Chemical substance

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. 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. 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. 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.

Elements

Main article: Chemical 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. 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 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

Chemical compounds

Main article: Chemical compound

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. 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.

See also: list of organic compounds

See also: list of inorganic compounds

Substances and mixtures

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 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.

Identification of a typical chemical substance

Common name	Systematic name	Chemical formula	Chemical structure	CAS registry number	InChI
alcohol, or ethyl alcohol	ethanol	C2H5OH	File:Ethanol-skeletal.png	[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). 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.