Solubility

for this time, i would share about solubility, hope my post gives many advantages for your duty or necessary :D

The solubility of a substance is the amount of that substance that will dissolve in a given amount of solvent. Solubility is a quantitative term. Solubilities very enormously (recall Table 8.2). The terms soluble and insoluble are relative. A substance is said to be soluble if more than 0.1 g of that substance dissolves in 100 mL solvent. If less than 0.1 g dissolves in 100 mL solvent, the substance is said to be insoluble or, more exactly, sparingly soluble. The terms miscible and immiscible may be encountered when considering the solubility of one liquid in another. Miscible means soluble without limits; for example, alcohol is miscible with water Immiscible and insoluble mean the same; oil is immiscible with water, as in oil and vinegar salad dressing (see Figure 11.1).

 

 

FIGURE 11.1 Soluble and insoluble. Alcohol is soluble in water; when added to water, it forms a clear solution. Oil is insoluble in water; when added to water, the two liquids form separate layers.

A. Determining Solubility
How is the solubility of a substance determined? A known amount of the solvent--for example, 100 mL--is put in a container. Then the substance whose solubility is to be determined is added until, even after vigorous and prolonged stirring, some of that substance does not dissolve. Such a solution is said to be saturated because it contains as much solute as possible at that temperature. In this saturated solution, the amount of solute is the solubility of that substance at that temperature in that solvent. Doing this experiment with water as the solvent and sodium chloride as the solute, we find that, at 20°C, 35.7 g of the salt dissolve in 100 mL water. The solubility of sodium chloride is, then, 35.7 g/100 mL water at 20°C. Sodium chloride is a moderately soluble salt. The solubility of sodium nitrate is 92.1 g/100 mL water at 20°C; sodium nitrate is a very soluble salt. At the opposite end of the scale is barium sulfate, which has a solubility of 2.3 X 10 ^-4 g/100 mL water at 20°C. Barium sulfate is an insoluble salt. See Table 8.2 for the solubility of other compounds and Table 8.3 for the rules predicting the solubility of ionic compounds.

B. Saturated Solutions
A saturated solution is one in which the dissolved solute is in equilibrium with the undissolved solute. The container in figure 11.2a holds a saturated solution of sucrose (cane sugar); at the bottom of the container is some undissolved sucrose. If we could see the individual molecules in this solution, we would see that some of the molecules of sucrose are moving away from the solid crystals at the bottom of the container as they dissolve. The same number of molecules are coming out of solution to become part of the undissolved sucrose.

FIGURE 11.2 Equilibrium in solutions: the equilibrium in a sucrose solution between dissolved and undissolved molecules; the equilibrium in an ionic solution between dissolved ions and undissolved sodium chloride.

\ Dissolution (dissolving) and precipitation are occurring at the same rate, thereby satisfying the requirement for a dynamic equilibrium. This requirement was set forth in Section 10.3, where we discussed the equilibrium between liquid and vapor. We can express the equilibrium in the sucrose solution with the equation

sucrose(s) sucrose(aq)

Two statements can be made about this solution: (1) the two processes dissolution and precipitation are going on at the same time, and (2) the number of molecules in the solution remains constant.
A saturated solution of an ionic compound is slightly different from that of a covalent compound like sucrose. The ionic compound dissolves and exists in solutions as ions; the covalent compound dissolves and exists in solutions as molecules. The equilibrium of sodium chloride with its ions in a saturated solution would be shown by the equation

NaCl(s) Na⁺(aq) + Cl^-(aq)

These equilibria are diagramed in Figure 11.2. An unsaturated solution contains less solute than does a saturated solution. No equilibrium is present. When additional solute is added to an unsaturated solution, it dissolves. When additional solute is added to a saturated solution, the amount of dissolved solute does not increase, because the limit of solubility has already been reached. Adding more solute to a saturated solution simply increases the amount of undissolved solute.
Remember in discussing these solutions that solubility changes with temperature. (This factor will be discussed further in Section 11.2C2.) A solution that is saturated at one temperature may be unsaturated at a different temperature.
This spot is perhaps appropriate for a few comments on preparing solutions. Dissolution requires interaction between the molecules (or ions) of the solute and the molecules of the solvent. A finely divided solute will dissolve more rapidly than one that is in large chunks, because more contact is made between solute and solvent. Constant stirring increases the rate of dissolution, because stirring changes the particular solvent molecules that are in contact with undissolved solute. Because the solubility of solids and liquids generally increases with temperature, solids and liquids are often dissolved in warm solvents.