Melting sharp

An interesting point is that AH itself varies with r [10].] As is the case when P is varied, the rate of nucleation increases so strongly with the degree of supercooling that a fairly sharp critical value exists for T. Analogous equations can be written for the supercooling of a melt, where the heat of fusion AH/ replaces AH . 

Typical results for a semiconducting liquid are illustrated in figure Al.3.29 where the experunental pair correlation and structure factors for silicon are presented. The radial distribution function shows a sharp first peak followed by oscillations. The structure in the radial distribution fiinction reflects some local ordering. The nature and degree of this order depends on the chemical nature of the liquid state. For example, semiconductor liquids are especially interesting in this sense as they are believed to retain covalent bonding characteristics even in the melt. 

Substance chemically pure. This is almost invariably the cause of a sharp melting-point. 

In just the proportion to give a sharp-melting eutectic mixture is so remote that this possibility may be neglected. [Occasionally arbitrary mixtures of two substances which (usually) are chemically related may melt fairly sharply at temperatures intermediate between the melting-points of the two components, but this phenomenon is rarely encountered.]... 

Checking the Purification. The purity of the dry re-crystallised material must now be determined, as it is possible that repeated recrystallisation may be necessary to obtain the pure material. The purity is therefore checked by a melting-point determination, and the recrystallisation must be repeated until a sharp melting-point is obtained. Should the compound have no well-defined melting-point e.g.y the salt of an organic acid or base), it must be analysed for one suitable component element, until its analysis agrees closely with that theoretically required. 

Being crystalline compounds which usually have sharp melting-points, they are used to characterise the parent aldehydes and ketones. 

Note that the amino-acids, because of their salt-like nature, usually decompose on heating, and therefore seldom have sharp melting-points. Furthermore, all naturally occurring amino-acids are a-amino-acids, and consequently, with the exception of glycine, can exist in optically active forms. 

Identification of Amines. Picric acid combines with many amines to give crystalline picrates, of general formula B,(NO )aCeHjOH, where B is a molecule of a monacidic base. These picrates have usually sharp melting- or decomposition-points, and serve to characterise the amines concerned. They may be formed either by (a) direct union of the acid and the base in a suitable solvent, or (6) by the interaction of sodium picrate and a salt of the amine in aqueous solution. 

The reason for the constancy and sharpness of the melting j)oint of a pure crystalline solid can be appreciated upon reference to Fig. 7,10, 1, in which (a) is the vapour pressure curve of the solid and (6) that of the liquid form of the substance. Let us imagine a vessel, maintained at constant temperature, completely filled with a mixture of the above liquid and solid. The molecules of the solid can only pass into the liquid and the molecules of the liquid only into the solid. We may visualise two competitive processes taking place (i) the solid attempting to evaporate but it can only pass into the liquid, and (ii) the liquid attempting to distil but it can only pass into the solid. If process (i) is faster, the solid will melt, whereas if process (ii) proceeds with greater speed the... 

S-Alkyl-iso-thiuronium picrates. Alkyl bromides or iodides react with thiourea in alcoholic solution to produce S-alkyl-iso-thiuronium salts, which yield picrates of sharp melting point ... 

The melting points of carbohydrates (sugars) are not usually sharp and they are perhaps best expressed as decomposition points. 

The sulphonyl chlorides are treated with concentrated ammonia solution to produce sulphonamides, which usually possess sharp melting points and are therefore useful as derivatives, for example ... 

These are crystalline compounds with sharp melting points, and possess the further advantage that their equivalent weights may be determined by dissolving in dilute alcohol and titrating with standard alkali. Nitro-phenols, however, give unsatisfactory derivatives. 

In chocolate, cocoa butter is the continuous phase. The characteristic meltabiUty of cocoa butter constitutes a puzzle in chemical stmcture and poses difficulty in replacement cocoa butter has a sharp melting point at body temperature. 

Very high melt strength is also observed in situations where the molten polymer contacts a sharp object. Skin packaging trials have successfliUy packaged objects such as arrowheads and fishhooks in ionomer films. 

The sharp melting poiat and the low melt viscosity also mean that nylon can give problems with nozzle drool and/or premature freeze-off For this reason, it is normally necessary to use either a reverse-taper nozzle (fitted with a heater to avoid freeze-off), a mechanical shut-off nozzle, or melt decompression. Melt decompression, or suck-back, iavolves the screw retractiag slightly after the screw retractioa time, pulling the material back and... 

Refractoriness. Most refractories are mixtures of different oxides, sometimes with significant quantities of impurities. Thus, they do not have sharp melting points but a softening range. Refractoriness is the resistance to physical deformation under the influence of temperature. It is determined by the pyrometric cone equivalent (PCE) test for aluminosiHcates and resistance to creep or shear at high temperature (see Analytical methods). 

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