Roger S. Macomber University of Cincinnati Cincinnati, Ohio 45221
A "Relevant" First Experiment for Freshman Chemistry Laboratory
It is fairly rare to find experiments for freshman chemistry laboratory v.hich combine all of the following desirable attributes: illustrative of one or more fundamental scientific principles or techniques, exemplary of the scientific method, readily understood by the student, and relatively simple and inexpensive to perform. We would like to offer the following experiment, which fulfills all of these criteria, for use as a "first-day" freshman experiment. It introduces the student to the processes of observation and inference, to the use of an analytical balance and gravimetric procedure, to a simple statistical analysis of experimental data, and moreover it is an enjoyable one to perform. Additionally the required apparatus is of the type ordinarily found in the freshman laboratory. The focus of the experiment is a very common experiencethe popping of corn. Each student (or small group) selects a sample of popcorn kernels. The kernels are individually weighed, popped, and reweighed to determine the mass lost during the process. Careful observations are made of the corn popping, and deductions made about the cause. The average mass of unpopped kernels and the standard deviation are calculated, and these values are used to estimatc the number of kernels in a bulk sample. Finally the mass loss, expressed as a percentage of the initial mass, is calculated for each kernel, then averaged and the standard deviation computed. The entire experiment is flexible and allows the instructor to stress those areas he deems especially important. The aesthetic advantages of the procedure are obvious: the laboratory is filled with a not-unpleasant odor and the students may eat their results!
burner. Using a few of the unweighed kernels the gronp develops its popping technique to minimize scorching of the corn. The most uniform results are obtained by preheating the flask, adding the kernel, then resuming gentle heating and agitation until the kernel flowers. The flask is removed from the heat and the flower carefully returned to the spot plate. Momentary agitation directly after popping prevents the flower from being scorched. Once perfected, the procedure is extended to the weighed kernels, one by one. The behavior of each kernel before and during the popping process is carefully noted, as is the appearance of any condensate near the top of the flask. If a fresh bag of popcorn is used, unexploded kernels and fragmentation during popping are rare; incompletely exploded or badly scorched kernels can be excluded from subsequent calculations. The popped kernels are then weighed and their masscs recorded in the table. Hygroscopicity of the flower does not appear to be a problem. The entire experiment can be carried out in an hour; calculations can be assigned as homework. The mass lost by each kernel is calculated, then expressed as a percentage of the kernel's initial mass. For both the initial masses and the percentagc mass losses, the mean value (5) and the sample standard deviation (u) are calculated in the normal manner
* = -u = "?
where n = number of observations. The significance of u in terms of thc reliability of experimental measurements can be explained, e.g., that Z * 2u represents Sample Data
Experimental
Prior to the actual experiment the students are instructed in the proper use of the local analytical balances. Depending on the class size, time available, and number of balances, groups can be established as necessary. A fresh hag of popping corn (its net weight noted) is opened and each group is given ca. 20 kernels. All handling of individual kernels should be carried out with tweezers to avoid moisture on the hands being transferred to them. The group then randomly selects -15 of its kernels and weighs them individually, recording their masses in tabular forms to the nearest 0.0001 g (or the balance limit). A convenient device to keep the kernels separated during the experiment is a porcelain spot plate. The reaction vessel consists of a loosely corked 250-ml Erlenmeyer flask, which is agitated by means of a test tube holder above an asbestos-centered wire gauze. Heat is supplied by a cool flame of a bunsen 714
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Journol of Chemiml Education
12
kernel
mass before, g
mass after, g
% mass
mass loss, g
lost
*
*
95y0 confidence 0.1089 9:3 2.0% interval 0.0390 g (u/f) X 100% 17.9% 10.470 net we~ght(contents) of package = 906 g (2.00 lb) No. kernels 906 g (95% confidence) = 0,1089 0.0390 glkernel 8320 2980 kernels
*
+
a 95% confidence interval. A comparison is then made of the relative magnitudes of +he standard deviations (u/5)for the initial mass versus that for the percentage mass loss. The latter is seen to bc significantly smaller. Based on the average mass of an unpopped kernel and thc net weight of the bag of popcorn (listed on the bag) an estimate of the total number of kernels in the unopened bag is made. Following the calculations, questions of the following type can be posed to the students.
(1) Describe in detail your observations of the corn popping. Do not confuse explanations with observations.
(2) What do you suppose is the nature of the mass lost? What evidence can you offerto mpport your answer? (3) What can you infer about how (why) corn pops when hertted? (4) Is it t o be expected that the relative magnitude of s of the initial mass will always exceed that of t,he percentsge mass lost? Why? (5) Is it valid to delete scorched or incompletely popped kernels from the calculations? Why?
Volume 49, Number 10, October 1972
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