Transition point, ordinary

At a given (low) temperature and pressure a crystalline phase of some substance is thermodynamically stable vis a vis the corresponding amorphous solid. Furthermore, because of its inherent metastability, the properties of the amorphous solid depend, to some extent, on the method by which it is prepared. Just as in the cases of other substances, H20(as) is prepared by deposition of vapor on a cold substrate. In general, the temperature of the substrate must be far below the ordinary freezing point and below any possible amorphous crystal transition point. In addition, conditions for deposition must be such that the heat of condensation is removed rapidly enough that local crystallization of the deposited material is prevented. Under practical conditions this means that, since the thermal conductivity of an amorphous solid is small at low temperature, the rate of deposition must be small. 

G. Tammann found that potassium and sodium chlorides form a continuous series of mixed crystals between 660° and 500°. Since neither salt has a transition point, the phenomena observed when the mixed crystals are cooled must be attributed to separation of the components. With diminishing temperature, therefore, either the attractive forces within the molecules of the respective chloride must increase, or those between the unlike molecules must be greatly weakened. The results obtained by etching the individual crystals at the ordinary temperature indicate that the intra-molecular forces of the potassium chloride crystals differ from those of the sodium chloride crystal, or, more precisely, that certain lattice regions are more closely united in the former, whilst such differences are not observed in the latter. In the light of these observations, it is surprising that the X-ray analysis indicates the same lattice for each crystal. 

Engel (1883) and Linck 1 (1899) stated that amorphous arsenic is transformed at 360° C., irreversibly and with considerable development of heat, into metallic arsenic Erdmann and Reppert gave 303° C. as the transformation temperature, while Jolibois 2 and Gaubeau 3 determined the point of irreversible transformation both of the brown and grey varieties to be 270° to 280° C. Erdmann gave the transition point between the brown form and the grey form as 180° C., but such a critical point has not been substantiated. Jolibois asserted that his thermal observations admitted only two allotropes, the ordinary grey... 

The relation between the octahedral and prismatic modifications has not yet been satisfactorily elucidated. The former is the stable form at ordinary temperatures and the latter at higher temperatures the transition point according to Rushton and Daniels is 250° C. and according to Smits and Beljaars 200° C., but the prismatic form is persistent at much lower temperatures and the change from octahedral to prismatic may be monotropic. Interesting information has been obtained from measurements of the vapour pressure of the oxide. The following values have been obtained ... 

The anhydrous salt is obtained1 by heating the crystals to 120° C, If crystallisation takes place at the ordinary temperature, the dodeca-hydrate, Na2HAs04.12H20, is obtained while if the crystals are formed above 36° C. the heptahydrate, Na2HAs04.7H20, is produced. The transition point determined from the solubility curve of sodium monohydrogen arsenate in water 2 is at 22° C. 

Cryoscopy. Souchay (40) has summarized the application of fused salt cryoscopy to ionic solutes. Obviously two limitations are inherent in this method. Under ordinary pressures, measurements are possible at only one temperature—namely that of the transition point (e.g., ca. 32.38°C. in the case of Na2S04 10 H20). Secondly, the solute is being examined in solutions of high ionic strength only. Isopiestic vapor pressure measurements have been used as a variation, which, in principle, eliminate both limitations. However, it does not appear that it is as yet possible to analyze such data to yield equilibrium constants (33). Furthermore, Tobias has cast doubt upon the inherent accuracy of the method when the polyions contain more than 3 or 4 metal ions (41). 

The decahydrate or ordinary borax forms monoclinic crystals of density 1-723 (Hassenfratz 16) or 1-694 at 17° C., and 1-728 at the temperature of liquid air (Dewar17). Its specific heat18 is 0-385 between 19° and 50° C. A pentakydrate belonging to the rhombohedral division of the hexagonal system crystallizes above 60° C. Its density19 is 1-815. The pentahydrate is stable from 60° C., the transition-point from the decahydrate, to 125° C. At 130° C. the stable form is the trihydrate ... 

In conclusion we may say that a conversion near 100% can only be reached within reasonable time if the polymerization temperature is above the glass transition point of the polymer. This conclusion holds for bulk, emulsion, and suspension polymerization but, of course, not for solution polymerization where the solvent-polymer mixture usually has a glass transition point which is well below ordinary polymerization temperatures. 

For the adhesive hard-sphere model, the theoretical phase diagram in the Tg-0 plane has been partially calculated (Watts et al. 1971 Barboy 1974 Grant and Russel 1993). According to this model, there is a critical point r. c = 0.0976 below which the suspension is predicted to phase separate into a phase dilute in particles and one concentrated in them (see Fig. 7-4). The particle concentration at the critical point of this phase transition is 0c = 0.1213. This phase transition is analogous to the gas-liquid transition of ordinary... 

Hydrides of the 3d, 4d, and 5d metals. We now comment briefly on the remaining hydrides of Table 8.1. All the (h.c.p.) elements Ti, Zr, and Hf form dihydrides with fluorite-type structures which are cubic above their transition points and have lower (tetragonal) symmetry at ordinary temperatures. Both phases have ranges of composition, which depend on the temperature, and the following figures (for room temperature) show that for Ti the cubic phase includes the composition TiH ... 

Ammonium fluoride, NH4F, crystallizes with a structure different from those of the other ammonium (and alkali) halides. The chloride, bromide, and iodide have the CsCl structure at temperatures below 184 3°, 137-8 , and — IT S C respectively, and the NaCl structure at temperatures above these transition points, but NH4F crystallizes with the wurtzite structure, in which each N atom forms N-H—F bonds of length 2-71 to its four neighbours arranged tetrahedrally around it. This is essentially the same structure as that of ordinary ice. 

The occurrence of polymorphic forms and the persistence of the metastable state are facts of the highest practical and theoretical importance. In the case not only of tin, but also a number of other metals, e.g. bismuth, cadmium, copper, silver, and zinc, allotropic modifications exist with transition points at temperatures above the ordinary and, owing to the slowness of transformation, these metals exist, at the ordinary temperature, in a metastable state. On this fact depends the practical, everyday use of these metals. ... 

Law of Successive Reactions.— When sulphur vapour is cooled at the ordinary temperature, it first of all condenses to drops of liquid, which solidify in an amorphous form, and only after some time undergo crystallisation or when phosphorus vapour is condensed, white phosphorus is first formed, and not the more stable form, violet phosphorus. It has also been observed that even at the ordinary temperature (therefore much below the transition point) sulphur may crystallise out from solution in benzene, alcohol, carbon disulphide, and other solvents, in the monoclinic form, the less stable crystals then undergoing transformation into the rhombic form a similar behaviour... 

Besides the ordinary white phosphorus which crystallises in the regular system, Bridgman has discovered the existence of a second form of white phosphorus, possibly belonging to the hexagonal system. These two forms of white phosphorus are enantiotropic, with a transition point at — 76 9° under atmospheric pressure. 

On following the solubility curve of the hexahydrate from the ordinary temperature upwards, it is seen that at a temperature of 29 8° represented by the point H, it cuts the solubility curve of the a-tetrahydrate. This point, therefore, represents an invariant system in which the three phases hexahydrate, a-tetrahydrate, and solution can coexist under constant pressure. It is also the transition point for these two hydrates. Since, at temperatures above 29 8°, the a-tetrahydrate is the stable form, it is evident from the data given before (p. 184), as also from Fig. 79, that the portion of the solubility curve of the hexahydrate lying above this temperature represents metastable equilibria. The realisation of the metastable melting-point of the hexahydrate is, therefore, due to suspended transformation. At the transition point, 29 8°, the solubility of the hexahydrate and a-tetrahydrate is 100 6 parts of CaClg in 100 parts of water. 

As already mentioned, the decomposition of copper calcium acetate into the single salts and saturated solution is accompanied by a contraction, and it was therefore to be expected that increase of pressure would lower the transition point. This expectation of theory was confirmed by experiment, for van t Hoff and Spring found that although the transition point under atmospheric pressure is about 75°, decomposition of the double salt took place even at the ordinary temperature when the pressure was increased to 6000 atm. ... 

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