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
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.
- 12 small test tubes
- test tube rack
- conductivity meter
- Buchner funnel
- filter flask and trap
- filter paper
- Three 150-mL beakers, one 250-mL beaker
- gas burner and flint igniter
- ring stand and wire gauze
- pipets of several sizes
- 100-mL graduated cylinder
- Develop a procedure to evaluate, in a qualitative sense,
the solubility of four possible solutes in three possible
- The solutes are: iodine, sodium chloride, glucose, and
naphthalene. The solvents are toluene, water, and ethanol.
- 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.
- 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.
- 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.
Evaluation of Conductivity
- 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.
- 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.
- Tabulate your data. State your conclusions as appropriate
generalizations about the relationships among conductivity,
solubility, and ionic dissociation.
Temperature Dependence of Solubility
- Working with a partner, you will be assigned a temperature
at which to evaluate the solubility of sodium carbonate in
- Devise an appropriate procedure. (See suggestions
below.) All of the equipment listed above will be available
and, within reason, the student may request additional
- 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.
- 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
- What is a saturated solution?
- 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.
- 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,
- Referring to the procedure you devised to determine the
solubility of sodium carbonate, what step leads to the
greatest uncertainty in the result?