Isomorphous substances

In the single case of isomorphous substances the mixed melting-point test fails. 

Not all isomorphous substances form mixed crystals. Calcite (CaC03) and sodium nitrate (NaN03) form similar atomic arrangements, their unit cells are both rhombohedra of very similar dimensions, and also the corresponding ions are closely similar in size but they do not form... 

Oriented overgrowth. Isomorphous substances which do not form mixed crystals may do the next best thing one crystal may grow on the other in parallel orientation. Sodium nitrate grows on calcite this way. Isomorphism is not, however, a necessary condition for oriented overgrowth it is sufficient if the arrangement of atoms on a particular plane of one crystal is similar, in type and dimensions, to the arrangement on one of the planes of the other crystal the two structures may be in other respects completely different from each other (Royer, 1926,... 

The -maxima and minima on viscosity-composition curves are reminiscent of those on vapour pressure-composition curves of binary, mixtures. 5 The vapour pressures and viscosities are equal at some temperatures, say T and To, and T and To respectively. Then To/T—To7T =C(T —T), where C is a constant. A plot of TojT—To IT against T—T gives a straight line in many cases, both for vapour pressure and viscosity in other cases, the vapour pressure shows a minimum and the viscosity a maximum, and the vapour pressure a maximum and the viscosity a minimum. Prasad, 6 from the relation with vapour pressure deduced the equation rj =rjjrio= +ac, where c=conc. of non-electrolyte. The theoretical value of a is 0 00652 the observed values were glucose 0 44, fructose 0 44, sucrose 0 78, independent of temperature. According to Errera, the curves depend on the electric dipolarity of the liquids if both are nonpolar, the curve is concave to the composition axis whilst if both are polar, it is convex. Wolkowa found that the viscosity of a solution is approximately proportional to its heat of dilution. There seems to be no relation between the viscosity and surface tension of a mixture of acetic acid and water (cf. salt solutions, 13.VIII E). Mixtures of isomorphous substances obey an approximately linear relation. 

Simon,22 with chromates and sulphates, concluded that equimolecular amounts of isomorphous substances, within limits, produce equal changes of viscosity. 

Isomorphous substances often crystallize together from a mixed solution to form solid solutions, single crystals containing both substances. In a solid solution the different ions (such as Mn++ and Ca+ +) are arranged at random in the positions occupied by one kind of ion alone in a pure substance. For example, solid solu tions of chrome alum, KCr(S04)2 12HoO, and ordinary alum, KA1(S04) 12H20,... 

Two melted isomorphous salts case where there is produced a single kind of mixed crystals.—We have just studied the ormation of mixed crystals in an aqueous solution containing two isomorphous salts. By side of this generation of mixed crystals in the wet way, one may study their formation in the dry way one may melt together two isomorphous substances and study the mixed crystals which the mixture furnishes on freezing the problem is thus quite simplified, because we have to deal no longer with three, but with only two independent components. 

Case in which there may be formed two kinds of crystals —In a great number of cases two isomorphous substances are isodimarphous they may give birth to two different kinds of mixed crystals which we shall denote by the indices a and... 

Supersaturation can be achieved by cooling, by evaporation, by the addition of a precipitant or by a chemical reaction that changes the nature of the solute. Supersaturation itself is insufficient to cause crystals to form the crystal embryos must form by collision of molecules of solute in the solution, or sometimes by the addition of seed crystals, or dust particles, or even particles from container walls. Deliberate seeding is often carried out in industrial processes seed crystals do not necessarily have to be of the substance concerned but may be isomorphous substances (i.e. of the same morphology). As soon as stable nuclei are formed, they begin to grow into visible crystals. 

We shall now consider briefly the formation of solid solutions by isomorphous substances the consideration of the formation of solid solutions of isodimorphous substances will, on account of the complexity of the relationships, not be undertaken here. ... 

Discussion The so-called double salts are compounds which are formed as the result of the chemical combination of two salts. They are usually prepared by dissolving the two constituents in water and allowing the crystals to deposit from the solution. The composition of a double salt is definite it does not vary with the proportions of the constituents present in the solution as is the case with the composition of the crystals obtained when mixtures of isomorphous substances are crystallized (see Experiment 107). 

If crystals of different species are isostructural and have the same type of bonding, they also will have very similar unit-cell dimensions and will macroscopically appear almost identical. This is known as isomorphism. Examples of isomorphic materials include ammonium and potassium sulfate and KH2PO4 and NH4H2PO4. In each of these materials, the potassium and ammonium ions can easily substitute for each other in the lattice since they are of almost the same size. This illustrates one of the properties of isomorphous materials, that is they tend to form solid solutions, or mixed crystals. Crystallization from a solution of two isomorphous materials, therefore, can result in a solid with varying composition of each species with unit-cell dimensions intermediate between the two components. The purification of isomorphous substances can, therefore, be difficult. 

Isomorphs - Substances of different chemical nature but having the same crystal structure. 

Seed crystals, however, do not necessarily have to consist of the material being crystallized in order to be effective isomorphous substances will frequently induce crystallization. For example, phosphates will often nucleate solutions of arsenates sodium tetraborate decahydrate (borax) can nucleate sodium sulphate decahydrate phenol can nucleate m-cresol and so on. The success of silver iodide, as an artificial rain-maker, is generally attributed to the striking similarity of the Agl and ice crystal lattices. However, there are many... 

For a tabulation of X-ray diffraction data of tetragonal isomorphous substances including Ge(CeH5)4, see [3]. 

Solid solution. Mixed crystals formed when isomorphous substances crystallize together out of a solution. Supersaturation. The departure from solution saturation usually caused by cooling of the mixture and/... 

In Fig. 1 the molecule of 2-amino-4,6-dichloropyrimidine is represented, as determined by Clews and Cochran (5). These authors also studied the isomorphous substance 2-amino-4-methyl-6-chloropyrimi-dine the molecule of this substance has essentially the same dimensions as those of the dichloropyrimidine. The interatomic distances shown in Fig. 1 are not those reported by the authors, but are the values calculated from the atomic parameters that they ve in their paper the distances as calculated from the parameters differ by as much as 0.05 A. from those reported by the authors, and the bond ai es differ by as much as 6°. Except for those in Fig. 12, the distances and bond an es reported for the other substances discussed below are those given by the respective authors some of them have been checked by us. 

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