Pure substance

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In chemistry, a pure substance is a substance that is composed of only one chemical compound or one chemical element ; the term is in contrast to a mixture . A pure substance can also designate an “isotopically pure” substance (such as pure heavy water ) or a compound that is completely isotope-marked in a defined position .

Examples of pure substances and mixtures

Basic substances:

Note: Chemical compounds are made up of several elements, but they are not a mixture of elements


Properties of basic substances

Basic substances can be elements or compounds . Basic substances have clearly defined physical properties that are used for characterization, e.g. B.

Ideal pure substance

The ideal of a pure substance cannot be achieved in practice. Substances designated as pure substances still contain a small amount of impurities. A number of purity definitions therefore apply to tradable basic substances (chemicals trade), e.g. B. "pro analysi", "reinst", "According to DAB (German Drug Book)" etc. These definitions are mostly earmarked and are supplemented by an analysis of those impurities that are of particular importance for the respective purpose of the pure substance .

Omnipresence of elements and fabrics

The term ubiquity of the (chemical) elements was coined in 1936 by the German chemists Ida Noddack-Tacke and Walter Noddack , who discovered rhenium .

The more general law of the ubiquity of substances expresses that any substance is at least minimally soluble in any other substance. This has the consequence that it is impossible to produce pure substances in the actual sense, since they would always have to be in contact with some vessel or the like, so that individual particles of the vessel wall would contaminate the substance again.

Thermodynamically , the law is underpinned by the concentration dependence of the chemical potential :

For infinite dilution with :

This limit value shows that under the assumption that there is a pure substance, every other substance would have an infinitely strong tendency to dissolve in the pure substance.

Classification in the scheme of chemical substances

Schematic classification of the substances
Pure substance
mixture Mixture of several
Mixture with metallic properties ,
contains at least one metal
Solidliquid ,
gas dissolved  in a liquid
Gaseous bubbles in
a liquid
Rigid foam
Gaseous bubbles in
a solid
Solid particles in
a liquid
Mixture of several
immiscible liquids
mixture of several not
mixable solid
Solid particles
in a gas
Liquid particles
in a gas

Individual evidence

  1. a b entry on substance. In: Römpp Online . Georg Thieme Verlag, accessed on January 3, 2012.
  2. ^ I. Noddack: About the omnipresence of chemical elements , Angewandte Chemie 49 (1936) 835-841.