substances versus mixtures 6 Chemicals versus chemical substances 7 Naming and indexing 8 Isolation, purification, characterization, and identification 9 See also 10 Notes and references 11 External links
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.
A chemical substance may well be defined as "any material with a
definite chemical composition" in an introductory general chemistry
textbook. 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 compounds 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 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, 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 -CH2- repeating units, and is
generally sold in several molar mass distributions, LDPE, MDPE, HDPE
The concept of a "chemical substance" became firmly established in the
late eighteenth century after work by the chemist
Native sulfur crystals.
Main article: Chemical element See also: List of 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 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. Around a dozen 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 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
Main article: Chemical compound
List of organic compounds and List of inorganic compounds
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
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
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
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.
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). "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 compositionsl, as "gasoline" is
primarily defined through source, properties and octane rating.
Naming and indexing
Every chemical substance has one or more systematic names, usually
named according to the
Identification of a typical chemical substance
Common name Systematic name Chemical formula Chemical structure CAS registry number InChI
Alcohol, or ethyl alcohol Ethanol C2H5OH
Isolation, purification, characterization, and identification Often a pure substance needs to be isolated from a mixture, for example from a natural source (where a sample often contains numerous chemical substances) or after a chemical reaction (which often give mixtures of chemical substances). See also
Chemical safety signs
Notes and references
^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "
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