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
- Pure chemical elements are e.g. B. pure hydrogen , oxygen or gold .
- Pure chemical compounds are z. B. pure, distilled water , pure carbon dioxide or pure sodium chloride ( table salt ).
Note: Chemical compounds are made up of several elements, but they are not a mixture of elements
- Air is a gas mixture of nitrogen , oxygen, argon, carbon dioxide, hydrogen and other gases in traces
- Milk is a liquid mixture ( emulsion ) of water, carbohydrates , fats , proteins and trace elements .
- Granite is a solid mixture consisting mainly of quartz , feldspars and dark, mafic minerals .
Properties of basic substances
- Melting point (melting temperature)
- Boiling point (boiling temperature)
- Refractive index
- electric conductivity
- Thermal conductivity
- Solubility in a solvent
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 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.
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|