Polar solvents dissolve polar and ionic solids, and nonpolar solvents generally dissolve nonpolar solids. This can be easily demonstrated and projected on an overhead projector.
Trichlorotrifluoroethane (TTE), 5 mL
Solid iodine, I2(s), several crystals
Solid copper(II) sulfate, CuSO4 * 5 H2O(s), several crystals
Place two Petri dishes on the stage of an overhead projector. Add enough water to each dish to cover the bottom. In each dish, carefully add about 2 mL TTE. (Notice that it forms a pool.) Carefully drop a few crystals of iodine in the TTE pool of one Petri dish, and the same number in the water near the pool. What do you observe? Now, in the second dish add a few crystals of copper(II) sulfate in the TTE pool, and the same amount in the water near the pool. Stir each gently with a toothpick. What do you observe?
TTE is a nonpolar solvent; it does not dissolve in polar water. Iodine is a nonpolar solid; it does not dissolve in polar water, but does dissolve in nonpolar TTE. Copper(II) sulfate is an ionic solid that dissolves in polar water, but not in nonpolar TTE.
TTE is an acceptable and safe organic solvent. All of these materials can be disposed of according to directions given in the SourceBook Safety section.
The solubility of most solids increases with an increase in temperature. Some compounds, however, show a decrease in solubility with increased temperature. Calcium acetate, Ca(CH3COO)2, is such a compoundit has a negative coefficient of solubility.
Calcium acetate solution, saturated (about 60 g/150 mL water)
Erlenmeyer flask, 250-mL
Hot plate or burner
Place about 150 mL calcium acetate solution in a 250-mL flask and cool it overnight. Heat the flask gently and note that a precipitate begins to form as it is heated (at about 80 °C). Remove the flask from the heat and cool it by placing it in a stream of tap water. Notice that the precipitate goes back into solution as the temperature decreases.
Collect waste chemicals in a special container and dispose of them according to the acceptable procedure for your school. (This solution can be stoppered and saved from year to year. You may need to readjust the amount of water as needed after long periods of standing.)
Students often get the impression that all precipitates are white, since this is what they usually see. This demonstration produces a nice blue precipitate.
Limewater (saturated calcium hydroxide solution), ~0.5 g in 200 mL H2O
0.1 M Cobalt(II) nitrate solution [2.9 g Cobalt(II) nitrate hexahydrate, Co(NO3)2 * 6 H2O, per 100 mL solution] in dropping bottle
Place the limewater in a large beaker. Add a few drops of cobalt nitrate solution. Note the immediate formation of a blue precipitate.
Cobalt nitrate should be handled with care. Wear gloves and dispose of the precipitate by drying and following directions in the SourceBook Safety section.
Cobalt ion reacts with the basic limewater solution to produce cobalt hydroxide, a blue precipitate.
Co2+(aq) + 2OH-(aq) <=> Co(OH)2(s)
Water that we use in our homes often comes from rivers, lakes, or ground deposits, and contains impurities such as bacteria, soil, and toxic substances that must be removed before we can use it. After water is allowed to stand in large tanks to allow large particles to fall to the bottom, it is treated to remove smaller suspended particles. This demonstration duplicates that treatment.
Lime solution (calcium hydroxide; about 3 tsp per liter water)
Alum solution (potassium aluminum sulfate, or aluminum sulfate, or ammonium aluminum sulfate; about 3 tsp per liter water)
Red litmus paper
Add enough soil to a container of water to make a muddy solution. Divide this in halfone will be treated and one will serve as a control. Place a piece of red litmus paper in one container. Add lime solution, a dropperful at a time, stirring after each addition until the paper turns blue. Add a dropperful of alum solution to the container. What do you observe? Continue adding alum solution until a thick, white gelatinous precipitate forms. Stir the solution thoroughly and observe the two containers side-by-side for several minutes.
The addition of lime solution, Ca(OH)2, produced a basic solution. Aluminum ions reacted with this solution to produce aluminum hydroxide. This compound has a low solubility, so it forms a gelatinous precipitate that settles to the bottom, taking with it much of the suspended matter, leaving clear water.
Al3+(aq) + 3OH-(aq) > Al(OH)3(s)
This material can be flushed down the sink.