SPECIAL EXERCISES for STUDENTS in GENERAL CHEMISTRY I. The
Preparation and Properties of an
Alloy
G. BRYANT BACHMAN The Ohio State University, Columbus, Ohio
The pre@ration of Wood's metal and. the molding of a low-melcing alloy sfioon from this material pr&e stimulating exfieriments for first-year chemistry students who have suficient interest and ability to forge ahead of their classmates i n laboratory work.
T
HE ordinary problems of teaching involve the instruction of students of average ability. The special problems involve the instruction of both gifted and backward students. In a fist-year chemistry laboratory course, especially if the class is a large one, certain factors cause a wide variation in the speed with which the assigned experiments are performed. These factors include previous training in the use of the hands as well as general intelligence. Obviously, it is pedagogically just to set the speed of
this type increase their interest in chemistry and return to them as much or more in knowledge and experience than they invest in time and energy. The desire for knowledge is strong, but the desire for knowledge over and above that acquired by fellow classmates may become an overpowering urge. The study of alloys in elementary chemistry is necessarily limited for the most part to textbook comments. The high temperatures necessary for preparing most alloys and the mechanical devices for measuring their special qualities are usually not available in first-year laboratories. Fortunately, however, there are a few alloys, among them Wood's metal, that are readily prepared and whose outstanding properties are easily observed. Wood's metal may be prepared by melting together in a small casserole or other suitable container 30 g. of bismuth, 15 g. of lead, 7.5 g. of tin, and 7.5 g. of cadmium, stirring constantly with a file to prevent the occurrence of partial oxidation before the formation of the alloy. When cooled this alloy is hard and crystalline in nature and melts a t about 6&70°C. To emphasize this latter property in a striking manner and a t the same time to create a greater interest on the part of the student, the alloy can be cast into the form of a spoon. For this purpose it is necessary to prepare a plaster of Paris mold such as is shown in the accompanying photograph.* Once made this mold will serve for the making of hundreds of spoons if care is used in its handling. It is important not to lay the face of the mold down on the table top as this will soon scratch and mar it. Likewise, the molten metal must not be poured too hot or the plaster will be dehydrated and crumble to a powder. In this laboratory where many of these spoons have been satisfactorily made by fist-year students the following procedure is recommended. The metals are weighed and placed in separate paper bags by the storeroom clerks. These bags, the mold, several bricks to hold down the top of the mold, a small casserole, a metal funnel and a thermometer are placed in a box of suitable size. This box with one or more instruction sheets is passed out to the student or students who are performing the exercise.
the course not on the basis of the rapid students nor on the basis of the slow students, but rather on the basis of the average students who make up by far the majority of the class. Such a plan has, however, its own drawbacks, not the least of which is the atrophy of interest on the part of the brilliant students who are required to repeat exercises, or a t best to "do some more of the same kind" with but few variations in procedure. For such students it is better to have a few exercises of a stimulating nature that will lead *The writer is indebted to Professor John Younger of the them to make use of the surplus time a t their disposal Department of Industrial Engineering of The Ohio State Unifor learning about new subject matter that they will versity for the preparation of the mold shown in the accompanyotherwise not encounter in the course. Ex~erimentsof ing photograph. 241
After the student has melted the four metals together according to the above description he allows the melt to cool until it is solid. While this takes place he examines the mold and carefully fits it together with the pouring hole or "sprue" on top. Bricks are laid on the top of the mold to hold it tightly together and to promote the formation of a more perfect spoon. The funnel is inserted in the sprue. When this has been accomplished the solid alloy is again heated in the casserole until it melts, the thermometer (best encased in a thin-walled glass tube to prevent breakage) is inserted, and the mass heated to a temperature of about 150°C. Experience has shown that this is a good pouring temperature. The liquid alloy is now poured as quickly as possible into the mold through the funnel, allowed to cool for five minutes, and then removed by separating the mold and gently tapping with a file on the sprue. The spoon is broken from the sprue and the rough edges removed with the aid of a sharp penknife or a metal file. The student is told that this spoon will melt if placed in a cup of good hot tea or coffee and the rest is left to his imagination. Needless to say the exercise seldom ends with the making of the spoon. The cost of the materials in each spoon runs about thirty-five cents. This may be defrayed by the institution or the metal remaining after the spoon has been melted may be returned to the storeroom and used again and again by other students. In these laboratories the students were almost always desirous of re-
taining their spoon and allowing the expense to be debited to their own storeroom accounts. In writing up the exercise the student is asked to describe what he has done briefly and concisely. One or more elementary texts containing chapters on alloys and an encyclop~diaare recommended for reading purposes, and several questions about alloys and eutectics are asked. The student is also asked to contribute anything from his own experience or from his readings which he believes to be of interest. I t is especially encouraging to read a report in which a good student has been stimulated sufficiently to delve farther into the subject of alloys than is required of him. Most good students will do this and will often bring to light facts new and interesting to the instructor. One student presented a veritable essay on alloys known and used in the middle ages and before. In any event the otherwise wasted or misdirected energies of the more apt students are applied to the task of acquiring special knowledge in a way that is interesting to them. SELECTED REFERENCES (1) McPmnso~AND
HENDERSON, "An elementary study of chemistry," rev. ed.. Ginn & Co., New York City, 1932, 729 pp., Chap. 32, "Auoys." "A textbook of inorganic chemistry." 3rd ed.. (2) PARTINGTON, The Macmillan Co.. Ltd., London, 1930, 1083 pp., Chap. 38. "Metals and alloys." (3) Any standard encydopgdia may be consulted for information on particular alloys or on alloys in general.
