Good examples of such discourse are provided in this issue. Parsons (1) suggests that an earlier paper on determining the volume fraction of oxygen in air was in error. He also provides calculations and arguments in support of his hypothesis regarding what really happens. Birk and Lawson (2) describe the long history of the idea that a burning candle in a closed container will not be extinguished until all the oxygen has been used up. Many published experiments for determining the fraction of oxygen in air depend on this hypothesis. Birk and Lawson's results, however, contradict it. A mouse confined with the burning candle remains active and unharmed after the candle goes out, showing no signs of oxygen deprivation. Quantitative measurement of the change in volume of the gas inside the container confirms that only part of the oxygen is consumed before the candle goes out.

It is quite clear that we teachers can fool ourselves into thinking we have demonstrated a scientific principle or fact when we have not. Often we show students a captivating visual display of a phenomenon, but interpret it incorrectly. Another of the many examples that have appeared in these pages involves diffusion. Graham's law is usually derived and related to the kinetic-molecular theory based on the speeds of molecules. It was shown some time ago in JCE that molecular speeds are related to rates of effusion of gases into a vacuum through an orifice in a thin barrier (3). A different derivation is required (but a similar result is obtained) when gases at equal pressure diffuse into one another through a porous medium. Still another derivation is required (and a different result is obtained) when two gases diffuse into a third gas from opposite ends of a constant-volume container.

Graham's law is typically demonstrated either by diffusion through a porous frit or by diffusion of HCl and NH₃ into air from opposite ends of a glass tube. Neither of these involves experimental conditions that satisfy the assumptions of the derivation presented in most texts. Also misleading are demonstrations in which perfume released in one place becomes detectable throughout a room, or a crystal of KMnO₄ dissolves and disperses to form a uniformly colored aqueous solution. Unless special precautions are taken, dispersion of a substance into a fluid depends more on convection than on diffusion (4). In one case a study of student misconceptions about diffusion was based on the researcher's misconception that dispersion of a dye in water during a period of only a few minutes was an illustration of diffusion (5).

If we can convince ourselves that we have accurately determined an expected result or demonstrated a principle, even though the experiment or demonstration should not give that result, then most students are also likely to be convinced. It is important that they learn that skepticism and courteous, rational discourse are important components of scientific progress. Persistent misconceptions such as the two described above provide a golden opportunity to involve students in such discourse.

We could, for example, demonstrate both a method that works and one that does not, compare results, and ask students to suggest additional experiments that might resolve the issue. (Steel wool and 0.25 M acetic acid can be used to achieve a reproducible and reasonably accurate determination of the fraction of oxygen in air [6]. Davis [4] reports that rates of diffusion in an agar gel, which minimizes convection, are essentially the same as in water, which provides a way of showing how slow diffusion really is.) Or we could ask students to make careful observations as an experiment is being carried out and then decide whether the proposed interpretation was correct. (If the only effect of burning a candle in a beaker inverted in a water bath is to use up the oxygen, then the water should rise slowly and steadily into the beaker as long as the candle burns; it does not.)

Getting the right answer is not nearly as important as getting an answer right- exploring and experimenting to eliminate alternative hypotheses and finding the best-supported explanation. Diffusion and the fraction of oxygen in air can be studied with simple, inexpensive equipment, and it is easy for students to experiment with them. If we use them appropriately, these two subjects have great potential for enhancing students' skills in critical thinking and experimental design. Many other phenomena reported in these pages provide similar opportunities. Let's apply our ingenuity and effort to making the most of them.

Literature Cited

1. Parsons, L. J. Chem. Educ. 1999, 76, 898.

2. Birk, J. P.; Lawson, A. E. J. Chem. Educ. 1999, 76, 914.

3. Mason, E. A.; Kronstadt, B. J. Chem. Educ. 1967, 44, 740. Kirk, A. D. J. Chem. Educ. 1967, 44, 745.

4. Davis, L. C. J. Chem. Educ. 1996, 73, 824.

5. Westbrook, S.; Marek, E. A. J. Res. Sci. Teach. 1991, 28, 649-660

6. Birk, J. P.; McGrath, L.; Gunter, S. K. J. Chem. Educ. 1981, 58, 804.