A solution is a homogeneous mixture of two or more pure substances on molecular level whose composition can vary within certain limits. A solution has two components (substances) known as solute and the solvent. A supersaturated solution contains more dissolved solute than required for preparing a saturated solution and can be prepared by heating a saturated solution, adding more solute, and then cooling it gently. Excess dissolved solute crystallizes by seeding supersaturated solution with a few crystals of the solute. For example, the object of pan boiling is the production of a fine even crop of sugar crystals. A primary condition for the attainment of this end is the maintenance of a control over the rate of crystallization during the growth. Basically, a crystal surface maintained in a solution will only grow if the concentration of the solution is maintained at a level greater than the saturation concentration. Such a solution is said to be supersaturated. Supersaturation in Phase Change (Crystallization and Condensation)
Applications of Supersaturated SolutionWhen a solution of a solid solute dissolved in a liquid solvent is saturated, it is in thermodynamic equilibrium. In order for crystallization to occur, the state of the system must be shifted to a nonequilibrium state in which the concentration of the solute in the solution exceeds its equilibrium concentration at the given solution conditions. Solutions that are in the nonequilibrium state are said to be supersaturated. The simplest method to create a supersaturated solution is by cooling.  A solution is initially prepared at point A. If this solution is cooled it will be saturated when it intersects the saturation line. If it is cooled past the saturation line to point B it will be supersaturated. Just because this solution is supersaturated, however, does not mean that it will immediately crystallize. Supersaturated solutions are metastable. This means that there is a free energy barrier which must be overcome for the phase transition to be overcome. The simplest and most common method of making a supersaturated solution is by cooling, but this is not the only method that can be used, there are many methods like solvent evaporation, temperature change, change in pH, chemical reaction and alteration in solvent composition. Examples of Supersaturated SolutionSupersaturated solution contains more dissolved substances than a saturated solution. For example, 40g NaCl in 100ml H2O. The additional 4.0g NaCl remains undissolved. Solved Examples1. What is the mass percent of sodium hydroxide in a solution that is made by dissolving 8.00g NaOH in 50.0g H2O? Solution: Knowns 8.00g NaOH 50.0g H2O Solving for mass percent = 8.00g NaOH/8.00g NaOH + 50.0g H2O = 13.8% NaOH solution. 2. Will a solution made by adding 2.5g of CuSO4 to 10g of H2O be saturated for unsaturated at 20oC? Solution: We first need to know the solution of CuSO4 at 20oC. From figure 14.4 we see that the solubility of CuSO4 at 20oC is about 21g per 100g of H2O. This amount is equivalent to 2.1g of CuSO4 per 10g of H2O. Since 2.5g per 10g of H2O is greater than 2.1g per 10g of H2O, the solution will be saturated and 0.4g of CuSO4 will be maintained.
A supersaturated solution is a more solute solution than can be dissolved by the solvent. If you haven’t learned what a solute / solvent is, the material that is dissolved in the solution, such as salts but not restricted to salts, is a solution. The most popular example is sodium acetate which is supersaturated.
A supersaturated solution is a solution that contains more than the average solvent that can be dissolved at a given temperature. The recrystallization of the excess dissolved solvent in a super-saturated solution can be started by inserting a tiny solute crystal, called a seed crystal.
According to the solubility of the substance a “supersaturated” solution produces more dissolved content than it should. In the case of sugar, whose chemical name is “sucrose,” approximately 211 grams of water can dissolve in 100 millilitres. Solubility is temperature dependent; more sugar dissolves in hot water than in cold.
Under appropriate conditions, solutions may often be formulated that contain a greater amount of solvent than the one required to form a saturated solution. Owing to the presence of the solute in a supersaturated solution in a concentration greater than the concentration of equilibrium, super-saturated solutions are unstable.
The solid crystals in the hydrated crystals will dissolve into the bath, forming a supersaturated solution. When the solution for sodium thiosulfate is gradually cooled the super-saturated solution should remain liquid. Placing a small crystal in the over-saturated solution would make the liquid solid.
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The major component of a solution is called the solventThe major component of a solution.. The minor component of a solution is called the soluteThe minor component of a solution.. By major and minor we mean whichever component has the greater presence by mass or by moles. Sometimes this becomes confusing, especially with substances with very different molar masses. However, here we will confine the discussion to solutions for which the major component and the minor component are obvious. Solutions exist for every possible phase of the solute and the solvent. Salt water, for example, is a solution of solid NaCl in liquid water; soda water is a solution of gaseous CO2 in liquid water, while air is a solution of a gaseous solute (O2) in a gaseous solvent (N2). In all cases, however, the overall phase of the solution is the same phase as the solvent.
A solution is made by dissolving 1.00 g of sucrose (C12H22O11) in 100.0 g of liquid water. Identify the solvent and solute in the resulting solution. Solution Either by mass or by moles, the obvious minor component is sucrose, so it is the solute. Water—the majority component—is the solvent. The fact that the resulting solution is the same phase as water also suggests that water is the solvent. Test Yourself A solution is made by dissolving 3.33 g of HCl(g) in 40.0 g of liquid methyl alcohol (CH3OH). Identify the solvent and solute in the resulting solution. Answer solute: HCl(g); solvent: CH3OH One important concept of solutions is in defining how much solute is dissolved in a given amount of solvent. This concept is called concentrationHow much solute is dissolved in a given amount of solvent.. Various words are used to describe the relative amounts of solute. DiluteA solution with very little solute. describes a solution that has very little solute, while concentratedA solution with a lot of solute. describes a solution that has a lot of solute. One problem is that these terms are qualitative; they describe more or less but not exactly how much. In most cases, only a certain maximum amount of solute can be dissolved in a given amount of solvent. This maximum amount is called the solubilityThe maximum amount of a solute that can be dissolved in a given amount of a solvent. of the solute. It is usually expressed in terms of the amount of solute that can dissolve in 100 g of the solvent at a given temperature. Table 11.2 "Solubilities of Some Ionic Compounds" lists the solubilities of some simple ionic compounds. These solubilities vary widely: NaCl can dissolve up to 31.6 g per 100 g of H2O, while AgCl can dissolve only 0.00019 g per 100 g of H2O.
Table 11.2 Solubilities of Some Ionic Compounds
When the maximum amount of solute has been dissolved in a given amount of solvent, we say that the solution is saturatedA solution with the maximum amount of solute dissolved in it. with solute. When less than the maximum amount of solute is dissolved in a given amount of solute, the solution is unsaturatedA solution with less than the maximum amount of solute dissolved in it.. These terms are also qualitative terms because each solute has its own solubility. A solution of 0.00019 g of AgCl per 100 g of H2O may be saturated, but with so little solute dissolved, it is also rather dilute. A solution of 36.1 g of NaCl in 100 g of H2O is also saturated but rather concentrated. Ideally, we need more precise ways of specifying the amount of solute in a solution. We will introduce such ways in Section 11.2 "Quantitative Units of Concentration". In some circumstances, it is possible to dissolve more than the maximum amount of a solute in a solution. Usually, this happens by heating the solvent, dissolving more solute than would normally dissolve at regular temperatures, and letting the solution cool down slowly and carefully. Such solutions are called supersaturatedA unstable solution with more than the normal maximum amount of solute in it. solutions and are not stable; given an opportunity (such as dropping a crystal of solute in the solution), the excess solute will precipitate from the solution. It should be obvious that some solutes dissolve in certain solvents but not others. NaCl, for example, dissolves in water but not in vegetable oil. Beeswax dissolves in liquid hexane but not water. What is it that makes a solute soluble in some solvents but not others? The answer is intermolecular interactions. The intermolecular interactions include London dispersion forces, dipole-dipole interactions, and hydrogen bonding (as described in Chapter 10 "Solids and Liquids"). From experimental studies, it has been determined that if molecules of a solute experience the same intermolecular forces that the solvent does, the solute will likely dissolve in that solvent. So, NaCl—a very polar substance because it is composed of ions—dissolves in water, which is very polar, but not in oil, which is generally nonpolar. Nonpolar wax dissolves in nonpolar hexane but not in polar water. This concept leads to the general rule that “like dissolves like” for predicting whether a solute is soluble in a given solvent. However, this is a general rule, not an absolute statement, so it must be applied with care.
Would I2 be more soluble in CCl4 or H2O? Explain your answer. Solution I2 is nonpolar. Of the two solvents, CCl4 is nonpolar and H2O is polar, so I2 would be expected to be more soluble in CCl4. Test Yourself Would C3H7OH be more soluble in CCl4 or H2O? Explain your answer. Answer H2O because both experience hydrogen bonding Key Takeaways
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What is a substance dissolved in a solution and contains greater amount of a solute than the other component?
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