Chemical substance
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Water and steam are two different forms of the same chemical substance |
A
chemical substance is any
material with a definite chemical composition, no matter where it comes from.
[ 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. ] For example, a sample of
water has the same properties and the same ratio of
hydrogen and
oxygen whether the sample is isolated from a
river or made in a
laboratory. A pure substance cannot be separated into other substances by any mechanical process,
[Ebbing, D. D.; Gammon, S. D. General Chemistry, 7th ed., p12, Houghton Mifflin, Boston, Massachusetts, 2002.]. Typical chemical substances found in the home are
water,
salt (
sodium chloride) and
sugar (
sucrose). Generally, substances exist as
solid,
liquid, or
gas, and may change between these
phases of matter with changes in
temperature or
pressure.
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.
[ 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, and it is one of the foundations of modern chemistry.
A chemical substance that cannot be broken down or transformed by ordinary chemical processes into a different substance is called a
chemical element (often referred to simply as an
element). An element consists of particles called
atoms, which consist of negatively
charged electrons centered about a
nucleus of positively charged
protons and uncharged
neutrons. 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. The majority of elements are 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.
[ 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
[The boundary between metalloids and non-metals is imprecise, as explained in the previous reference.] 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.
Two or more elements combined into one substance form what is called a
chemical compound, which consists of atoms
bonded together in
molecules. An enormous number of chemical compounds are possible by combining the roughly 120 chemical elements; currently, about thirty million have been characterized and identified.
[Chemical Abstracts substance count] Compounds based on
carbon and
hydrogen atoms are called
organic compounds, and those based on other elements are called
inorganic compounds. Compounds containing bonds between carbon and a
metal are called 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.
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.
Every chemical substance carries a unique
systematic name, 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, which have more explicit information about the
structure of the compound. Computer-friendly systems have been developed for substance information, such as the
CAS registry number,
SMILES and more recently the
International Chemical Identifier or InChI. Besides being used on computer
databases, these systems, especially the CAS number, have also become useful in paperwork as unique codes for identifying specific substances.
Often a chemical 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.
