Chemical substance
A chemical substance is any material object that can undergo transformations responsible for a phenomenon such as a fire or an explosion. In chemistry text books it is defined as a material with a definite chemical composition.[1] The example often cited is a sample of water that has the same properties and the same ratio of hydrogen to oxygen whether the sample is isolated from a river or made in a laboratory.
However, this text book definition does not cover mixtures of elements and compounds, which are the most common form of chemical substances and which do not have a universal definite composition. Thus it becomes imperative to differentiate between pure substances and mixtures of substances.
Forms of chemical substances
The various forms in which a chemical substance can be found in nature are:
Pure substance
A pure substance is either an element or a compound. In case it is an element it can either be natural or synthetic (obtained through a nuclear reaction). Same is the case if it is a compound, it is either a natural or obtained through a chemical reaction.
Examples of typical pure chemical substances found in the home, often cited in text books, are water, salt (sodium chloride) and sugar (sucrose). However, strictly speaking most of the household samples are never really pure substances, they are mixtures with several impurities. Pure substances can exist as solid, liquid, gas or plasma.
The concept of a pure 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.[2] 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 pure chemical element can be transformed to a compound when it reacts with another element or a compound. A chemical compound can be converted into another chemical compound through a chemical reaction.[3]. An element can be transformed into another element only through a nuclear reaction. However, most chemical or nuclear reactions seldom yield a pure substance as their products.
Mixtures
A chemical substance can be a mixture of various elements and/or chemical compounds. Chemical mixtures contain more than one pure chemical substance, and they do not have a fixed composition. In principle, they can be separated into the component substances by a process that does not involve a chemical reaction. 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 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.
A mixture can be in the form of a gas, liquid, solid or a colloid.
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) . However, there are chemical substances known by a traditional name such as air, steel, petroleum, brass etc. All of these chemical substances are invariably mixtures.
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.
| 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). 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.
References and notes
- ^ 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.
- ^ 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.
- ^ Ebbing, D. D.; Gammon, S. D. General Chemistry, 7th ed., p12, Houghton Mifflin, Boston, Massachusetts, 2002.