SEPTEMBER, 1948
0
SOME REACTIONS USING TEST TUBES WILLIAM G. KESSEL Indiana State Teachers College, Terre Haute, Indiana
IT
IS A rather common practice of teachers to use terminology and ideas that are out of the range of a student's experience, particularly the beginning students. As is suggested by the Chinese proverb, "A picture is worth 10,000 words, little test-tube demonstrations and experiments may help to clarify and crystallize new ideas in the student's mind. For example, we glibly speak of the solvent ability of various solvents and of the variations in solubility of different compounds. This does not mean very much to the average student as he probably never has had an opportunity to observe these facts. A simple demonstration can be given using carbon tetrachloride, water, and ether as solvents, which form three distinct layers: Then drop a few fine crystals of iodine .into the tube. Immediately three distinct differences in color are apparent showing the variation of solubility of iodine in these solvents. This same fact can be satisfactorily shown by numerous other solvent and solute systems. One that works well is carbon tetrachloride, glycerol, and amyl acetate with methyl redcrystals. It takes a longer time, but other factors, such as the effect of density and importance of mixing, are easily demonstrated. One can note the spangled, streamer effect of the methyl red in the glycerol layer. If you use carbon disulfide, water,
JOURNAL OF CHEMICAL EDUCATION
and nitropropane as solvents, with methyl red, another relationship is easily demonstrated. The nitropropane is soluble in the carbon disulfide, somewhat soluble in the water and the water slightly soluble in the nitropropane. It is soon shown that the addition of a small amount of methyl red produces no color with dry carbon disul-
fide, only a slight color with water and noticeably more color with nitropropane. However, if the water and nitropropane are mixed the color becomes much deeper. The same is true with the carbon disulfide and water mixture. But if all three are mixed, the carbon disulfide-nitropropane layer has almost all the methyl red. This little demonstration shows the additive effect of mixed solvents and calls attention to the fact that small amounts of water may change the solvent characteristics of a given solvent. Most of our students in qualitative analysis have become familiar with the solubility of lead chloride in hot water-actually, about 3.34 g. per 100 rnl. in hot water, and only 0.673 in cold water. If this solution is allowed to cool, some entirely different crystals are seen. This is more effectivelyshown with lead iodide which is less soluble, only 0.436 g. in hot water and 0.044 in cold. On cooling some 20 to 30 minutes, a beautiful display of golden crystals results. The use of carbon tetrachloride as a solvent to give a color test for the presence of free iodine or bromine is common. It also is a good example of selective extraction, a process which we frequently speak of, but the student has most likely never seen such an operation. Just calling it to his attention or using it as a demonstration will enable him to visualize' this procedure. Following the suggestion by Dr. Baker on use of scrap and broken glassware, I constructed the conductivity cell shown in Figure 1. We often speak of hydrogenation of some mateterial, but only a few students will ever see or realize how simple this is unless it is shown them early in their work. It is easy to hydrogenate cottonseed or corn oil'with the apparatus shown in Figure 2. The catalyst, Raney nickel, will work very well.
