Mixtures and compounds

A compound has a fixed composition, whereas the composition of a mixture may be varied. There are always two H atoms for each O atom in a sample of the compound water, but sugar and sand, for instance, can be mixed in any proportions. Because the components of a mixture are merely mingled with one another, they retain their own chemical properties in the mixture. In contrast, a compound has chemical properties that differ from those of its component elements. The formation of a mixture is a physical change, whereas the formation of a compound requires a chemical change. The differences between mixtures and compounds are summarized in Table G.l. 

Boyle believed there were many elements. He defined them as primitive and simple, or perfectly unmingled bodies, which are the ingredients of all mixtures and compounds. Boyle thought these elements accounted for all the different properties of chemicals. From Boyle s pioneering work it became clear that elements cannot be changed into one another by ordinary methods. Therefore, the attempts to convert base (nonprecious) metals into gold were discontinued. 

Detection.—Sulphur in the free state is readily recognisable by its general appearance and characteristics, and especially by its combustion to sulphur dioxide. Both in mixtures and compounds the presence of the element can be demonstrated by heating with charcoal and an alkali carbonate,2 or even better, on a small scale, by heating with an equal bulk of sodium or potassium,3 or with powdered iron 4 in each case some of the sulphur is converted into sulphide, which may be detected by the action of an aqueous extract on mercury or silver, or on sodium nitroprusside the metals are blackened, whilst the nitro-prusside is very sensitive in giving a purple coloration (see p. 62). Alternatively, the solution of the alkali sulphide may be acidified and tests applied for hydrogen sulphide to the vapours evolved on warming. 

The lipid moiety of the sialoglycolipids from the hepatopancreas of P. pectinifera includes a-hydroxy fatty acids, among which, the C22, C23, and C24 acids account for >90% of the mixture, and compounds whose major... 

Table G.l Differences Between Mixtures and Compounds Mixture Compound...

Mixtures and Compounds movie Mixtures and Compounds activity... 

Use the Mixtures and Compounds activity (eChapter 2.7) to develop a flow chart for identifying substances as heterogeneous mixtures, homogeneous mixtures, compounds, or elements. 

Table 2.7 The major differences between mixtures and compounds.

In iron(n) sulfide, FeS, one atom of iron has combined with one atom of sulfur. No such ratio exists in a mixture of iron and sulfur, because the atoms have not chemically combined. Table 2.7 summarises how mixtures and compounds compare. 

Cut the substance into smaller pieces and test them with the same magnet. The magnet has no effect. Iron loses its property in the new substance. This is a compound called iron sulfide containing iron and sulfur. The same elements, iron and sulfur, can form both mixtures and compounds. Can you think of other elements which forms both mixtures and compounds Compounds contain two or more elements. The elements chemically combine together to form a compound with different properties. 

Mixtures and Compounds.—In the olden days, no distinction was drawn between a compound and a mixture. Indeed, all impure substances artificially prepared were termed mixts. It was only after the true idea of elements had been arrived at, and indeed not until Dalton had formulated the laws which go by his name, that the distinction was drawn. The ultimate criterion for combination is definiteness of proportion, and this is generally connected with uniformity in properties, or homogeneity. A substance is said to be homogeneous when no one part of it differs from any other part in composition. But this may be predicated of glass, or of air, which are mixtures, and not compounds. A mixture may be homogeneous a compound must. 

The fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

Distinguish between elements and compounds. Distinguish between pure substances and mixtures. Classify mixtures as homogeneous or heterogeneous. Explain the difference between mixtures and compounds. 

A compound is composed of two or more elements chemically joined together. A mixture is composed of two or more substances physically mixed together but not chemically joined. As a result, there are two major differences between mixtures and compounds. 

Jones and co-workers used this scheme to study C-C versus C-H bond cleavage reactions involving Ni(0) complex 2, which is reminiscent of the chemistry involved with hydrocyanation of 1,3-butadiene.126 At room temperature, compounds 3 and 4 form more rapidly than 5. Over time, especially at temperatures above 25 °C, compound 5 disappears from the reaction mixture and compounds 3 and 4 are the only products. When BPh3 was added to complex 2, compound 6 formed exclusively and rapidly, even at -30 °C. There was no formation of complexes 3 and 4, which would involve C-H activation, indicating that in the presence of a Lewis acid, C-C activation is faster than OA of a C-H bond. 

Even though a few elements, such as carhon and gold, are sometimes found in elemental form in nature, most of the substances we see around us consist of two or more elements that have combined chemically to form more complex substances called compounds. For example, in nature, the element hydrogen is combined with other elements, such as oxygen and carbon, in compounds such as the water and sugar used to make a soft drink. (Perhaps you are sipping one while you read.) In this chapter, you will learn to (1) deftne the terms mixture and compound more precisely, (2) distinguish between elements, compounds, and mixtures, (3) describe how elements combine to form compounds, (4) construct systematic names for some chemical compounds, and (5) describe the characteristics of certain kinds of chemical compounds. The chapter will also expand your ability to visualize the basic structures of matter. 

Figure 2.19 The distinction between mixtures and compounds. A, A mixture of iron and sulfur can be separated with a magnet because only the iron is magnetic. The blow-up shows separate regions of the two elements. B, After strong heating, the compound iron(ll) sulfide forms, which is no longer magnetic. The blow-up shows the structure of the compound.

Taylor s book is quite detailed on a great variety of applications of PAD other than for ejection. It deals, of course, mainly with the matmals and applications in the United Kingdom where apparently a greater use is made of mixtures and compounds of high nitrogen content. 

A mixture of elements, without chemical bonding between the different types of atoms, is not a compound. Mixtures of elements can often be separated by simple physical means, e.g. a magnet removes iron from a mixture of iron filings and sulphur. But the main distinction between mixtures and compounds is that the proportions of the elements in a mixture may vary considerably from sample to sample in a given compound the proportions are always the same. 

Iron, gold, sulfur, tin, and a few other substances that occur in Earth s crust were recognized as elements long before the modem meaning of the term developed. Other naturally occurring elements were identified only after long and laborious efforts to separate them from the compounds in which they are found. Although it isn t always easy, chemists can now separate and identify the elements present in the most complex mixtures and compounds. 

For mixtures and compounds the stopping power can be calculated from stopping power data for the elemental components by the additivity rule of Bragg and Kleeman ... 

NPK/NP/PK Fertilizers - Terms similar to and often used in conjunction with mixtures and compounds. They signify that the fertilizer contains nitrogen (N), phosphorus (P), and/or potassium (K). 

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