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 A chemical substance is any
material with a definite chemical composition, no matter where it comes
from.[1] 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.[2] 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.[3] 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.
Element:
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 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.[4] Around a dozen elements[5] 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 :
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.[6] 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 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.
Substances and 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.
Naming chemical substances :
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.
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.
Sources : Internet
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