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ROBERTREEVES Marlborough School 107 South Wllson Orlve LOJAngeles. CA 90004
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Focus on Forensic Experiments Kern 0. Berry University of Puget Sound, Tacoma, WA 98416
Applications of chemistry to the solution of real problems provide a source of interest for students a t all levels. Many teachers search for examples, demonstrations or illustrations in which chemistry plays an important role. This article will provide a number of applications of chemistry to problems encountered in the forensic science laboratory. For a complete discussion of forensic science and its place in the curriculum, see the Symposium on Forensic Chemistry published in this issue (1).
Figure 1. Three otthe passibleorientations forthe "2." "3." and "6" sidesof a Common die. These are the only sides that lack a center of symmetry.
An Experlrnent in Observallon and Logic
I t is useful to help students become good ohservers and thereby be able to recognize differences between systems and subtle changes that occur as events proceed. Piaget has taught that students are a t first concrete thinkers. They need to understand science in terms of things they can experience through touch, sight, or other sensory inputs (2-7). One concept that students can become familiar with by observing some common objects is the concept of symmetry. In doing so, the student can develop his or her powers of observation. A Unique Characteristic of Dice
In this experiment, the student will examine a set of dice. Each die has a series of dots representing a number on each side of the cube. Some of those dots are arranged in a very symmetrical manner. For instance, the single dot of the "1" lies at the center of the square face of the cubic die. The four dots of the "4" lie essentially at the corners of a square. The dots on the "5" are a combination of those two, having dots a t the corners of the square and also a t the center of that imaginary square. Each of these exhibits a high degree of symmetry.
1060
Journal of Chemical Education
The other faces have their marks oriented in some special arrangement. The three dots of the "3" all lie along one of the diagonals for that face. Since there are two such face diagonals, there are two possible arrangements for those dots (see Fig. 1).The same can be said about the "2" and the "6". Although the sum of the values on the opposite sides of a die must always be seven, manufactured dice use a limited number of spatial orientations and it is possible t o make dice with different patterns than are ordinarily found. For instance, if one imagined exchanging the "3"and "4" sides on a die, several new possibilities are generated, one of which are exactly like the first. The examination of a set of dice made from wood blocks or styrofoam cubes provides an excellent exercise in helping students make careful observations, develop hypothesis for problem solving, and provides a rapid means by which those hypotheses can be tested. Procedure
Construct a set of dice from wood blocka or Styrofoam cubes. Wood blocks produced from 2 X 2-in, boards, sanded, and finished
Fiaure 2. Targets representing the four possible combinations of precision and accuracy: A, gaod precision, good accuracy: 8, poor preclsion. gaod accuracy: C. g o d precision, poor accuracy: D, poor precision, poor accuracy
are ideal. The dots may he painted or gummed circles obtained from an office supply store may he placed where appropriate to represent the dots. Mark eaeh die with an identification number. This number should not be considered when making comparisons. Students observe the dice. notine the orientation of sides on the hlock. and the orientation of numbers on a side. The ohiect. and onlv ~~~.~ direction to the students, is to determine which two dice aie exactli identical in all of their details, including orientation of sides and orientation of numbers on a side. ~~~
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Notes Styrofoam cuhpr sometimes haw chinks. holes. or other nirkr in the surfacr. 911 he sure to let ~tudrncsknow they are tocompnre only the "dicey" properties when comparing the objects, and not the properties ofthe materials from which the dice are made. Those students who have the quickest recognition of the problem or are ahle to recognize the symmetry properties most easily will almost always start by having the "2" and "3" sides on the top and front as the dice lie on the lah table. Thus, they are ahle to see two of the three unique sides at the same time. By placing the dice so that they can see the "6"side also, they can quickly visualize the orientation differences of all three of the unique sides at the same time. Once students have been cautioned to make careful observations, they sometimes become too discriminating; they are ahle to find subtle differences where none were intended. This has been a very good experiment bywhich toget students to begin toevaluate which ohservations are imoartant and which are trivial. It offers the advantage l s Further, rhr mnterials are prrmanmr, inexpensive, and rQquirt little prrpararicm h ~ f o reach ~ . use and little cleanup rifort following each use. ~~
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Gunshot Patterns As with many other experiments, graphic examples may he desirable t o illustrate t h e concents of accuracv. orecision. - .. and experimental error. This demonstration or experiment offers a simole.. ranid. . . inexnensive. and somewhat dramatic example of t h e applicationbf those ideas in the context of a crime laboratory examination. It also may serve t o alert students to recognize possible sources of experimental error in their own experiments, n o matter how carefully they work. Procedure For thisexprrimrnr. onrshould have a t leait fuurshooting IaIgQtS preicrahlv \\ithsr~rralertmsrorerveas"unknowns." l'argrrs may he purchased at stores sellingguns and ammunition, hut may also be ohtainedfrom shootingcluhs.) The four basic targets (Fig. 2) should serve as examples of: Good precision and guud accuracy Poor prerisim but gwld acrurnr).. 31 Good prec~iimbut poor nrrurncy. 4) Poor precision and poor accuracy 1) 21
Students should know that each target the results of a .. reoresents . diitrrent individual using his own gun and shunting on a standard range under condition4ns similar to rhoit. exprrienrrd by fhr orher ~ shoutrrs as pomhle. It is important I
