A chemical substance is any material that has a definite chemical composition. Moreover, a pure substance cannot be separated into other substances by any mechanical process. Chemical elements and compounds are substances, mixtures are not.
Examples of familiar chemical substances are pure water, table salt (sodium chloride), and sugar (sucrose). Generally, substances exist in the solid, liquid, or gas phase, and they may change from one phase to another during 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. For example, a sample of pure water has the same properties and same ratio of hydrogen to oxygen whether the sample is isolated from a river (and then purified) or produced by a chemical reaction in a laboratory.
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 distributed around 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.
The majority of elements are classified as metals. These elements—such as iron, copper, and gold—have a characteristic luster. Generally speaking, metals are good conductors of heat and electricity, and they are malleable and ductile.
Around a dozen elements—such as carbon, nitrogen, and oxygen—are classified as nonmetals. They lack the metallic properties described above. They also have a high electronegativity value and a tendency to form negative ions called anions.
Certain elements, such as silicon, have some properties of metals and some of nonmetals. They are known as metalloids.
An enormous number of chemical compounds can be produced by combining the roughly 120 chemical elements. Currently, about 30 million compounds have been identified and characterized. 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 the atoms of elements share electrons are known as covalent compounds. Compounds in which oppositely charged ions are held together by electrostatic interactions are known as ionic compounds (or salts).
Chemical substances are often intimately mixed together to form what are called mixtures. Mixtures do not have a fixed composition. In principle, they can be separated into the component substances by purely mechanical processes. A solution of sugar (sucrose) in water is a mixture. Butter and soil are other examples of mixtures.
Gray iron metal and yellow sulfur are both chemical elements, and they can be mixed together in any ratio to form a yellow-gray mixture. No chemical process occurs, and the material can be identified as a mixture by the fact that the sulfur and iron can be separated by a mechanical process, such as using a magnet to attract the iron away from the sulfur.
By contrast, if iron and sulfur are heated together in a certain ratio (56 grams (one mol) of iron to 32 grams (one mol) of sulfur), a chemical reaction takes place and a new substance is formed, namely, the compound iron(II) sulfide, with chemical formula FeS. The resultant 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.
To name chemical substances, chemists use rules formulated by the International Union of Pure and Applied Chemistry (IUPAC) or the Chemical Abstracts Service (CAS). In this manner, a unique systematic name is given to every chemical substance. Many compounds are also known by their more common, simpler names, many of which predate their systematic names. For example, glucose is a common name for a type of sugar that has long been known. Its systematic name is 6-(hydroxymethyl)oxane-2,3,4,5-tetrol. In addition, many natural products and pharmaceuticals have been 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 (Simplified molecular input line entry specification), and the International Chemical Identifier (InChI). Besides being used on computer databases, these systems, especially the CAS number, have become useful in paperwork as unique codes for identifying specific substances.
|Common name||Systematic name||Chemical formula||Chemical structure||CAS registry number||InChI|
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.
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