A Definition and substances

It was made clear in Chapter II that the surface tension is a definite and accurately measurable property of the interface between two liquid phases. Moreover, its value is very rapidly established in pure substances of ordinary viscosity dynamic methods indicate that a normal surface tension is established within a millisecond and probably sooner [1], In this chapter it is thus appropriate to discuss the thermodynamic basis for surface tension and to develop equations for the surface tension of single- and multiple-component systems. We begin with thermodynamics and structure of single-component interfaces and expand our discussion to solutions in Sections III-4 and III-5. 

In the method of classification of matter based on composition, a given specimen of material is regarded as either a pure substance or a mixture. An outline of this classification scheme is shown in Table 1-2. The term pure substance (or merely substance) refers to a material all parts of which have the same composition and which has a definite and unique set of properties. In contrast, a mixture consists of two or more substances and has a somewhat arbitrary composition. The properties of a mixture are not unique, but depend on its composition. The properties of a mixture tend to reflect the properties of the substances of which it is composed that is, if the composition is changed a little, the properties will change a little. 

For a defence of the view that chemical substances may be regarded as energy-complexes, and that this view is equally as valid as the older notion of a chemical substance as an inertia-complex, i.e., as something made vp entirely of different units or atoms each characterised by the possession of a definite and constant weight at a fixed point on the earth s surface, see an article by the present writer, entitled "The Claims of Thermochemistry," Knowledge and Scientific News, vol. vii. (New Series), pp. 227 et seq. (July, 1910). 

Lavoisier uses the word principle, here and elsewhere, to mean a definite homogeneous substance he uses it as synonymous with the more modern terms element and compound. 

Leaching Studies. A determination of the adequacy of RO system flushing and a definition of substances that could leach from the system are important to the use of the sampling method. Both of these questions were addressed in these studies. 

A definite and distinct, electrically neutral group of bonded atoms. Examples H2 NH3 CH3COOH. mole fraction x The number of moles of molecules (or ions) of a substance in a mixture expressed as a fraction of the total number of moles of ions and molecules in the mixture. 

Minerals are natural, usually inorganic, substances with a definite and predictable chemical composition and physical properties (O Donoghue 1990). Some organic substances (e.g., oxalates, amber, etc.) are also considered as minerals. 

Since the vapour pressure increases rapidly with temperature, it is essential in accurate measurements to use a definite and constant temperature, and as it is affected by impurities it is equally necessary to employ highly purified liquids. Neglect of the second requirernent is probably the main source of error in older measurements, since traces of volatile substances (air, benzene) produce appreciable errors, In an unequally heated space, the vapour pressure generally corresponds with that at the lowest temperature Watfs principle ). This is the principle of Wollaston s 12 cryophorus (Greek kqvos, frost, I carry). 

The melting point—apart from a very shght variation with the pressure—is a definite and unique temperature for every pure substance. 

In the method of classification of matter based on composition, a given specimen of material is regarded as either a pure substance or a mixture. The term pure substance refers to a material all parts of which have the same composition and that has a definite and unique set of properties. 

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