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Water and steam are two different forms of the same chemical substance
A 'chemical substance' is a material with a definite chemical composition. It is a
concept that 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.
[ 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 became known as the
law of constant composition[1]. 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.
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. A pure chemical substance cannot be separated into other substances by a process that does not involve any
chemical reaction and is rarely found in nature. Some typical chemical substances can be
diamond,
gold,
salt (
sodium chloride) and
sugar (
sucrose). Generally, chemical substances exist as ''
solid'', ''
liquid'', or ''
gas'', and may change between these ''
phases of matter'' with changes in
temperature or
pressure.
Defining
Chemical substances (also sometimes referred to as a pure substances) are often defined as ''any material with a definite chemical composition'' in most introductory general chemistry textbooks.
[ 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. 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'.
[2] and the chemical substance index published by
CAS also includes several
alloys of uncertain composition.
[3]
Elements
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.
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 as oxygen and
ozone. The majority of elements are classified as ''
metals''. These are elements with a characteristic such as
iron,
copper, and
gold. Metals typically conduct electricity and heat well, and they are
malleable and
ductile.
[ 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. ] Around a dozen elements
[4] 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 called anions. Certain elements such as
silicon sometimes resemble metals and sometimes resemble non-metals, and are known as ''
metalloids''.
Chemical 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 what is called a 'chemical compound'. 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.
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 (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
While the term chemical substance is a somewhat technical term used most often by professional chemists, the word 'chemical'
[5] 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
[6]; and is most commonly used only for artificial or processed substances, such as the products of the
chemical industry.
Naming and Indexing
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
[7]. 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.
[8]. 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 the
IUPAC and the Chemical Abstract Service (CAS) have intiated steps to make such tasks easier. CAS that provides the abstracting services of the chemical literature, 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.
Isolation, purification, characterisation 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). This is usually done using physical processes such as
distillation,
filtration,
liquid-liquid extraction, and
evaporation. These same techniques, along with others such as
recrystallisation, may also be used to purify the substance. Once the pure material has been prepared, its chemical and physical properties may be examined in order to characterise the substance. Finally,
chemical analysis may be used to identify the
chemical composition and to
assay the purity.
References and notes
1. http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/LawofDefiniteProportion.html
2. http://www.diracdelta.co.uk/science/source/p/u/pure%20substance/source.html
3. http://www.cas.org/ASSETS/58D34DD3892142D18F5C3B0A004D3A0C/indexguideapp.pdf
4. The boundary between metalloids and non-metals is imprecise, as explained in the previous reference.
5. What is a chemical
6. http://www.bfr.bund.de/cd/569
7. http://www.rz.uni-karlsruhe.de/~ed01/Jslit/eduquim.htm
8. Chemical Abstracts substance count
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