Forming , solid-phase definition

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

The term bioavailability has different meanings in different contexts and disciplines. Numerous definitions of bioavailability exist. Research on the relationship between bioavailability and chemical speciation (forms) originated in the field of soil fertility in the search for a good predictor for the bioavailability of essential plant nutrients (Traina and Laperche 1999). It is well accepted that dissolved nutrients are more labile and bioavailable to plants than solid-phase forms (including sorbed species). The same has been considered to be true for organic contaminants and their availability for microbial degradation. 

Although, the true density of solid phase p=m/Vp (e.g., g/cm3) is defined by an atomic-molecular structure (/ ), it has become fundamental to the definition of many texture parameters. In the case of porous solids, the volume of solid phase Vp is equal to the volume of all nonporous components (particles, fibers, etc.) of a PS. That is, Vp excludes all pores that may be present in the particles and the interparticular space. The PS shown in Figure 9.17a is formed from nonporous particles that form porous aggregates, which, in turn, form a macroscopic granule of a catalyst. In this case, the volume Vp is equal to the total volume of all nonporous primary particles, and the free volume between and inside the aggregates (secondary particles) is not included. 

The humic/organic matter coatings of different solid phases (i. e., SPm /SP0M), such as soils, sediments, suspended solids, colloids, and biocolloids/biosolids, interact with organic pollutants in aqueous systems in various ways. Adsorption is an important interaction mode. The reversibility and/or irreversibility of the adsorption processes is of major importance. The question whether the bound residues of pollutants are to be considered definitely inactivated has been the focus of extensive research. This question was posed as follows. Have the adsorbed pollutants become common components incorporated into the humic polymer coating of solid phases (i. e., being absorbed), or are they only momentarily inactivated in reversibly bound forms thus representing a possible source of pollution by a time-delayed release of toxic units ... 

As in the example of magnesian calcite, measurement of the IAP for Al-goethite as a function of the mole fraction of A1 would permit the calculation of the activities of the end-member solid phases, goethite and diaspore, in the mixed solid (Eq. 3.35). Alternatively, simply by way of an illustration, it may be assumed that the two solid phases mix to form an ideal solid solution, which, by definition,24 exhibits solid-phase activities always equal to the respective mole fractions of the components (FeOOH) = 1 - x and (AIOOH) = x. Under this simplifying assumption, Eq. 3.34 can be integrated directly, with the following result ... 

Water and ionic compounds are very different types of substances, and it is not unnatural that they do not form solids of variable composition. The reason why water solutions of ionic substances exist is that the water molecules can rotate so as to be attracted to the ions this is not allowed in the solid, where the ice structure demands a fairly definite orientation of the molecule. But as soon as we think about solid phases of a mixture of similar components, we find that almost all the solid phases exist over quite a range. Such phases are often called by the chemists solid solutions, to distinguish them from chemical compounds. This distinction is valid if we mean by a chemical compound a phase which really exists at only a quite definite composition. But the chemists, and particularly the metallurgists, are not always careful about making this distinction for this reason the notation is misleading, and we shall not often use it. 

SOLUBILITY OF PRECIPITATES A large number of reactions employed in qualitative inorganic analysis involve the formation of precipitates. A precipitate is a substance which separates as a solid phase out of the solution. The precipitate may be crystalline or colloidal, and can be removed from the solution by filtration or by centrifuging. A precipitate is formed if the solution becomes oversaturated with the particular substance. The solubility (5) of a precipitate is by definition equal to the molar concentration of the saturated solution. Solubility depends on various circumstances, like temperature, pressure, concentration of other materials in the solution, and on the composition of the solvent. 

A strict definition of a phase is a domain with a homogenous chemical composition and physical state (Atkins 1998). Typically, phase behavior is associated with equilibrium mixtures of hquid, vapor and solid phases in a mixture of two or more substances as a fimctiorr of temperature, pressure and compositiorr. hr fats, when discussing phases, it usually refers to their polymorphic form, sohd/hquid states, or compositionally differerrt ffactiorrs within a complex mixtrrre. Therefore, when phases of fats are discussed it is important to carefully define what is being used as the definition for... 

What we mean in this report by equilibrium and disequilibrium requires a brief discussion of definitions. Natural physicochemical systems contain gases, liquids and solids with interfaces forming the boundary between phases and with some solubility of the components from one phase in another depending on the chemical potential of each component. When equilibrium is reached by a heterogeneous system, the rate of transfer of any component between phases is equal in both directions across every interface. This definition demands that all solution reactions in the liquid phase be simultaneously in equilibrium with both gas and solid phases which make contact with that liquid. Homogeneous solution phase reactions, however, are commonly much faster than gas phase or solid phase reactions and faster than gas-liquid, gas-solid and... 

A review of some of the features of these diagrams in terms of the Phase Rule is enlightening (Findlay 1951). A system composed of two different solid forms of a substance will have one component and two solid phases. In the absence of a further definition of the system there will be one degree of freedom. In Fig. 2.7 this is either the temperature or the pressure along the I<->II line. Choosing either variable fixes a point and defines the system. However, suppose that we are interested in the situation when the two phases are in equilibrium with the liquid or the vapour. Each one of those is an additional phase, making three in total and, by virtue of the Phase Rule, rendering the system invariant. Invariance results in a triple point for each case, defined by the intersection of the I<->II curve with the I<->v. and n<->v. curves on the one hand and the intersection of the I<->II curve with the I<->1. and II<->1. curves on the other hand. 

The solid phase is a body of definite composition formed at the expense of the components 0, 1, 2, 3 ice, simple salt—anhydrous or hydrous—double anhydrous or hydrous salt we shall denote this solid by the letter C. 

These phenomena are very sharply produced when the temperature does not exceed a certain limit, near the fusing-point of gold beyond this limit the two oxides exist in contact as a fused mass instead of forming two solid phases, th form but one liquid phase from mono variant the system becomes bivariant at a given temperature there can be no longer any definite dissociation tension. 

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