Solids solutions of, in liquids

A solution that contains as much of a dissolved species as it can at equilibrium is said to be saturated with that species. A solution in equilibrium with solid solute must be saturated with that solute if it were not, more solute would dissolve. 

If a saturated solution is cooled, the solubility of the solute generally decreases in order for the cooled solution to return to equilibrium, some solute must come out of solution as solid crystals. The crystallization rate may be slow, however, so that a metastable condition can exist in which the concentration of the solute is higher than the equilibrium value at the solution temperature. Under such conditions, the solution is said to be supersaturated and the difference between actual and equilibrium concentrations is referred to as supersaturation. Ail problems involving solid-liquid separations in this text assume that equilibrium exists between the solid and liquid phases, so that supersaturation need not be considered. 

Both the filtrate and the liquid retained in the filter cake are in equilibrium with solid AgNO cr s-tals and must therefore be saturated with AgNOa at 20 C The compositions of saturated solutions of silver nitrate at 100 C and 20 C are given at the beginning of this section and are used in the flowchart. 

The simultaneous solution of these three equations yields 

The Gibbs phase rule shows that specifying temperature and pressure for a two-component system at equilibrium containing a solid solute and a liquid solution fixes the values of all other intensive variables. (Verify this statement.) Furthermore, because the properties of liquids and solids are only slightly affected by pressure, a single plot of solubility (an intensive variable) versus temperature may be applicable over a wide pressure range. 

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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. 

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

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. 

Unless otherwise stated, solutions of solids in liquids are expressed as percentage w/v, and solutions of liquids in liquids as percentage v/v. When acids of various strengths are specified, e.g. 50% sulphuric acid, this implies the appropriate dilution by volume of the strong acid in water. 

Miscible. The above terms refer principally to solutions of solids in liquids. When one liquid dissolves in another liquid to form a solution, the term miscible is used. The solvent will be the component that is present in the larger quantity. Ethanol is completely miscible with water - a property used to produce the wide strength range of alcoholic drinks. Immiscible. When one liquid is insoluble in another hquid, i.e. the hquids do not mix, the term used is immiscible. The two hquids will form separate layers with the one with the lower density on top. For example, a puddle in the street often has an iridescent or rainbow-like appearance due to a thin layer of oil on the top. Oil is immiscible with water, and has the lower density, so it floats (see Figure 4.6.1). 

In dealing with the solution of solids in liquids, the solubility or mole fraction of the solute in the solvent is the primary variable of interest. The solubility depends on the intermolecular forces between the solute and the solvent, which are represented by the activity coefficient. However, equally important in determining the solubility is the standard state to which the activity is referred and the fugacity of the pure solid. In writing the equilibrium equation, it is assumed that there is no solubility of the solvent in the solid. Then, Eq. (68) can be written. 

In assessing the above treatment of non-ideal solutions, it must be kept in mind that it is applicable to a limited number of systems. This follows from the fact that we are often dealing with solutions of solids in liquids, and also because not all liquids are miscible over the whole composition range. Under these circumstances it is not convenient to define the standard state for one component in terms of the pure substance. Thus, for the majority of solutions, the majority component is treated as the solvent and its thermodynamics discussed with respect to its pure state within the context of Raoult s law. The other minority component, which is the solute, is discussed using a standard state based on the properties of an ideally dilute solution. These systems are considered in more detail later in this chapter. 

This equation has the same form as that obtained for ideal solubility but AHfvs has been replaced by the enthalpy of solution AHmjx. In non-ideal solutions of solids in liquids which do not follow either Henry s or Raoult s Laws, AHmix is the differential enthalpy of solution of the solute in the saturated solution. Both AGmix and AHmix are for non-ideal solutions similar to the reaction free energy we introduced when studying equilibrium in chemical reactions. They are all differential quantities AHmix is the enthalpy change when one mole of solute is added to an infinite volume of nearly... 

Solution of solid in liquid evaporation or (better) crystallization to recover solid distillation to recover liquid... 

Furthermore, the discussed methods allow the calculation of heat effects emerging for solution of solids in liquids. With the activity a, it follows... 

Two things happen when a solid dissolves (a) Solute molecules are pulled apart, as in melting (or even as in evaporation), which requires an input of heat, (b) Solute molecules become associated with solvent molecules, with a resultant output of heat. This attachment of solvent molecules to solute molecules is known as SOLVATION or, when the solvent is water, HYDRATION, For most solutions of solids in liquids, the... 

Lewis and Whitman [16,17] based on the data fisr solution of solid in liquid phase accept that on both sides of the int % betwc and liquid, an... 

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