Pure substance

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:

• 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

Mixtures

• 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

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

• Melting point (melting temperature)
• Boiling point (boiling temperature)
• density
• 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 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 :

${\displaystyle \mu _{i}=\mu _{i}^{\circ }+RT\ln \left\{a_{i}\right\}}$

For infinite dilution with : ${\displaystyle a_{i}\to 0}$

${\displaystyle \lim _{a_{i}\to 0}{\mu _{i}}=\lim _{a_{i}\to 0}{\mu _{i}^{\circ }+RT\ln \left\{a_{i}\right\}}=-\infty }$

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

material                                                                                                                               mixture                       Pure substance                                                                                               homogeneous mixture                         connection   element                                                                                           Gas mixture Mixture of several gases   Alloy Mixture with metallic properties , contains at least one metal   Solution Solid ,  liquid , gas dissolved  in a liquid             molecular   Ionic                                                       heterogeneous mixture                                                                                                                             Foam Gaseous bubbles in a liquid   Rigid foam Gaseous bubbles in a solid   Aerosol   Suspension Solid particles in a liquid   Emulsion Mixture of several immiscible liquids   Batch mixture of several not mixable solid                                                                             Smoke Solid particles in a gas   Fog 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.