Measurement of Eggs A General Chemistry Experiment m o m a s A. Newton University of Southern Maine. Portland, ME 04103 The existence of the term "cookbook chemistry" indicates that many experiments do not require students t o think. This article describes a simple experiment that requires beginning students t o be creative before, during, and after the laboratory. I t involves measuring the mass, dimensions, and volume of an egg, as well as the masses of the shell, white, and yolk. We have used this experiment in our oneyear course in General, Organic, and Biological Chemistry as well as in the General Chemistry course for science majors. Dlscusslon
Each student selects an egg to measure. Most of the eggs are medium sized, but approximately 10%of them are small. They weigh their egg on a triple-beam and an analytical balance. They measure its dimensions with a micrometer and determine its volume by displacement. Finally, they separate theshell, yolk, and white and determine the mass of each part of the egg. Before the lab each student is required to devise his or her own method for separating the three components of their egg. One week in advance they must submit a brief written description of their procedure, inrludine a-~-list thev will need to perform the .. ---- ~of all the ecuiomeut separation. They then execute their procedurd using only the materials from their list. At the end of the day, all the student data is compiled in a class data sheet that they use in their nost-laboratorv exercises. These calculations involve the assessment of significant figures, statistical analysis, percent composition, and, if desired, mathematical modeling. Eggs are sold by weight. Small eggs weigh 18-21 oz per dozen, medium 21-24, and large 24-27 oz per dozen. An average medium egg should weigh 1.9 oz or 53.1 g. Student results confirm these expectations: in a typical sample, the average mass was 53.63 & 2.58 g. Assuming an egg whose mass is more than two standard deviations from the average mass is not a medium-sized egg allows students to identify the small eggs in the class sample. A more valuable feature of this experiment, from our perspective, is the opportunity students have to devise their own procedure t o separate the shell, white, and yolk of their eggs. Advanced planning is an essential component in all experimentation, yet few lab manuals offer students a chance t o design and execute their own procedure. As mentioned earlier, we require each student to submit a written description of his or her procedure, including a list of all equipment he or she will need, one week before performing the experiment. A large majority of students decide to separate the components of their eggs by cracking the shells and pouring the white from one half of the shell to the other, similar to the way a cook would do it. Some students include an egg separator in their equipment list. A minority of each class proposes to hard boil their eggs before they separate the parts. In the laboratory most students, whether they cook their eggs or not, initially perceive this as a superior approach since the separation is cleaner with a cooked egg than i t is with a raw one. However, cooking the egg introduces an indeterminate error: the mass of the egg changes when it is ~
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cooked. This ~ r o v i d e an s excellent opportunity for students to discuss the.merits and demerits oidiffereniexperimental terhnioues and illustrates clearly that there is more than one way to perform an experiment. A third notahle feature of this exercise is that the ralculationof themass percentage of shell, yolk, and white of an egg provides an excellent analogy t o help students understand how to figure the percent composition of a compound. The post-lab problems illustrate this parallel. Another extension, suitable for science major, involves approximating the volume of an egg by taking linear combinations of the volumes of reeular solids. The idea is analogous to formingmolecular orgitals bytaking linear combinations of atomic orbitals. Consider the situation shown in Figure 1. This diagram compares the volumes of two spheres whose diameters are equal to the major and minor axes of a regular ellipsoid, a reasonable model for the shape of an egg. I t is obvious to students from these drawings that the small sphere underestimates the volume of the egg white the large oneoverestimates it. I t is also obvious that averaging the two spherical volumes will give a value that is closer t o the volume of the egg. In fact, the volume calculated in this manner is approximately 20%too high when student data is used. From this result students can deduce that an average that ~
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Flgure 1. The shaded areas represent the projections of two spheres whose diameters equal the major and minor axes of a regular ellipsoid whose projection is shown unshaded.
Figure 2. The shaded areas in (a) are projections of two hemispheres whose diameters are the same as the minor axis of the projection of the ellipsoid lb). The unshsded area in (a) is the projection of a cylinder whose height is the difference between the malor and minor axes of the eiiipsold (b). The superpasition of (b) on (a) gives (c).
weights the contrihution of the small sphere more heavily than that of the larae one will yield a result that approximates the volume oi' the egg more closely. For example, a weighting factor of 3 for the small sphere gives a volume that is within 3%of the experimental value. Figure 2 shows another approximation in which a sphere whose diameter is equal to the minor axis of the egg is
divided into two hemispheres that are then added to the ends of a cvlinder whose height is equal to the maior axis of the egg minus the minor a&. The volume calculated using this model is within 10% of the measured volume. Many other combinations are possible. Experimental details and additional information are available from the author.
Volume 67
Number 7 July 1990
605
