VOL. 4, No. 10
STUDY OP SIMPLE CHEMICAL CHANGES
1289
THE STUDY OF A SIMPLE CHEMICAL CHANGE. A LECTUREDEMONSTRATION FOR BEGINNERS M. BOLANDER, ARSENALTECHNICAL HIGHSCHOOL, INDIANAPOLIS, INDIANA LESTER Beginners in chemistry are often introduced to the idea of a simple chemical change by the study of the effect of heating a strip of copper in the air. They observe the formation of a black crust and, by variation of the conditions under which it is formed, that the presence of air is necessary for the change. They readily understand from their experiments and the reading of the text that the oxygen of the air is responsible for the change, but fail to grasp fully the true nature of it. To aid them in the formulation of a clearer idea of what has happened the following demonstration experiment has been successfully used. A piece of sheet copper with a total surface of about fifteen or twenty square centimeters is carefully counter-balanced on a rather sensitive balance. Since i t is not the purpose to make actual weighings but only to notice weight changes the counter-poise is not disturbed throughout the experiment. The strip is removed and heated until a good coating of oxide is formed and then allowed to cool, care being taken to save all of the crust which has a tendency to peel off. The strip, with any crust which may have fallen off, is put back onto the balance pan and it will be evident that it has increased in weight. This shows that the oxygen of the air has become a part of the blackened strip. The crust is removed and the strip returned to the balance which will show the strip to be lighter than the original piece. This observation shows that the crust contains copper in its make-up. In order to add interest to the experiment and to establish more clearly the fact that the oxide contains copper, it can be reduced, and the freed copper shown as evidence. This experiment has been used with a great deal of success and has never failed to increase the interest of the beginning students in their endeavors to understand the nature of chemical change. This experiment to be most useful should follow some work of the students themselves on the effect of heating of metals in the air. It should not be put into the hands of the student for greater care is needed for its performance than the average beginner is likely to exercise.
