Pure substances, distinguishing mixtures from

How is a mixture distinguished from a pure substance A mixture always contains two or more substances that can be present in varying concentrations. Let s consider two examples. 

We generally distinguish between two methods when the determination of the composition of the equilibrium phases is taking place. In the first method, known amounts of the pure substances are introduced into the cell, so that the overall composition of the mixture contained in the cell is known. The compositions of the co-existing equilibrium phases may be recalculated by an iterative procedure from the predetermined overall composition, and equilibrium temperature and pressure data It is necessary to know the pressure volume temperature (PVT) behaviour, for all the phases present at the experimental conditions, as a function of the composition in the form of a mathematical model (EOS) with a sufficient accuracy. This is very difficult to achieve when dealing with systems at high pressures. Here, the need arises for additional experimentally determined information. One possibility involves the determination of the bubble- or dew point, either optically or by studying the pressure volume relationships of the system. The main problem associated with this method is the preparation of the mixture of known composition in the cell. 

The distillation technique is not used to separate complex mixtures, but finds its acceptance more for the preparation of large quantities of pure substances or the separation of complex mixtures into fractions. The technique depends on the distribution of constituents between the liquid mixture and component vapors in equilibrium with the mixture two phases exist because of the partial evaporation of the liquids. How effective the distillation becomes depends upon the type equipment employed, the method of distillation, and the properties of the mixture components. The distinguishing aspects of distillation and evaporation are that in the former all components are volatile, whereas in the latter technique volatile components are separated from nonvolatile components. An example of distillation would be the separation of ethyl alcohol and benzene. An evaporative separation would be the separation of water from an aqueous solution of some inorganic salt, for example, sodium sulfate. 

Microscopic investigation of the solid formed when a eutectic freezes indicates that it is a mixture of very fine particles of pure A and pure B, rather than a solid solution of A and B. If a solution that has the eutectic composition is cooled, freezing occurs at a single concentration, just like a pure component. A eutectic can be distinguished from a pure component either by microscopic inspection of the solid or by adding one of the pure components. In the eutectic case, the freezing point of the solution will be increased by this addition, rather than decreased as occurs for a pure substance. 

A chemical mixture is a collection of molecules or atoms of different types. A mixture is distinguished from a pure substance, which has constant composition (is composed of a only one type of molecule or atom), and a unique set of physical properties (no matter how large or small a sample is observed). The properties of a mixture depend not only on the types of substances that compose it, but also on their relative amounts the composition of a mixture is not constant. The separation of mixtures is big business, and separation science is a subdivision of chemistry. 

Discussion Some substances are perfectly homogeneous, that is all particles of the substance have identical characteristic properties. In the case of other substances, different particles have different characteristic properties these are mixtures. Often the particles of a mixture are so minute and so evenly distributed throughout the whole mass that a substance appears to be homogeneous when in reality it is not. In such cases properties other than appearance must be depended upon to distinguish a pure substance from a mixture. Differences in density, in solubility, in action upon heating, and in behavior in many other tests may prove an apparently pure substance to be in reality a mixture. 

As mentioned in Sect. 8.1.2, the term melt can only be vaguely distinguished from the term solution. In the general usage, melts are rather pure substances. Solutions are liquid mixtures, which contain considerable amounts of one or more components which are not to be crystallized. From a procedural point of view solution and melt can be distinguished in the following way a melt can be frozen to a large extent even by a small decrease in temperature below the liquidus hne for a... 

Figure 2.10 You cannot distinguish a pure substance from a mixture of uniform appearance by observation alone at the macroscopic level. On the particulate level, however, the difference is readily apparent, (a) Pure bromine consists of bromine molecules, (b) Pure water consists of water molecules, (c, d) Mixtures of bromine and water consist of bromine and water molecules. 

Recall from Section 2.4 that constant physical and chemical properties are what distinguish pure substances from those that are impure-mixtures. Pure substances are either elements or compounds. 

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