Chemical substance
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Chemistry Portal 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.<1> 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<2>. 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.<3> 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.<4> and the chemical substance index published by CAS also includes several alloys of uncertain composition.<5> Nonstoichiometric 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.
Elements
Main article: Chemical element
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 lustre such as iron, copper, and gold. metals typically conduct electricity and heat well, and they are malleable and ductile.<6> Around a dozen elements<7> 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.
See also: List of elements by name
Identification of a typical chemical substance
Common name
Systematic name
Chemical formula
Chemical structure
CAS registry number
InChI
alcohol, or
ethyl alcohol
ethanol
C2H5OH
<64-17-5>
1/C2H6O/c1-2-3/h3H,2H2,1H3
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).
More abstracts about the Chemical Substance