11. SOLUTIONS AND SOLUBILITY

The mechanism by which (and the extent to which) a solute dissolves in a solvent depends upon the nature of each substance and the temperature of the system. Most often we think of solutions of a solid solute in a liquid solute. However, solutions need not be limited to such a pattern. Metal alloys (solutions of solids in solids) and carbonated water (solution of a gas in a liquid) are other possibilities. (When dealing with gaseous solutes, the pressure of the system has an effect upon the solubility as well as the temperature.)

As a generalization, the more chemically similar two substances are the more likely they are to form a solution. "Like dissolves like." Another way of saying this is that a polar substance would be more likely to dissolve in a polar solvent than in a nonpolar one. Likewise, a nonpolar substance would be expected to show greater solubility in a nonpolar solvent than in a polar one.

Benzene, C6H6, is a nonpolar substance that dissolves completely in a nonpolar solvent such as ether, CH3OCH3. In the polar solvent water, benzene is insoluble (well, to be exact, 0.08 g of benzene dissolve in 100 g of water). Glucose is a sugar with a great deal of polar character, and glucose dissolves readily in water, to the extent of 83 g per 100 g of water, and not at all in ether.

In addition to the process of dissolution, ionic solutes exhibit the phenomenon of dissociation. In other words, when an ionic substance dissolves, the positive and negative ions of which the substance is composed actually separate and exist as positively and negatively charged ions in the solvent. Since those ions are free to move in the solution as charge carriers, solutions of ionic solutes can conduct an electrical current. This property of ionic solutions was utilized in Experiment 6, Electrolysis of Copper. Compounds that dissociate in solution and yield ions are known as electrolytes; those that dissolve without formation of free ions are known as nonelectrolytes.

In this exercise the student is asked to develop her or his own experimental procedure to perform both qualitative and quantitative investigations on the properties of a group of potential solutes and solvents.

EQUIPMENT

  1. 12 small test tubes
  2. test tube rack
  3. conductivity meter
  4. Buchner funnel
  5. filter flask and trap
  6. filter paper
  7. Three 150-mL beakers, one 250-mL beaker
  8. gas burner and flint igniter
  9. ring stand and wire gauze
  10. pipets of several sizes
  11. 100-mL graduated cylinder

PROCEDURE

Qualitative Analysis

  1. Develop a procedure to evaluate, in a qualitative sense, the solubility of four possible solutes in three possible solvents.
  2. The solutes are: iodine, sodium chloride, glucose, and naphthalene. The solvents are toluene, water, and ethanol.
  3. For the purpose of a qualitative evaluation of solubility, it is satisfactory to determine whether or not a small amount of the possible solute, say a few crystals, will dissolve in a small amount of possible solvent, say 5 mL or so.
  4. Your laboratory instructor will identify the solvent waste cans in which any waste toluene and toluene solutions should be placed. Ethanol and water solutions may be disposed down the laboratory drains.
  5. Tabulate your results appropriately and make any appropriate generalizations about the influence that the relative polarity of a substance has upon its role as a solvent or solute.
  6. Evaluation of Conductivity

  7. Working with a partner, prepare 10% (w/v) solutions of glucose in water and sodium chloride in water. Also prepare a 1% (w/v) solution of glucose in ethanol. Fifty milliliters of each of these solutions is sufficient.
  8. Under the direct supervision of the laboratory instructor, evaluate the ability of each of your solutions and of the two solvents themselves to conduct an electrical current.
  9. Tabulate your data. State your conclusions as appropriate generalizations about the relationships among conductivity, solubility, and ionic dissociation.
  10. Temperature Dependence of Solubility

  11. Working with a partner, you will be assigned a temperature at which to evaluate the solubility of sodium carbonate in water.
  12. Devise an appropriate procedure. (See suggestions below.) All of the equipment listed above will be available and, within reason, the student may request additional equipment.
  13. Using the procedure you have developed, determine the solubility of sodium carbonate at the assigned temperature. Report your results to the laboratory instructor in terms of the mass of solute that would dissolve in 100 g of solvent at your assigned temperature.
  14. The instructor will collect all the data from the class and post this on the blackboard. Using the pooled data, prepare a graph of the relationship between solubility and temperature.

QUESTIONS

  1. What is a saturated solution?
  2. The solubility of sodium carbonate exhibits a positive temperature dependence, that is, the solubility increases as the temperature increases. Are there substances which show the opposite behavior ? Give an example.
  3. Based upon your deductions about the polarity of benzene, C6H6, which of the following materials would be the best solvent for that material - water, toluene, ethanol ?
  4. Referring to the procedure you devised to determine the solubility of sodium carbonate, what step leads to the greatest uncertainty in the result?