<Li> Fine chemicals are produced at a high cost in small quantities for special low - volume applications such as biocides, pharmaceuticals and speciality chemicals for technical applications . </Li> <Li> Research chemicals are produced individually for research, such as when searching for synthetic routes or screening substances for pharmaceutical activity . In effect, their price per gram is very high, although they are not sold . </Li> <P> The cause of the difference in production volume is the complexity of the molecular structure of the chemical . Bulk chemicals are usually much less complex . While fine chemicals may be more complex, many of them are simple enough to be sold as "building blocks" in the synthesis of more complex molecules targeted for single use, as named above . The production of a chemical includes not only its synthesis but also its purification to eliminate by - products and impurities involved in the synthesis . The last step in production should be the analysis of batch lots of chemicals in order to identify and quantify the percentages of impurities for the buyer of the chemicals . The required purity and analysis depends on the application, but higher tolerance of impurities is usually expected in the production of bulk chemicals . Thus, the user of the chemical in the US might choose between the bulk or "technical grade" with higher amounts of impurities or a much purer "pharmaceutical grade" (labeled "USP", United States Pharmacopeia). "Chemicals" in the commercial and legal sense may also include mixtures of highly variable composition, as they are products made to a technical specification instead of particular chemical substances . For example, gasoline is not a single chemical compound or even a particular mixture: different gasolines can have very different chemical compositionsl, as "gasoline" is primarily defined through source, properties and octane rating . </P> <P> Every chemical substance has one or more systematic names, usually named according to the IUPAC rules for naming . An alternative system is used by the Chemical Abstracts Service (CAS). </P>

Which of the following is a pure substance made entirely of one type of atom