SPECIAL EXERCISES for STUDENTS in GENERAL CHEMISTRY* 111. Crysfal Growth
G. BRYANT BACHMAN The Ohio State University, Columbus, Ohio
The purification of soid materials by warious methods familiarity. On the other hand the uninitiated finds in of crystallization is so familiar a process to the average the dmelopment of individual crysta2s a display both chemist that he usually sees only the practical significance interesting and beautiful. This fact can be put to adof his work. His esthetic appreciation of this splendid vanlage in the instruction of students in the first-year phenomenon is, one might say, deadened by too great a laboratory.
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N THIS, the third of a series of special exercises, it seems advisable to call attention once more to the objective in mind. I t was pointed out previously that in order to care properly for students of superior ability the instructor in a general chemistry laboratory must provide extra experiments which are interesting as well as instructive. I t is believed that a successful method of doing this has been described. It consists. namely, in so coloring new material with experiments which per se are interesting that the student will readily tackle the more serious points in his enthusiasm for the seemingly more trivial ones. For example, in this exercise the observation of crystallization taking place and the preparation of a crystal-encrusted ornament provide the interest which leads the student into a consideration of the subject of crystal growth. For the successful development and maintenance of such an interest it is sufficient that something be created which is tangible and also novel, something which can be taken home if desired and exhibited to friends and relatives. A great pride naturally arises in the student from this event and this pride serves to stimulate him to study his subject and to make himself capable of explaining what he has done. Naturally, in order to do this he must digest and learn a t least part of the material his instructor is interested in h a v i n ~him learn.
Although only three exercises have been described to illustrate the objective outlined above, i t will be apparent that a little thought on the part of the laboratory instructor will lead to other wercises illustrating other fields. So long as a t least one part of the experiment consists in the making of something novel to the student, the other parts may be largely confined to more serious material, with the safe assurance that they also will receive greater consideration. THE CRYSTALLINE STATE
A motto of the chemist is that "anything that is crystalline is pure." While this statement is not strictly true, it owes its origin to the fact that a change from a non-crystalline or amorphous form to a crystalline form is usually accompanied by an elimination of impurities in the material a t hand. For this reason methods of obtaining good crystals have been rather carefully studied, and the facts so learned have for a long time been put to practical use. For example, the pure white crystalline structure we associate with ordinary table sugar is so familiar that a conscientious housewife would not think of either purchasing or using a sample of sugar inferior as to color or crystalline form. Nevertheless, the culinary departments of our homes have not always been able to exercise such a careful
was developed the technic of purification and crystallization which has made possible the uniformly highgrade crystals of sucrose we now enjoy. The important points which must be observed in obtaining good crystals may be simply summarized as follows. (1) The material must be sufficiently pure. (2) The rate of growth must be relatively slow and even. Various methods are known for attaining both of these aims although it is in the second that we are more particularly interested here. First-year chemistry students are rarely acquainted with even a few of the procedures whereby slow and even growth of crystals may be accomplished. SLOW EVAPORATION OF A SOLUTION
When a substance is relatively soluble in water good crystals may usually be obtained by exposing the solution to the air to evaporate and a t the same time protecting it from dust, drafts, and sudden changes of temperature. This may be illustrated by placing a few cubic centimeters of a saturated solution of copper sulfate in a watch glass and allowing i t to stand for an hour or so in a protected spot. Beautiful blue, flat plates, all of the same general shape will form, varying in size according to the speed of evaporation. Working with larger volumes of solution and carefully controlling the rate of evaporation so that it remains slow and even i t is possible to prepare very large crystals by this method. SLOW COOLING OF A HOT SATURATED SOLUTION
This is the method which is frequently the most convenient for crystallizing matter. It depends upon the fact that most substances are less soluble in a given liquid a t low temperatures than they are a t high temperatures and that a solution saturated a t one temperature will deposit some of the dissolved material of its own accord when cooled to a lower temperature. Here again the temperature must be lowered slowly if large crystals are to be obtained. A beaker of hot water
nearly saturated with potassium dichromate, if allowed to stand undisturbed until cool, will deposit fairly large crystals, but a portion of the hot solution if cooled quickly by shaking in a test-tube under running water will deposit only a fine powder which bears little formal resemblance to the larger crystals. Hot, saturated solutions of some substances do not deposit their dissolved material on cooling as rapidly as it seems they should. In the case of a compound such as sodium thiosulfate the hot saturated solution may even be cooled to room temperatures without depositing a single crystal. When this occurs the liquid is obviously holding more material in solution than it is capable of dissolving a t the lower temperature and hence the solution may be said to be sufiersaturated. So rapidly does precipitation occur upon addition of a crystal of the solute that large crystals do not have time to form, and usually only very small ones result. Evidently, then, it is important to avoid supersaturation when trying to grow large, well-formed crystals. Smgularly enough crystals will start to form in a solution not only on a crystal of the material in solution but frequently on fine particles of some foreign material also. The hairlike projections of an ordinary pipe cleaner furnish admirable starting points for crystallme growth, most of the material crystallizing on the pipe cleaner and literally covering it with an incrustation of well-formed crystals. Many beautiful forms may be obtained in this manner if one is clever a t designing. The accompanying photograph shows the result of allowing some of these pipe-cleaner designs to act as crystallization centers in a hot, saturated solution of potassium dichromate. The board shown in the photograph served as a designing board With its aid all sorts of ornamental figures may be formed from two or more pipe cleaners wired together. The picture cannot of course transmit the beautiful orange-red color of the crystals satisfactorily. SLOW REACTION BETWEEN A SOLUTION AND A SLIGHTLY SOLUBLE SUBSTANCE
A product formed as the result of a slow reaction will have ample time to form good crystals. One method of realizing such a slow reaction is to select a solvent in which one reactant is soluble while the other is but slightly soluble. As a well-known example of this method one might mention the fact that a strip of metallic zinc if hung in a ten per cent. lead acetate solution will gradually go into solution, and, as a result of its position in the electrochemical series, displace the lead from solution. A magnificently beautiful lead crystal aggregate known as a "lead tree" will form and hang suspended from the zinc strip. Similarly, a '
