Chemistry Department | Guided Inquiry-Model

Organization of a Discovery Exercise.

At the College of the Holy Cross a series of one-week modules introduce the fundamentals of chemistry. Each module begins with a discovery exercise that consists of a mix between discussion and experimentation. A typical Discovery exercise contains three parts.

First there is a prelaboratory discussion. Here the exercise is introduced in the form of a question. While the students are given considerable freedom in making predictions, offering hypotheses, and ultimately in suggesting experiments, the instructor exerts a subtle but important control over the discussion. His or her goal is to encourage students to draw upon earlier lectures and exercises; to promote good experimental design; to ensure that all students are vested in the outcome of the experiment; and perhaps most importantly, to steer the disucssion toward the preplanned experiment.

The data collection phase of the exercise is characterized by collaborative efforts. To establish an information base extensive enough to reveal the desired trends which in turn allow the discovery of a principle, each student must contribute unique data to the class pool. Experiments are often designed so that each student or small groups of students can carry out an individualized modification of the general experimental procedure.

At a post-laboratory Discovery session the students examine the pooled data and look for trends. These data and the associated trends are used to answer the prelaboratory questions and, ideally, to support a series of related discoveries. The instructor plays an important role in this aspect of the exercise. Students are introduced to the carefully structured way of thinking and evaluating data that suppports scientific creativity. Occasionally, the post-laboratory discussion will call for a short-follow-up experimentation session. More often, it will serve as the basis for further presentations in the lecture portion of the course.

Representative Exercise.

The Discovery exercise, "Trends in the Mass of a Collection of Pennies," illustrates many aspects of the program. In addition to introducing students to the scientific method, this laboratory-based exercise exposes them to several important processes and fundamental principles of the discipline.

Consistent with good experimental design, we try to initiate Discovery Exercises with well-defind questions. Ideally, the question should draw upon prior experiences of the students. Since the Pennies Exercise is designed for use early in the first course, it is difficult to rely upon a common base of chemical knowledge. Consequently, the focal question for this initial exercise is the seemingly non chemical question of what happens to the mass of a penny as it ages. Students have the experiential basis to offer an informed hypothesis on the question. Some speculate that the effect of "wear and tear" will cause the pennies to gradually lose mass. Others are confident that through corrosion and accumulation of grime pennies gain mass as they age. Students are prompted to settle the debate by collecting experimental data. They readily devise an appropriate experiment based on measuring the mass of several hundred pennies selected to represent different years of minting. Students are asked to consider the best way to visualize trends in the data. When they decide on a graph of mass as a function of year they are asked to predict the expected shape of the curve on the basis of whichever hypothesis they favored. This step --relating the possible outcome of the experiment of their predictions -- stimulates the students' interest during the subsequent data collection stage.

At this point each student has a vested interest in the outcome of the experiment and has thought about how the results might be interpreted. They are ready to enter the laboratory and weigh pennies. Rather than taking enough data to discern the trend, each student weighs ten pennies and contributes these data to an overall class pool. The group data are entered into a computer graphing program and a plot of mass as a function of year of minting is presented on an LCD system and overhead projector.

The trend in mass over time is not clear to each student from the ten points that they individually accumulate. However, a graph of the combined data clearly reveals an unexpected drop in mass in 1982 when Zn replaced Cu as the major component in pennies. In addition, for any given year, students observe a considerable range in penny mass. Unexpected results like these stimulate student discussion and sometimes can serve as the focus for further experimentation. In this case, students speculate that the dramatic decrease in mass resulted from either a change in amount of material used or a change in the composition of the material. Their discussion of how to distinguish between these hypotheses leads to the introduction of the concepts of intensive and extensive properties of matter. In general, one of the students will suggest measuring the intensive property density as a way to check for a change in composition.

After the instructor discusses experimental procedures, the students return to the lab and work cooperatively to obtain mass-volume data. Each student contributes a single point that is used in the preparation of a graph of mass as a function of volume for both old and new pennies. The instructor then discusses extracting information from the graph. The slope, for example, represents the desired density. The concept of relative versus absolute error and the propagation of error through an experiment are also readily illustrated from the data. The difference in slopes (densities) for new and old pennies verifies the hypothesis that the composition of pennies was changed in 1982. Students are often interested in returning to the original graph of mass versus year to discuss the causes for the non-uniformity in penny mass for a given year. They identify the inherent uncertainty of a balance, inconsistencies between balances, uneven degree of circulation, differences in location of minting, and the inherent uncertainity in the manufacturing process as worthy of consideration. Thus, in a single exercise students experience the process of forming, testing and refining a hypothesis. They also learn basic techniques of data analysis. Finally, they build an experiential basis for understanding several fundamental concepts.