SOME EXPERIMENTS WITH CRYSTALS CHARLESH. STONE,MEMORIAL HIGHSCHOOL,POR BOYS,BOSTON,MASSACNUSETTS
The variety of colors displayed by crystal substances, the different crystal shapes exhibited, and the curious relation between their water of hydration and heat combine to make crystals an interesting subject for experimentation and study. This applies not only to the teacher but also to any able and interested students who may find the time for such work. A few experiments dealing with the above topics follow. Crystal Form and Color Prepare moderately concentrated water solutions (25-50 cc.) of the following: sodium chloride, barium chloride, copper chloride, copper sulfate, lead nitrate, sodium nitrate, ferric ammonium sulfate, potassium chromate, potassium sulfate, potassium hydrogen sulfate, potassium aluminum sulfate, potassium chromium sulfate, zinc sulfate, magnesium sulfate. Any other substances which crystallize well may, of course, be included in the above list. In each case, if the solution is not clear it must be filtered, remembering that substances like copper chloride which dissolve cellulose must be filtered through asbestos. The clear filtrates should then be placed in crystallizing dishes and set aside in a place free from dust. A label bearing the name of the solute should be placed under each dish. (Since crystallizing dishes are rather expensive, a satisfactory substitute may be obtained a t one of the well-known ten-cent stores in the form of plain glass coasters.) After several days a considerable crop of M t a l s should have formed in some of the dishes. The length of time required for this will depend upon the amount of water used in making the solution, upon the temperature of the room, and upon the movement of air over the several dishes. It is better not to allow the liquid in any dish to dry up completely since some of the crystals are efflorescent and if left exposed to air will crumble to powder. When a good crop of crystals has formed in any dish, drain off the liquid, and dry the crystals on blotting or filter paper. If the crystals formed in any case are unsatisfactory, redissolve them in hot water, filter, and recrystallize. In each case note the following. Which of your crystals are: colorless, white, blue, amber, green, yellow, purple, Which of them take the shape of: a cube, a rhombus, a hexagon, etc.? How do you account for the little dark spot in the center of the common salt crystals? Do you note any similar condition in other crystals? In any two crystals of the same substance is the angle between the same two adjacent sides a constant, Which of the crystals show beveled edges' In what crystals is the upper plane surface the reverse of the lower plane surface? Do any two dierent substances crystallize in the same or similar shape? 1107
1108
JOURNAL OF CHEMICAL EDUCATION
JUNE,
1932
Crystal Repair Select as nearly perfect a copper sulfate crystal as you can find and break off a small bit from one of its sharp ends. Drop this mutilated crystal into a saturated solution of copper sulfate a t room temperature. A flat-bottomed dish should be used to hold the solution. After several days observe the following. Has the part broken off been replaced? Has the crystal grown? If so, has it grown uniformly? Effect of Heat In a dry test tube put one or two grams of copper sulfate crystals and heat them over the flame of the Bunsen burner, taking care to hold the tube with closed end slightly higher than the open end. Use very moderate heat, too much heat will spoil the experiment. Note: if water escapes, whether much or little comparatively; any color change. Explain observed result. Is the crystal shape retained when no further color change is seen? Lay the tube to cool across a support with hot end slightly higher than the cold end. Why is this necessary? Proceeding similarly heat a gram or two of copper chloride crystals, using very little heat. Too much heat will cause copper oxide to form. What results do you note as to water and color change? Why should too much heat cause the formation of copper oxide? Lay the tube to cool as in the previous case. In the same way heat a small quanti&y of barium chloride crystals. Does water escape? Is there any color change? Heat carefully a little cobalt chloride in a dry tuhe. How do you explain the color change? Will the residue remain blue if exposed to air? Why? What use can be made of the above fact? Effect of Various Liquids on the Anhydrous Residues Dry out the copper sulfate tube used immediately above and pour out the white solid into the palm of the hand. Powder the substance in the palm, collect it into a little heap and let one dro$ of water fall upon the heap. What color change do you notice' Explain it. What other change do you notice? Explain it. Dry out the copper chloride tube with rolled filter paper and make three portions of the brown powder in three separate tubes. To the first add a little (5 CC.)of carbon tetrachloride, to the second 5 cc. of 957& alcohol, to the third add 5 cc. water. Note the different results. Has carbon tetrachloride any ionizing power, Has alcohol? Has water, Dry out the barium chloride tuhe and pour the powder into the palm of the hand. Make a little heap of the powder and let one drop of water fall
V ~ L 9, . No. 6
SOME EXPERIMENTS WITH CRYSTALS
1109
on it. Is there any color change? Is there any heat change? When plaster of Paris sets does it grow warm? Why? Add water to the blue residue of cobalt chloride. Explain results. Other Properties Heat some of your common salt crystals in a tuhe. How do you explain the crackling noise (decrepitation) ? Will salt which has once been heated crackle if heated again? Do any other of your crystals decrepitate when heated? Why? Which of your various crystals contain no water of hydration? Which of them are' efflorescent? Which deliquescent? Which crystals contain the highest percentage of water?
Purification Pulverize in a clean, dry mortar 5 g. ordinary alum and 0.5 g. copper sulfate. Transfer the mixture to a clean evaporating dish with 25-30 cc. water and heat gently with stirring until complete solution is obtained. Filter into a clean crystallizing dish and set aside. In a few days when a good crop of alum crystals has formed drain off the liquid into another dish and wash the alum crystals with a little cold water to remove adhering mother liquor. Transfer one or two of the crystals to a clean test tuhe and dissolve in hot water. Test the liquid for copper ion with potassi&n ferrocyanide solution. Should any test for copper appear, redissblve the remaining crystals of alum in hot water and recrystallize. A pure.product should be obtained. What is the test for copper ion? The liqnid:n the first dish should not be allowed to evaporate completely. Why? What is the limit to which the evaporation should be allowed to proceed? Can all of the alum be recovered in pure form by this method? Why? What becomes of the copper sulfate?
