For example, polyethylene is a mixture of very long chains of -CH 2- repeating units, and is generally sold in several molar mass distributions, LDPE, MDPE, HDPE and UHMWPE. However, the polymer may be defined by a known precursor or reaction(s) and the molar mass distribution. Polymers almost always appear as mixtures of molecules of multiple molar masses, each of which could be considered a separate chemical substance. Therefore, although the exact chemical identity is unknown, identification can be made with a sufficient accuracy. For example, charcoal is an extremely complex, partially polymeric mixture that can be defined by its manufacturing process. The latter two consist of multiple chemical substances however, their identity can be established either by direct chemical analysis or reference to a single manufacturing process. For example, the EU regulation REACH defines "monoconstituent substances", "multiconstituent substances" and "substances of unknown or variable composition". In law, "chemical substances" may include both pure substances and mixtures with a defined composition or manufacturing process. Feldspars are a common example: anorthoclase is an alkali aluminum silicate, where the alkali metal is interchangeably either sodium or potassium. Many minerals, however, mutually dissolve into solid solutions, such that a single rock is a uniform substance despite being a mixture in stoichiometric terms. In geology, substances of uniform composition are called minerals, while physical mixtures ( aggregates) of several minerals (different substances) are defined as rocks. Broader definitions of chemicals or chemical substances can be found, for example: "the term 'chemical substance' means any organic or inorganic substance of a particular molecular identity, including – (i) any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature". Non-stoichiometric 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. The chemical substance index published by CAS also includes several alloys of uncertain composition. But, 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. According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. Chemical substances may be combined or converted to others by means of chemical reactions.Ĭolors of a single chemical ( Nile red) in different solvents, under visible and UV light, showing how the chemical interacts dynamically with its solvent environment.Ī chemical substance may well be defined as "any material with a definite chemical composition" in an introductory general chemistry textbook. However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical.Ĭhemical substances exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure and time. Other chemical substances commonly encountered in pure form are diamond (carbon), gold, table salt ( sodium chloride) and refined sugar ( sucrose). 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.
Chemical substances can be simple substances (substances consisting of a single chemical element), chemical compounds, or alloys.Ĭhemical substances are often called 'pure' to set them apart from mixtures. Chemical substances cannot be separated into their constituent elements by physical separation methods, i.e., without breaking chemical bonds. Steam and liquid water are two different forms of the same pure chemical substance, that is, water.Ī chemical substance is a form of matter having constant chemical composition and characteristic properties.
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