The Failed Experiment as a Teaching Aid It has often been observed that when an experiment fails, the participation of the students increases and, as a consequence, they learn more and better.' As a result of this observation, some years ago one of the authors (J.A.C.) started to use planned failed experiments in a course in physical chemistry. A perturbation would be introduced, withnut the students' knowledge, into experiments that were designed to illustrate a given law. The initial shock that was produced by the unexpected results was followed by renewed interest and attempts to explain the discrepancy. Several hypotheses usually were presented, and they were warmly discussed in a scientific way. Finally, the most probable one would be selected and tested by additional experiments. Often the perturbation would be detected and the abnormal results explained, even by the law that had heen questioned. More important than the fact that the class was interesting for both the students and the teacher was the initiation of the student into scientific analysis. As an example we describe here a "failed" experiment intended to exemplify Faraday's laws of electrolysis. The perturbation consisted of the use of a diluted (0.3%)KI solution containingonly a slight excess of iodide with respect to the amount of electricity to be passed through thecircuit Thus, the amount of free iodine found by titration with thiosulfate was less by approximately 50% than the amount expected. To determine whether the error was accidental or systematic, the experiment was repeated with approximately the same result. The students ohserved the system carefully and noted the presence of dark particles forming a t the anode. They postulated that the particles were solid iodine and suggested that orohablv it reacted slowlv with thiosulfate. thus causing an early and poorly defined end point. They proposed to test this i y p t h k i s by adding to ihe media some chemical to render more saiuble the solid. he$ determined from the literature that the solubility of I* could he increased by the use of organic solvents or by concentrated KI. The latter presented several advantages over the solvents. The experiment was repeated using a 3% KI solution and the results agreed well with those expected from Faraday's laws. 1 Koons,
L. F., J. CHEM. EDUC., 55,528 (1918).
FurdacHa Universidade de Rio Grande Ria Grande R.S. Rrasil
J u a n A. Coch Frugoni Alvaro Roeha Figueira
Volume 57, Number 3, March 1980 / 179
