Solids phase changes

From 160°C to room temperature. The lead-rich phase becomes unstable when the phase boundary at 160°C is crossed. It breaks down into two solid phases, with compositions given by the ends of the tie line through point 4. On further cooling the composition of the two solid phases changes as shown by the arrows each dissolves less of the other. A phase reaction takes place. The proportion of each phase is given by the lever rule. The compositions of each are read directly from the diagram (the ends of the tie lines). 

Solid-solid phase changes. Solid-solid phase changes have the same characteristics as solid-liquid phase changes, but usually do not posses a large phase change enthalpy. However, there are exceptions. 

Solidification Stefan Number St = cplAT/AHm Compare sensible to latent heat in liquid-solid phase change Matson et al. [409]... 

Types of intermolecular forces Properties of liquids Surface tension Viscosity Capillary action Structures of solids Phase changes and diagrams... 

Liquids, vapor pressure, solids, phase changes 5.8-5.11... 

The 355 nm emission is sharp and intense at the start of irradiation, and the intensity decreases with prolonged irradiation time. The 440 nm emission is weak and broad, and the intensity does not change with the irradiation time. Emission spectra of PMPrS obtained at ion fluences of 0.15,0.76, and 1.53 p,C/cm2 shows emission bands at 350 nm and 440 nm. The decrease in the intensity of the main peak indicates that main chain scission (photolysis) occurs under ion beam irradiation. Intense and sharp emission at 340 nm and weak broad emission at 440 nm for PDHS at 354 K are observed at the beginning of the irradiation and decrease on further irradiation. At 313 K and 270 K, sharp intense main emissions at 385 nm are seen. The 340 nm and 385 nm emission bands are assigned to a - a fluorescence. Experimental results have shown the presence of a phase transition at 313 K for PDHS.102,103 Below 313 K, the backbone conformation of PDHS is trans-planar, and above the solid-solid phase change temperature, a disordered conformation is seen. Fluorescent a -a transitions occur at 355 nm for PMPS, 350 nm for PMPrS, and 385 nm and 340 nm for PDHS. Emissions around 440 nm are observed at all temperatures examined and are assigned to defect and network structures induced by ion beams. 

During the eutectic reaction, the mean composition of the solid phases changes along the line yq. The solid composition reaches point q when the liquid is completely consumed by the eutectic crystallisation. 

On the surfaces of geological formations, different precipitation processes can be observed. The first one occurs when the concentration of some components reaches the value of the solubility product, the solution becomes oversaturated, and a new solid phase is precipitated (Section 1.2.3). The quantity of the precipitate depends only on the concentration of the solution. The precipitation takes place in a solution without the necessary presence of a solid surface. When, however, a solid phase, rocks, or soil is originally present, the precipitate is formed on it, and thus the total composition of the solid phase changes. When the precipitation forms colloidal particles, especially in diluted solutions, they can be adsorbed on the solid, if it is present. This process is governed by the so-called theory of colloid adsorption (Derjaguin and Landau 1941 Verwey and G. Overbeek 1948). 

In this section consideration is restricted to reactions between gaseous species adsorbed on a solid or liquid surface. Principally, we shall consider the adsorption process, adopting the kinetic viewpoint originally developed by Langmuir. The method is illustrative of kinetic procedures that can also be used to analyze processes involving gas-liquid and gas-solid phase changes. We shall not discuss reactions occurring at surfaces with steps that do not involve adsorption. 

The saturation vapor pressures of HDO and H2 0 are lower than those of H2 0, both over liquid and solid phases. These differences play an important role in the course of the atmospheric water cycle as they cause fractionation effects at vapor/liquid and vapor/solid phase changes, with the condensed phase in equilibrium with vapor being enriched in heavy isotopes. The fractionation coefficient a is defined as the ratio of D/H or in the condensed phase to the value of... 

Fractionation within the hydrosphere occurs almost exclusively during vapor-to-liquid or vapor-to-solid phase changes. For example, it is evident from the vapor pressure data for water (21.0, 20.82, and 19.51 mm Hg for H2 0, H2 0, and HD O, respectively) that the vapor phase is preferentially enriched in the lighter molecular species, the extent depending on the temperature (Raleigh distillation). The progressive formation and removal of raindrops from a cloud and the formation of crystals from a solution too cool to allow diffusive equilibrium between the crystal interior and the liquid, that is, isotopic reactions carried out in such a way that the products are isolated immediately after formation from the reactants, show a characteristic trend in isotopic composition. 

Fusion = fus (solid to liquid) vaporization = vap (liquid to vapor) sublimation = sub (solid to vapor) -> Solid-to-solid phase changes for example, Sn(gray) Sn(white)... 

The reduction is combined with a chemical vapor transport of tungsten via the volatile W02(0H)2. The morphology of the solid phases changes significantly. The reaction can be separated into three sequential stages ... 

In [68DOB/SUV] it was shown that the vapour pressure at 338 K kept constant when the solid phase changed from the composition H2Se03 to Se02. Thus formation of polymeric selenious acids is not indicated. 

Despite the unlikelihood of secondary mineral formation by ion substitution into or movement within an existing solid, some secondary 2 1 layer silicates apparently are formed by solid-phase changes of mica fragments inherited from the parent material. Hydrous mica, for example, is a product of chemical weathering as well as mechanical breakdown of mica. Hydrous mica, in turn, can be modified directly to vermiculite, montmorillonite, or chlorite. The process is not completely understood, but seemingly involves the outward diffusion of K+ from between the layer lattices and a subsequent or simultaneous reduction of charge within the layer lattice. 

CONCENTRATION PATTERNS IN FIXED BEDS. In fixed-bed adsorption, the concentrations in the fluid phase and the solid phase change with time as well as with position in the bed. At first, most of the mass transfer takes place near the inlet of the bed, where the fluid first contacts the adsorbent. If the solid contains no adsorbate at the start, the concentration in the fluid drops exponentially with distance essentially to zero before the end of the bed is reached. This concentration profile is shown by curve in Fig. 25.5a, where c/cq is the concentration in the fluid relative to that in the feed. After a few minutes, the solid near the inlet is nearly saturated, and most of the mass transfer takes place further from the inlet, The concentration gradient becomes S shaped, as shown by curve /j- The region where most of the change in concentration occurs is called the mass-transfer zone, and the limits are often taken as c/cq values of 0.95 to 0.05. 

Equilibrium Treatment of Solidification. As an example of liquid-solid phase change in solid-fluid flow systems with the assumption of local thermal equilibrium imposed, consider the formulation of solid-fluid phase change (solidification/melting or sublimation/frosting) of a binary mixture. For this problem, the equilibrium condition extends to the local thermodynamic equilibrium where the local phasic temperature (thermal equilibrium), pressure (mechanical equilibrium), and chemical potential (chemical equilibrium) are assumed to be equal between the solid and the fluid phases. This is stated as... 

It was suggested by Ulrich et al. (1988) that whenever the expression solution is used the mass transfer effects should dominate a process, and whenever the heat transfer is dominating a process of liquid solid phase change it should be called melt crystallization. An additional explanation could be found in the different growth rates that have their origin in the rate domination effect. 

The solubility of over 300 common inorganic compounds in water is tabulated here as a function of temperature. Solubility is defined as the concentration of the compound in a solution that is in equilibrium with a solid phase at the specified temperature. In this table the solid phase is generally the most stable crystalline phase at the temperature in question. An asterisk on solubility values in adjacent columns indicates that the solid phase changes between those two temperatures (usually from one hydrated phase to another or from a hydrate to the anhydrous solid). In such cases the slope of the solubility vs. temperature curve may show a discontinuity. 

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