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Which laboratory activity best simulates the first-order kinetics of radioactive decay?

Article Excerpt
ABSTRACT--A review of the literature indicates that many different types of activities have been used in undergraduate science laboratories to illustrate the concept of half-life. The choice of a specific activity will depend on the desired educational outcomes, the sophistication of the students, and the cost of materials. In this paper, I compare and contrast four half-life experiments that have been used in undergraduate science laboratory and lecture courses. The experiment giving the best verification results, based on agreement between experimentally determined values and theoretical values, was the decreasing volume model.

In undergraduate chemistry or general education science courses, students commonly perform an experiment investigating radioactive decay. The experiment usually involves a relatively simple, single decay process, such as [.sup.14]C[right arrow][.sup.14]N+[[beta].sup.-] and illustrates the important concepts of reaction kinetics and half-life. For legal and financial reasons, undergraduate students will probably not use radioactive isotopes. Instead, an experiment simulating the mathematical characteristics of radioactive decay will often be substituted.

A search of the Journal of Chemical Education online index for the topic "Kinetics" produced 639 papers. These papers included chemical experiments to measure kinetics in the undergraduate laboratory, half-life experiments using various radioactive isotopes, computer programs to simulate kinetics, calculation methods, classroom activities, and various physical simulations.

Some of the physical simulations that obey first-order kinetics include the flow of gas through an orifice (Coffin, 1948; Kahn, 1957) or the flow of liquids through capillaries (Lemlich, 1954; Davenport, 1975; Erwin, 1992). Other methods take a games approach (Harsch, 1984) and include such activities as flipping coins (Sanger et al., 2002; Sanger, 2003) or dice shaking (Schultz, 1997). Others involve the transfer of water using dippers of various shapes (Burk and Gunter, 1977), or the cooling of hot water (Birk, 1976).

With all of these choices, one is bound to ask "Which is the best simulation for my students?" The answer to this question depends on several factors. Is the experiment intended as a simple verification experiment, or is it intended to be a guided discovery experiment? How sophisticated are the students conducting the experiment? How complicated is the experimental procedure? Does the experiment require extensive preparation and expensive materials?

All of the above simulations use relatively inexpensive materials. The experimental procedures given are clear and straightforward. Neither the preparation time for the experiment, nor the laboratory time required to conduct the experiment are excessive. In selecting the simulation to be used, one important choice for the instructor is "Will the students have to measure elapsed time?" In many of the simulations described above, such as flipping coins, shaking dice, or dipping water, iteration can be substituted for elapsed time. For students having little practical laboratory experience, this may be a desirable simplification.

Another critically important question for the instructor is "Is this a simple verification experiment or a guided discovery experiment?" Verification experiments are important exercises, allowing students to develop and gain confidence in their laboratory skills. Guided discovery experiments are significantly different from verification experiments, providing students the opportunity to experience the "full" scientific process: observation, hypothesis, experiment to test hypothesis, revision of hypothesis. A verification experiment may not...

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