# 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:

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

Mixtures

## 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 :

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

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.