Dissolution of salts

Salts dissolve in water with dissociation of the constituent ions, this concept having been proposed originally by S. Arrhenius in 1887. His first idea was that all salts, including those of what would now be regarded as weak acids or bases, are completely dissociated at extreme dilution (Hall, 1985). It was eventually realized that substances such as NaCl, KCl, etc, are effectively completely dissociated at all concentrations. 

Dissolution of an ionic salt is essentially a separation process carried out by the interaction of the salt with water molecules. The separation is relatively easy in water because of its high dielectric constant. Comparison of the energies needed to separate ions of NaCl from 0-2 nm to infinity shows that it takes 692-89 kJ mol in vacuum, but only 8-82 kJ moF in aqueous solution (Moore, 1972). Similar arguments have been used to try to estimate solvation energies of ions in aqueous solution, but there are difficulties caused by the variations in dielectric constant in the immediate vicinity of individual ions. 

In order to dissolve ionic solutes so readily, water molecules must solvate the ions as they enter solution. Consequently, water molecules lose their translational degrees of freedom as a result of their association with specific ions. It is possible to estimate the number of water molecules in clusters of the type (H20) using mass spectrometry (Kebarle, 1977). 

The number of water molecules in such a cluster, the hydration number, varies with ionic size it is four for Li, three for Na, but only one for Rb.  

Mass spectrometry has been used to study the energetics of solvation and has shown that the enthalpies of attachment of successive water molecules to either alkali metal or halide ions become less exothermic as the number of water molecules increases (Kebarle, 1977). The Gibbs free energies of attachment for water molecules have also been found to be negative. 

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Solvent. The solvent properties of water and steam are a consequence of the dielectric constant. At 25°C, the dielectric constant of water is 78.4, which enables ready dissolution of salts. As the temperature increases, the dielectric constant decreases. At the critical point, the dielectric constant is only 2, which is similar to the dielectric constants of many organic compounds at 25°C. The solubiUty of many salts declines at high temperatures. As a consequence, steam is a poor solvent for salts. However, at the critical point and above, water is a good solvent for organic molecules. 

Between 1865 and 1887, Dmitri 1. Mendeleev developed the chemical theory of solutions. According to this theory, the dissolution process is the chemical interaction between the solutes and the solvent. Upon dissolution of salts, dissolved hydrates are formed in the aqueous solution which are analogous to the solid crystal hydrates. In 1889, Mendeleev criticized Arrhenius s theory of electrolytic dissociation. Arrhenius, in turn, did not accept the idea that hydrates exist in solutions. 

Considerable dissolution of salts may occur in the high pressure steam. As the steam density decreases through the... 

Since the only chemical bonds broken during the dissolution of salt are the ionic bonds, water is not truly a reactant in Eq. 2.5. Thus, Eq. 2.5 is more commonly written as... 

The dissolution of salt in water (2) is endothermic (AH > 0)—i. e., the liquid cools. Nevertheless, the process still occurs spontaneously, since the degree of order in the system decreases. The Na"" and Cl ions are initially rigidly fixed in a crystal lattice. In solution, they move about independently and in random directions through the fluid. The decrease in order (AS > 0) leads to a negative -T AS term, which compensates for the positive AH term and results in a negative AG term overall. Processes of this type are described as being entropy-driven. The folding of proteins (see p. 74) and the formation of ordered lipid structures in water (see p. 28) are also mainly entropy-driven. 

Future penetration of the repository by man is one of the several potential failure scenarios which has been calculated. One particular scenario which will be described assumes an open, unplugged borehole penetrates through the repository and connects aquifers above and below the salt. This case is of much greater concern than for a hole which terminates within the salt. In this latter instance, there is no mechanism to continue dissolutioning of salt and the hole will gradually be squeezed closed. Using the hydrologic parameters... 

Anderson, Roger Y., Deep Dissolutioning of Salt - Northern Delaware Basin, New Mexico, to be published, 1978... 

A lack of water in parts of Argentina explains why sulfide oxidation and mine drainage contribute little to the maximum 0.140mgL 1 of arsenic in local surface waters (Williams, 2001, 274). The sodium chloride-dominated waters are rich in lithium and boron, which suggests that the arsenic is associated with the dissolution of salt deposits rather than sulfide weathering (Williams, 2001, 274). Excessive evaporation in arid climates would initially concentrate arsenic in briny lake water and eventually precipitate it in salt... 

A further exceedingly important mixing operation consists of whirling up solid particles ( suspension of solids ) to obtain their surfaces completely accessible to the surrounding liquid (dissolution of salts, solid catalyzed reactions in a S/L/G system, and so on). To work out the criteria important for this task, research concentrated on measuring the critical stirrer speed necessary for the flow state in which no particle lingered longer than 1 second on the bottom of the vessel. 

So Watt s engine work and De Luc s meteorology certainly had plenty of opportunity to intersect. In fact, substantially similar chemical ideas were deployed in each domain.14 This intersection is apparent retrospectively in De Luc s Idees sur la Meteorologie. A centrepiece of the Idees was De Luc s critique of the solution theory of evaporation as put forward by Senebier, Franklin, Hutton and others. According to that theory, water vapour was produced - that is, evaporation occurred - by the dissolution of water in air in a way precisely analogous to the dissolution of salt in water. This theory faced the difficulty that evaporation occurred in vacuo. De Luc s alternative theory was that water vapour was in fact a chemical product of water and heat. In recounting how he became confident of that alternative theory De Luc gave a prominent place to Watt s steam experiments. 

The precipitation of salts according to Rule I is accompanied by a large favorable entropy change, as the strongly hydrated cations and anions release numerous waters of hydration. In contrast, the dissolution of salts according to Rule II is accompanied by very little entropy change, since one ion is a stmctme breaker, while the other is a structure maker the dissolution occurs because the ions are mismatched... 

CUCURBITA COECA — A Vessel in which dissolutions of salts and other substances are thoroughly filtrated by vapour. 

Thermal Phenomena in Dissolution of Salts Supplementary Experiments... 

Another source of urban salinization is the use of road de-icing salts. Salt has been used for road de-icing for several decades, particularly in the eastern and northeastern states in the United States and Canada. The use of road salt improves fuel efficiency and reduces accidents at the same time, it causes salinization of associated groundwater. In 1990, more than 10.5 Mt of salt were used for road de-icing (Richter et al., 1993). Brine generated in storage piles of salt (Wilmoth, 1972) and from the dissolution of salts that are applied directly to the roads (Howard and Beck, 1993 Williams et al., 1999) can contaminate water resources. If salt is applied as a powder, its particles may become airborne and transported for considerable distances downwind (Jones and Hutchon, 1983 Richter et al., 1993). 

DISSOLUTION OF SALT ON THE EAST FLANK OF THE PERMIAN BASIN IN THE SOUTHWESTERN U.S.A. 

Johnson, K.S., 1981. Dissolution of salt on the east flank of the Permian Basin in the southwestern U.S.A. In W. Back and R. Letolle (Guest-Editors), Symposium on Geochemistry of Groundwater — 26th International Geological Congress. J. Hydrol., 54 75-93. 

Fig. 7. Generalized cross-sections in Red River study area of Texas Panhandle, showing dissolution of salt units in vicinity of principal rivers and remnant of salt in syncline northeast of Plaska Dome (A—B, well 5). See Fig. 6 for lines of cross-sections.

The Na/Cl ratio of brines formed by dissolution of salt in western... 

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