Solids in Liquids

Dissolving sugar in water is another example of an endothermic process. Hot coffee drinkers are probably aware of the fact that the solubility of sugar increases at higher temperatures. 

It is more common for the solubility of solids in water to increase with increasing temperatures than for solubility to decrease with increasing temperatures. 

If raising the temperature increases solubility, then decreasing the temperature should decrease solubility. This is what happens when lava cools. Many of the minerals that deposit from lava beds do so because they become less soluble at cooler temperatures. Crystals begin to grow and will do so until the supply of ions needed is finally depleted. 

On the other hand, consider what happens when CaCOj dissolves in water. This time the process is exothermic, which we can show in the following equation  

Since the system must give offbeat in order for CaCO to dissolve, increasing the temperature works against the solution process. This time, as the temperature increases, the solubility decreases CaCOj becomes less soluble. In solution processes that are exothermic, lowering the temperature—not raising the temperature— increases solubiUty. 

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Solutions of solids in liquids can sometimes be separated by distilling off the liquid and leaving a residue of the solid, e.g., acetone and acetamide. 

These mixers are particularly suited for rapid mixing of powders and granules with liquids, for dissolving resins or solids in liquids, or for removal of volatiles from pastes under vacuum. Scale-up is usually on the basis of maintaining constant peripheral velocity of the impeller. 

Batch Stirred Tanks Tanks agitated by coaxial impellers (turbines, paddles, or propellers) are commonly used for batch dissolution of solids in liquids and may be used for leaching fine solids. Insofar as the controlhng rate in the mass transfer is the rate of transfer of mate-... 

Figure 2-12B and Table 2-2 present several typical valves and connections, screwed and flanged, for a variety of sizes or internal diameters. These do not apply for mixtures of suspended solids in liquids rather specific data for this situation is required (see [2]). Reference [3] presents data for specific valves. 

In terms of the two-phase system which comprises dispersions of solids in liquids, the minimum energy requirement is met if the total interfacial energy of the system has been minimized. If this requirement has been met, chemically, the fine state of subdivision is the most stable state, and the dispersion will thus avoid changing physically with time, except for the tendency to settle manifest by all dispersions whose phases have different densities. A suspension can be stable and yet undergo sedimentation, if a true equilibrium exists at the solid-liquid interface. If sedimentation were to be cited as evidence of instability, no dispersion would fit the requirements except by accident—e.g., if densities of the phases were identical, or if the dispersed particles were sufficiently small to be buoyed up by Brownian movement. 

The term two-phase flow covers an extremely broad range of situations, and it is possible to address only a small portion of this spectrum in one book, let alone one chapter. Two-phase flow includes any combination of two of the three phases solid, liquid, and gas, i.e., solid-liquid, gas-liquid, solid-gas, or liquid-liquid. Also, if both phases are fluids (combinations of liquid and/or gas), either of the phases may be continuous and the other distributed (e.g., gas in liquid or liquid in gas). Furthermore, the mass ratio of the two phases may be fixed or variable throughout the system. Examples of the former are nonvolatile liquids with solids or noncondensable gases, whereas examples of the latter are flashing liquids, soluble solids in liquids, partly miscible liquids in liquids, etc. In addition, in pipe flows the two phases may be uniformly distributed over the cross section (i.e., homogeneous) or they may be separated, and the conditions under which these states prevail are different for horizontal flow than for vertical flow. 

Surface Damage and Reaction Rates. Erosion of surfaces resulting in higher surface area and removal of inhibiting impurities are two effects of cavitation on solids in liquid media, both of which lead to increased reaction rates. The high temperatures and pressures are sufficient to deform and pit metal surfaces (even cause local melting of some metals) and to fracture many nonmetal lie solids, in particular, brittle materials. 

The dissolution of many ionic solids in liquids is endothermic (requires heat) due to the large solute-solute attractions that must be overcome relative to the solvent-solute attractions. 

Other models allow for a distribution of site reactivities. Similar considerations apply for reactions of solids in liquid solution. 

The solubility of solids in liquids is an important process for the analyst, who frequently uses dissolution as a primary step in an analysis or uses precipitation as a separation procedure. The dissolution of a solid in a liquid is favoured by the entropy change as explained by the principle of maximum disorder discussed earlier. However it is necessary to supply energy in order to break up the lattice and for ionic solids this may be several hundred kilojoules per mole. Even so many of these compounds are soluble in water. After break up of the lattice the solute species are dispersed within the solvent, requiring further energy and producing some weakening of the solvent-solvent interactions. 

Take melting points on dry, solid substances ONLY, never on liquids or solutions of solids in liquids or on wet or even damp solids. 

In this chapter, you learned about solutions. A solution is a homogeneous mixture composed of a solvent and one or more solutes. Solutions may be unsaturated, saturated, or supersaturated. Solution concentration units include percentage, molarity, molality, and mole fraction. The solubility of solids in liquids normally increases with increasing temperature, but the reverse is true of gases dissolving in liquids. The solubility of gases in liquids increases with increasing pressure. 

When analyzing for solids in liquid samples, such as water and wastewater samples, the analysis is based on a volume of the sample rather than a weight. Thus, while this is considered an example of a gravimetric analysis because the weight of the solids is determined, a volume of the sample is measured rather than a weight. There are several categories of solids that may be determined. 

Work on the rate of dissolution of regular shaped solids in liquids has been carried out by Linton and Sherwood(1), to which reference is made in Volume 1. Benzoic acid, cinnamic acid, and /3-naphthol were used as solutes, and water as the solvent. For streamline flow, the results were satisfactorily correlated on the assumption that transfer took place as a result of molecular diffusion alone. For turbulent flow through small tubes cast from each of the materials, the rate of mass transfer could be predicted from the pressure drop by using the 1 j-factor for mass transfer. In experiments with benzoic acid, unduly high rates of transfer were obtained because the area of the solids was increased as a result of pitting. 

Conduction is the principal mechanism of heat transfer in solids. In liquids, conduction is frequently augmented by convection as circulating currents are set up due to density differences, while in gases, because of... 

Liquid-Solid Mixtures Liptak [Chem. Eng, 74(4), 151-158 (1967)] discusses a variety of techniques that can be used for the measurement of solids-in-liquid suspensions or slurries. These include metering pumps, weigli tanks, magnetic flowmeter, ultrasonic flowmeter, gyroscope flowmeter, etc. 

The rate of solution of solids in liquids is found to be a reaction of zero order provided that the concentration of the solution remains unchanged and the surface exposed to solution constant, as is indicated by the early experiments of Wenzel Lehre der Verwandtschaft, xxviii. 1777) and of Veley J.G.8. LV. 361, 1889 Phil. Mag. CLXXXii. 279, 1891) on the rate of solution of metals in acids. 

In any discussion of solubility, it is important to remember solubility is temperature dependent. Generally, the solubility of solids in liquids increases with temperature. The variation of solubility in water varies greatly for different solutes. Figure 11.2 demonstrates that the solubility may increase... 

Aerosols may be produced by atomizing liquids or suspensions of solids in liquids. Nebulizers are a type of atomizer in which both large and small particles are initially produced but in which the large particles are removed by impaction within the nebulizer. As a result, only particles with diameters < 10 /xm exit most nebulizers. 

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