Why Does Popcorn Pop? An Introduction to the Scientific Method Frederick C. Sauls King's College. Wilkes-Barre. PA 18711 As part of our core curriculum, all nonscience majors are required t o take a two-course sequence in science. The first course introduces the fundamentals of the scientific method, compares the scientific approach to problems with other approaches, examines some of the limitations of science, introduces some of the fundamental laws (e.g., Newton's laws, atomic theory, consewation laws, quantization, relativity), and discusses them in the context of some problems of current interest. I t will surprise no one that this is a challenging course to teach and that examplesfor discussion that are readilv intelligible to the nonscientist are critically important h;t hard t o find. In discussing the "scientific method" of knowledce acquisition, an accessible model is particularly important, as the emphasis must be on the process itself, rather than on factual knowledee that mav be unfamiliar to the audience. I have found the question "why does popcorn pop?" an excellent focus for this discussion. I begin by introducing the Greek geometric model of knowledce, summarized bv the Drocess in Fieure 1. Possible sources of error are improper induction, incorrect abstraction, and poordeducrive rensonina. leadine to invalid laws, incorrect postulates, and erroneo& predlrctions, respectively. The first and last can, in principle, be eliminated with sufficient care; the second cannot. As examples I cite the Aristotelian conclusions that (a) a heavy body will fall faster than a light one and (b) a vacuum cannot exist. Both arise from the postulates that the rate of fall of a body is proportional to the weight and inversely proportional to the resistance. (A vacuum must have zero resistance; a body must therefore fall through a vacuum with infinite velocity-a clear impossibility.) A little prodding of the students can usually elicit the idea of some sort of "reality check" to test the results of this process. They readily agree that a procedure such as that in Figure 2 might have a better chance of producing reliable results. T o implement this method of knowledge acquisition, observations on popcorn popping are demanded from the students. Some hints are generally helpful, hut a list similar to that below is readily produced on the blackboard. Students usually appear surprised that they knew so much about popcorn popping.
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OBSERVATION
I
induction
t LAW
I t
Abstraction
POSTULATES
I
Deduction
1 PREDICTIONS Figure 1. (left) Geometric model of knowledge acquisition.
a. Heat is required h. Size increases c. Noise produced d. Sudden process e. Some do not pop f. Odor noted g. Initial density high h. Final density low i. Shape change j. Tastes better with salt and butter k. Oil often used 1. Kernels acquire kinetic energy m. Water (?) condenses on lid n. Color change o. Hull appears torn, inverted p. Texture changes from hard to soft
These statements may he considered some laws of popcorn popping, a s each is a generalization from experience. This is a good time to send the students home with the
Volume 68
Number 5 May 1991
415
OBSERVATIONS
assignment t o (1)generate a written hypothesis explaining why popcorn pops, (2) think of some testable predictions of their hypotheses, and (3) design experimental tests. Requirine that the hvoothesis he written (a n a r a e r a ~ h otwo1 r helm hienforcing &me discipline to the& thouhht. 1 streks th'at "correctness" of the hypothesis is not what is critical here but that consistency with the observations and testability are. It also helm to uoint out that the hv~othesis need not .. explain every dbservition (e.g., (j)). The students' usual hypotheses can generally he summarized by "steam explodes the kernel". By further questioning, one can elicit some further hypotheses hidden within this: i.e., that the hull must he a tight, tough, and flexible cover, that the stuff inside must be softer, etc. Eventually, a more complete explanation of the process (our hypothesis) is produced: 1. As temperature rises, water inside the kernel becomes steam, 2. Trapped steam exerts an increasing pressure on the hull,
I
Induction
t LAW
I
Abstractron
t HYPOTHESIS Ded~cfron
1
3. The hull tears at its weakest point,
I
. ..
4. Esea~inesteam ex~andsout the hole. a. pushing the wntentr outand fluffing them.
h. invrrting the hull. r. pushing the kvrnel in the opposite direction.
We then discuss which ohservationsmav be ex~lainedon the basis of this model: (a)-(dl, (g)-(i), (11-(m), and (0) fit &ely. I note that we have generated a model of a popcorn kernel without having looked inside and point out that this is how we have acauired our models of atoms and molecules-hv reasoning hackward from their behavior under various sets of conditions. Students will suggest several tests of this model, for example I. Is the condensate water? 11. Does the mass of a kernel decrease after popping? 111. Is the mass of the water lost (recover by trapping) equal to the mass lost by the kernel? (briefly discuss the conservation of mass). How does the odor detected fit into this? IV. Does drilling a hole in the kernel prevent popping? Are the kernels which did not pop cracked or broken? V. Will kernels pop at less than 100 "C? VI. Will they pop in apressure cooker? (Discuss how apressure cooker works, and note that water is added to produce the steam.) VII. Can the popping process he captured by high-speed photography? VIII. Can we cut a kernel in half and see the structure? Finally, as the students think they understand popcorn popping pretty well, I ask what popcorn is. "Seeds of the DoDcorn ~ l a n t .of course." Therefore. there is an embrvo hive inside the kernel, and the stuff inside is food and water to sustain its life. When the students realize that air is also required for the embryo to live, interesting hypotheses arise: (1) Is the air necessary stored inside the kernel? The high density
implies little free air is present, and the known long lives of seeds (months to decades) render it nnlikelv that enough could he present. The required air probably came8 from the atmosuhere. (We note that this reasonine does not dimrove the hyporhexie, but i t isdiscouragmgand ~uggestsasearch for a morr plausible one., 12) Is there a one-waytrap door that allows gases m but not out? Since the net reaction involved is
this implies a buildup of COz inside the kernel and eventual popping without heat. As this phenomenon has, as far as I know, never been observed, we are again encouraged to seek a better hypothesis.
418
Journal of Chemical Education
\
TESTABLE PREDICTIONS Test AGREE
A
[CONFIRMATION ]
Modify DISAGREE
[DISPROOF1
/
Figure 2. (right) Idealized outline d me scientific method
(3) Is there a hole small enough to let oxygen in but not steam out? This sounds attractive until I point out that an oxygen molecule is nearly twice the size of a water molecule. (4) Does heating cause the hole(s) present in the kernel to swell shut, allowing steam pressure to build up? (5) Is there a small hole that allows oxygen to enter slowly hut cannot vent the rapid steam build-up? Hypotheses (4) and (5)are the most attractive. T h e idea of using an experiment to discriminate between competing hypotheses is conveniently introduced here. Gentle nudging can usually get one of the students to suggest a slow heating or holding the kernels a t a temperature just below 100 O C for an extended time before popping. Requiring the students to think of and write out some tests of hypotheses (1)-(3) above is alsoa useful assignment. Alternatively, hypotheses (1)-(5) can be elicited with minimal discussion and thestudents assigned the task of devising some experiments to distinguish among these competing hypotheses. There are several advantages to this example: (1) Virtually all the information required can he drawn out of the audience. It is within the experience of every student, so the
focus is clearly on the scientific method, not the factual content. All haveseen the popping process, yet few have seriously considered it. (2) Scudcntsrnjoy the topic. I hsr,eoverhesrdgroupsnf students in vigorous debate in hallways, cafeterm. eM. (31 It is flexible emugh to he used with a aide range of audiences-I have used it with success for groups from grade school to college students in freshman chemistry. (4) It is fun to lead, as the discussion has never gone the same way twice. It reauires mental aeilitv - . and the abilitv to adant the presentation in midstream. Its obvious spontaneity is appreciated by the students. ( 5 ) It can readily be used as a lead-in to the laboratory to test their predictions.
