A CHEMISTRY CLASS CONSTRUCTS a LABORATORY APPARATUS WALTER V. BURG
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University of Toledo, Toledo, Ohio
Exercises that engage a class in the construction of apfwratus for labmatory experiments can be arranged so as to constitute a valuable addition to the customary pogramfor courses in ekmentary general chemistry.
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OST of the laboratory manuals for elementary general chemistry contain rather comprehensive directions as to the setting up of apparatus for the various student experiments. Such an arrangement, although generally welcomed by the instructor as a decided help in his work, bears the danger that throughout the entire course the students may be kept from using their own ingenuity in combating even
minor technical difficulties, This would be especially regrettable with regard to those students who are mainly interested in theoretical problems and to whom the chemistry course could offer a valuable opportunity for developing a more favorable attitude toward the practical side of things. As a means to avoid those disadvantages the author wishes to suggest adding to the laboratory program a few exercises that engage the class in a technical problem, such as the construction of an apparatus to be used by the students for their chemical experiments. Although exercises of this kind generally require more painstaking preparations than the customary laboratory experiments, their high educational value fully justifies the increased effort on the part of the instructor. This applies particularly to the selecting of the object, which has to be done with
more than the usual care in order to assure a sufficient degree of difficulty and a t the same time enable the students to accomplish their task with a minimum of help. In the following lines the successive steps are de-. scribed by which the students of a first-year chemistry course-under the guidance of their instructor-carried out an exercise of the kind mentioned above. The exercise was given in the early part of the course after the chapter on oxygen had been concluded and the study of hydrogen had been started. The students had just learned of the various methods for preparing this element and were also familiar with its combustibility and the explosive character of its mixtures with oxygen or air. Now the study of hydrogen was to be continued, which made the use of a convenient gas generator very desirable. Thus the topic of the next exercise suggested itself. It was formulated as follows: to construct a small but efficientlaboratory apparatus for the preparation of hydrogen. At the outset the students were confronted with an important question; which of the methods for generating hydrogen should they choose for their purpose? Briefly the class reviewed the different possibilities. Neither the reaction between sodium and water nor the decomposition of steam by heated coke or iron was found to be suitable, the former on account of the hazards involved, the latter because of the complicated apparatus required. On the other hand, the interaction between certain metals and acids offered a harmless as well as convenient way of preparing the gas. Therefore, i t was decided to design the generators for the use of this method. The reacting substances were selected by the instructor; they were zinc and sulfuric acid. Now the students turned to their actual jobs, the construction of the generator. What kind of container would serve best as the reaction vessel for the two substances? The experiments with oxygen (preparation of the gas from potassium chlorate) suggested a testtube supported by means of a damp and a ring stand. However, as several students objected, there was no need in this case for such an arrangement since the reaction did not require application of heat. A flat-bottomed vessel that could be placed on the desk without a supporting device would be entirely su5cient. After b r i d y examining their stock of containers the students decided in favor of wide-mouthed bottles which combined a sturdy construction with a satisfactory capacity. The bottles were provided with one-hole rubber stoppers and delivery tubes.' After each student had arranged his apparatus and connected it with a pneumatic trough, zinc and sulfuric acid were distributed by the instructor. Only small amounts of acid were given out in order to prevent the reaction from becoming too violent. The generators were charged, closed, and the escaping gas was collected in test-tubes over water. As was expected, the first portions of hydrogen were mixed with air and exploded Delivery tubes had already been used for the preparation of oxygen.
slightly when ignited a t a distant flame. After a while all air had been expelled from the generators, as was evidenced by the quiet combustion of the gas. In the meantime, however, the reaction between zinc and acid had slowed down perceptibly. What could be the cause of this change? The bottles were opened and their contents examined. The students found that there was still enough zinc but that most of the acid had been used up. It became clear that, in order to obtain a continuous evolution of gas, fresh acid had to be added a t frequent intervals. But there a difficulty arose since each time the stoppers were lifted from the bottles, the hydrogen was again contaminated with air. The students saw that their construction had to be modified so as to permit the addition of acid without opening of the generators. Therefore, it was decided to replace the one-hole stoppers by two-hole stoppers, each carrying a delivery tube and a funnele for introducing the acid. However, the new model proved quite unsatisfactory since the gas now escaped through the funnel and none of i t passed to the pneumatic trough. After deliberating briefly the class found a workable solution. The stems of the funnels were connected with short glass tubes by means of rubber tubing and the funnels thus extended into the liquid a t the bottom of the containers. Now the generators seemed to be perfect. As soon as the gas evolution slowed down, fresh acid was added through the funnels and the reaction sped up again. Although most of the students appeared to be well satisfied with their models, a few of them voiced the opinion that the apparatus could be greatly improved by pro$ding some device5 for regulating the gas flow. As the generators were now constructed, there was no means to shut off the steam of hydrogen wben the gas was not needed. Several proposals were made and discussed. The one finally agreed upon was to cut the delivery tube into two,parts and to join the latter by means of a short piece of rubber tubing that carried a screw clamp. The changes were quickly performed and their effectiveness tested. However, the result was not quite what had been expected. Each time the gas flow was stopped by operating the clamp, part of the acid rose in the stem of the funnel. The phenomenon was studied and soon explained. The hydrogen, being prevented from escaping through the delivery tube, developed a pressure and thus forced the liquid upward into the funnel. Several interesting observations were made by the class. Not all of the acid was driven out of the bottle; there was just enough left to touch the end of the stem. From time to time a large gas bubble rose in the latter and burst wben reaching the surface of the liquid that had collected in the funnel. Again the explanation was simple: the acid left a t the bottom of the container was still in contact with zinc and, therefore, the liberation of hydrogen continued. But the effect was not quite the same in the different apparatus. There were a few generators in which the The use of funnels w a s proposed by the students since they had not as yet worked with thistle-tubes.
stems of the funnels extended into the zinc itself, and once more. The bottles were opened again, cleaned, here, as the students observed, the acid did not cover all and dried. Then the plates were prepared and inserted. of the metal and the evolution of gas was much slower. But now an unexpected difficulty seemed to arise. How A lively discussion started which led to an important could the apparatus he charged with zinc without the conclusion: if i t were possible to extend the funnels latter falling through the hole in the plate? It did not below the zinc, all of the acid could be forced away from take long until one of the students found a satisfactory the metal and the reaction hrought to a complete stop. solution. While adding the metal, the funnel had to The class now entered upon the most interesting be held in such a position that the hole was just covered phase of the exercise. Many ideas were advanced, some by the end of the stem. Now the bottles were closed of which showed considerable ingenuity. Thus one and fresh acid introduced through the funnels. Again, student proposed to replace the simple container by a some hydrogen was collected and then the gas stream vessel with a thick bottom, the latter having a narrow shut off by means of the clamps. The generators recavity for the stem of the funnel. This suggestion, sponded a t once. Al.! of the acid was forced away from however, like several others, had to be rejected because the zinc and the gas evolution hrought to a complete the required apparatus was not available. Finally, a stop. The problem was solved and the exercise finished. member of the class pointed out that the generators might he used in their original form if some device of Great was the satisfaction of the students! But now a acid-resisting material could be inserted which would surprise awaited them. Out of one of the cases the keep the zinc above the bottom of the stem. This idea instructor took a big apparatus, the Kipp generator. appealed to the class, and the' requirements were At first the attention of the class was diverted by the studied which such a support had to meet. Several unfamiliar shape, but soon the principle of the apparadevices were proposed, one of them, a perforated cir- tus was recognized. Yet there was no disappointment cular plate, resting upon a cylinder of glass and touch- among the students. They understood the purpose of ing the walls of the generator. But how to insert such the exercise--and they liked their generator much beta plate? How to pass i t through the contracted opening ter than the "big one." of the bottle? The class pondered this question. I t S W Y became evident that a flexible material had to be .used. Since the exercise just described was the iirst of its "How about rubber," a student asked. The idea was examined from all angles. One by one the advaneages kind given to this particular class, its object had been which rubber offered became apparent. It could easily purposely selected so as to present but a moderate debe cut to size and provided with a bole for funnel and gree of difficulty. However, after having once successacid. If the diameter of the plate were made slightly fully completed such a project, the students will be larger than that of the bottle, i t could be forced into eager to match their ingenuity-'against more compliTherefore, it will he of interest that place and would stay there on account of the friction cated produced. After all these points had been thoroughly adequate topics can be found a t almost every stage studied, the class proceeded to rebuild the generators of the general chemistry course.
