Chemistry
Saturated Unsaturated and Supersaturated
Saturated, unsaturated, and supersaturated are terms used to describe the concentration of a solution. A saturated solution contains the maximum amount of solute that can be dissolved at a given temperature. An unsaturated solution can dissolve more solute, while a supersaturated solution contains more solute than should theoretically be possible at that temperature, often achieved through careful manipulation of conditions.
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6 Key excerpts on "Saturated Unsaturated and Supersaturated"
eBook - PDF- David A. Ucko(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
At this point a balance or equilibrium exists. The rate at which the sugar dissolves in the water exactly equals the rate at which it returns from solution to form solid sugar. (This dynamic situation is similar to the liquid-vapor equilibrium described in Section 12.4.) The solution then is said to be saturated. (See Figure 13-4.) A saturated solution contains the maximum amount of dissolved solute possible under normal conditions at a particular temperature. The amount of solute needed to make a saturated solution depends on the solubility of the solute in the solvent. A saturated solution does not necessarily contain a large quantity of solute. If a solute is only slightly soluble, not much of it will dissolve, and we can therefore prepare a saturated solution with relatively little solute. As shown in Table 13-4, a saturated solution of calcium hy-droxide at 20°C contains only 0.165 g of Ca(OH) 2 per 100 g of water. When a solution contains less solute than required for equilibrium, it is said to be unsaturated. More solute can therefore be dissolved in an unsaturated solution. For example, a solution prepared with 20 g of sodium chloride in 100 g of water at 20°C is unsaturated. As shown in Table 13-4, the solution becomes saturated only when the mass of sodium chloride reaches 36 g per 100 g water at this temperature. A solution can contain a large amount of solute and still be unsaturated if the solubility of the solute is great. Under certain conditions, we can make a solution containing more solute than is required to form a saturated solution at a particular temperature. Such a solution is said to be supersaturated. A supersa-turated solution can be prepared by first making a saturated solution and then slowly lowering its temperature (assuming that the solubility of the solute decreases upon cooling). If the temperature is lowered slowly enough, excess solute will remain dissolved. Another method 13.4 SATURATION FIGURE 13-4 A saturated solution.
eBook - PDF- Allan Blackman, Steven E. Bottle, Siegbert Schmid, Mauro Mocerino, Uta Wille(Authors)
- 2022(Publication Date)
- Wiley(Publisher)
In this chapter, we will focus on the energetics of formation of solutions and the quantification of physical properties of solutions. FIGURE 10.2 A saturated solution. In a saturated solution, a dynamic equilibrium exists between the undissolved solute and the dissolved solute in the solution. A solution can be defined as a homogeneous mixture of two or more pure substances (‘homogeneous’ means that all regions of the solution have exactly the same composition). Despite the fact that we often think of a solution as involving some type of liquid, we can have gaseous solutions com- prising two or more gases (e.g. air) and even solid solutions comprising two or more solids (e.g. alloys such as brass and solder) in addition to the ‘usual’ solutions containing a gas, liquid or solid dissolved in a liquid. In the case of liquid solutions, we call the liquid the solvent, while the dissolved substance is called the solute; the solute is present in smaller amounts than the solvent. A solute is said to be soluble in a particular solvent if it dissolves completely in that solvent at the specified temperature. We define the solubility of the solute in a particular solvent as the maximum amount of the solute that dissolves completely in a given mass or volume of the solvent at a particular temperature, T, and a particular pressure, p. A saturated solution is one in which no more solute will dissolve. The most common type of saturated solution we will encounter is that illustrated in figure 10.2, in which excess solid solute is in equilibrium with its dissolved form. The process of dissolving a solute in a solvent to give a homogeneous solution is called dissolution. We therefore talk of the dissolution of a solute in a particular solvent. 10.2 Gaseous solutions LEARNING OBJECTIVE 10.2 Describe why gases mix spontaneously. We will begin our investigation of solutions by looking at mixing two gases to form a gaseous solution.
eBook - PDF- Allan S. Myerson, Deniz Erdemir, Alfred Y. Lee(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
Any supersaturation simplifica- tions should be justified before use. In very nonideal solu- tions and in precise studies of crystal growth and nucleation, activity coefficients should be used as described in Section 1.5.1. Further caution should be used if these alternate supersaturation units are employed in ternary or Solutions and Solution Properties 19 higher-order solutions because differences in solvent com- position can affect the calculation of solute concentration in the solution. Supersaturation also may be expressed as a percentage, where 0 percent supersaturation corresponds to a saturated solution and 100 percent supersaturation corresponds to twice the saturation concentration, although this is more common in vapor–liquid applications that in solid–liquid applications. Finally, supersaturation may be characterized in terms of degrees. This refers to the difference between the temperature of the solution and the saturation temperature of the solution at the existing concentration. A simpler way to explain this is that the degrees of supersaturation are simply the number of degrees a saturated solution of the appropriate concentration was cooled to reach its current temperature. This is generally not a good unit to use, yet it is often mentioned in the literature. 1.5.3 Metastability and the Metastable Limit As we have seen previously, supersaturated solutions are meta- stable. This means that supersaturating a solution some amount will not necessarily result in crystallization. Referring to the solubility diagram in Figure 1.17, if we were to start with a solution at point A and cool to point B just below saturation, the solution would be supersaturated. If we allowed that solu- tion to sit, it might take days before crystals form. If we took another sample, cooled it to point C, and let it sit, this might crystallize in a matter of hours. Eventually we will get to a point where the solution crystallizes rapidly and no longer appears to be stable.
eBook - PDFIndustrial Crystallization
Fundamentals and Applications
- Alison Lewis, Marcelo Seckler, Herman Kramer, Gerda van Rosmalen(Authors)
- 2015(Publication Date)
- Cambridge University Press(Publisher)
Chapters of books are available on the crystallization of biomolecules (Myerson, 2002, Pessˆ oa Filho et al., 2011). 1.5 Supersaturation Consider a homogeneous solution that is not necessarily in an equilibrium state, for example the pharmaceutical intermediate D-(p-hydroxy)phenylglycine in water, the sol- ubility of which is shown in Figure 1.12. If the thermodynamic state of the solution is situated below the solubility line (region A in the figure), the solution is said to be undersaturated. If the solution is at the solubility line (line B), the solution is said to be saturated and if it is above the solubility line (region C), the solution is said to be supersaturated. The elementary processes related to crystallization, such as nucleation and molecular crystal growth, take place in supersaturated solutions. The degree of supersaturation often determines their mechanism and rate. Therefore, in order to properly characterize these rate processes, variables that quantitatively express the degree of supersaturation are needed. A rational basis found in thermodynamics is presented next. Thereafter, other practical quantities that describe the degree of supersaturation are introduced, based on easily measurable variables such as concentrations or temperatures. 1.5.1 General thermodynamic expression for the supersaturation Consider a multi-phase system with allowed mass transfer of components between the phases. A requirement for thermodynamic equilibrium in such a system is equal Supersaturation 15 chemical potentials of each component through all phases. Let μ liquid and μ solid [J mol −1 ] represent the chemical potential of the crystallizing compound in a solution and as a solid, respectively. If the system is in equilibrium then: μ liquid,eq = μ solid (1.1) Suppose the system leaves its equilibrium state due to some external action, e.g. cooling or evaporation of part of the solvent, so that the solution becomes supersaturated.
eBook - PDF- Amos Turk(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
But there is nothing wrong with a supersaturated solution, any more than with a book standing on end; in each case, there exists another situation which is more stable, * Strictly speaking, the heat effect in question is that associated with the addition of solute to a nearly saturated solution. There are rare instances in which heat is emitted on forming the dilute solution and absorbed on adding solute to a concentrated solution (for example, NaOH in H 2 0), or conversely. 222 • SOLUTIONS -Η Ο Η ammonia Η Η ethanol acetone namely, saturated solution plus excess solid, or the book lying flat. Just as the book needs a push in order to reach its more stable position, the supersaturated solution may not disgorge its excess solute without a little encouragement. Supersaturation can be observed in any kind of solution, provided a finite solubility exists, but the phenomenon is most commonly observed when one component is a crystalline solid. It is not easy to initiate the growth of a crystal. The embryonic crystal must be formed by several molecules or ions that come together in the correct configuration and stay that way long enough for other particles to deposit on them. Such an event is not impossible, but remains im-probable until the solution is considerably supersaturated. Super-saturation is much more likely to be relieved by deposition of solute molecules on a dust particle, the container wall, or any other solid present. The best crystallization nucleus, of course, is a fragment of the solute itself, or of something else with a similar crystal structure. When such a seed is introduced, crystals usually form very rapidly, leaving a saturated solution. 13.7 • SOLUBILITY AND MOLECULAR STRUCTURE When a solid or a liquid dissolves in a liquid, the molecules of each kind must lose some of their like neighbors, obtaining in exchange neighbors of the other kind.
- G. T. Hefter, R. P. T. Tomkins, G. T. Hefter, R. P. T. Tomkins(Authors)
- 2003(Publication Date)
- Wiley(Publisher)
The important qualifier saturated implies equilibrium with respect to the processes of dissolution and crystallization for solubility of a solid in a liquid, of phase transfer for solubility of a liquid in another liquid, or of vaporiza- tion and dissolution for solubility of a gas in a liquid. The equilibrium may be stable or metastable, that is, the composition of a system may maintain a particular value for a long time, yet may shift suddenly or gradually to a more stable state if subjected to a specific disturbance. An example of a metastable equilibrium is a supersaturated solution of a solid in a liquid; a perturbation of the system, such as addition of an appropriate seed crystal, can initiate crystallization and bring the system to its final state of stable equilibrium. The solubility of a substance in metastable equilibrium is usually greater than that of the same substance in stable equilibrium. (Strictly speak- ing, it is the relative activity of the substance in metastable equilibrium that is greater.) Thermodynamics of Solubility 21 3 THERMODYNAMICS OF SOLUBILITY 3.1 General Thermodynamic Equation for Solubility The general solubility equilibrium is that of a component B in phase in equilibrium with the same component in phase : (1) The basic thermodynamic relation is the equality of the chemical potentials on both sides of equation (1), as shown in equation (2). For complete thermo- dynamic equilibrium, each phase must be in thermal equilibrium (uniform temperature, T ) and mechanical equilibrium (uniform pressure, p). (2) In equation (2), are the mole fractions of C 1 components in the system. The amount fraction of component C is not an independent variable; the amount fractions sum to unity. If the analytical expressions for the chem- ical potentials are introduced, we obtain: (3) The standard chemical potentials must be compatible with the definitions of the activities a B of substance B for a particular composition scale.
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