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HIGH-SCHOOL CHEMISTRY Crystallization CHARLES H . STONE Vermont Junior College, Montpelier, Vermont
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HEN a dilute solution is allowed to evaporate spontaneously, a point is presently reached a t which the solvent is saturated with the solute. At that point crystallization begins and continues until the solvent is completely removed. Interesting and beautiful results may be obtained in the experiments that follow. I . General Results. Prepare dilute solutions of potassium sulfate, potassium hydrogen sulfate, sodium chloride, lead nitrate, potassium chromate, and other salts. Filter (if necessary) into crystallizing dishes and let stand until the solvent has completely evaporated. Some solutes may develop the creeping effect; a little vaseline rubbed around the upper inside rim of the dish will prevent this. K2SOa gives hard, glassy, diamondshaped crystals; KHSOh yields long white needles; NaCl produces white cubes with a dirk spot (water) in the center of each; Pb(N0s)~gives shapes resembling an equiangular triangle with the vertices cut off on a line a t right angles to the bisector of the angle, the sides being alternately long and short. Others will, of course, yield varying shapes. 2. Crystallization on a Form. Procure some pipe cleaners and cut two of them in halves. Using ordinary sewing thread, tie four ends firmly together, leaving a long thread to be used later. Soak in water. Remove and bend the free ends outward and downward. Suspend this form in a hot saturated solution of potassium dichromate to which potassium sulfate has been added. Support the form by means of the thread to a support above. When the solution is cold, remove the form; i t will be covered with fine red crystals of the dichromate with white crystals of the sulfate here and there, giving the effect of a shrub with red branches bearing white flowers. 3. Copper Formate. Stir powdered copper carbonate into formic acid until no more will dissolve. Filter and allow the filtrate to evaporate. Cubic blue crystals form. Since these are hydrated, they may be preserved in a tightly stoppered vial to which one small drop of water has been added. 4. Cobalt Formate. Proceed as above, using cobalt carbonate. The dry crystals preserved in a tube will glow like live coals when exposed to bright sunlight. 5. Sugar Crystalsin Jellies. Beautiful crystals may
often be noted in glass jars containing jams, jellies, preserved fruits, etc. 6 . Purple Chrome Alum. Stir powdered potassium dichromate into 25 cc. of dilute sulfuric acid in a beaker until the action is complete. Stand the beaker in a cooling mixture and stir in alcohol, using a thermometer, a little a t a time; the odor of formaldehyde will be noted. Keep the temperature below 30% When action is complete, let stand. Eventually beautiful purple crystals of chrome alum will form. Let the sunlight shine through some of them. 7. Ammonium Oxalate. To a hot solution of oxalic acid add ammonium hydroxide until the whole is neutral. When cold, the beaker will contain white needles of ammonium oxalate: 8. Lead Iodide Spangles. Fill a 50-cc. graduated cylinder with hot water to-warm it. In a beaker add a gram or twoof powdered lead iodide to 50 cc. of water near the boiling point. Pour the contents of the cylinder into a filter to warm it. Discard this liquid and pour the contents of the beaker into the filter, catching the filtrate in the warmed cylinder. As the hot liquid cools, brilliant spangles of lead ioilide form. 9. Projecting' Crystal Growth. Lay a clean glass plate across the ring of a stand or a tripod. On the center of the glass place a drop of saturated barium chloride solution. An electric lamp below will warm the glass so crystallization will soon begin. A magnifying mirror may be so placed as tb'project the process onto a screen where the crystallization may be observed by a class. Using another glass, other saturated solutions may be used and the differences in crystal forms noted. 10. Sulfur. The size of a crystal obtained as above depends upon the rate of evaporation of the solvent; the Gower the rate, the larger the crystal. Dissolve a gram or two of powdered roll sulfur in carbon disulfide and filter into a crystallizing dish. Cover the dish with three or four thicknesses of blotting paper and lay a wood block on top. In this way evaporation of the solvent is much delayed since it can take place only through the pores of the blotting paper. After several days the solvent will have completely disappeared, and crystals of sulfur much larger than those obtained by the usual evaporation will be found.
