A Lesson in Trace Evidence
Trace evidence can be a challenging unit to teach high school students with Early Onset
Senioritis. The microscope work can be tedious and students' attention to detail wanes easily.
However, I know that my students come to class thinking that dirt is dirt and sand is sand; they
fully believe that there is little difference between samples or that telling the differences involves
intricate analysis that is beyond their comprehension.
"Murder on the Beach" Laboratory
The Murder on the Beach lab is relatively
easy to prepare, uses tests and data beyond microscope work, and includes a mystery for students
to solve. In part one of the lab, the students review known samples of sand – those that were
taken from the suspects. I usually give the students about 50 minutes for this part. Part two
requires the students to analyze a sample of sand found on the victim and to identify which of the
suspects committed the crime.
Collecting and Preparing Sand Samples
If you live near a beach, an initial sand sample is easy to obtain. The rest can come from just
about any pet or aquarium store – several varieties are available for reptiles and fish. Most of
these samples are going to be a relatively neutral pH, so I treat the samples ahead of time to give
the students some measurable differences. I generally start with three or four very different
samples, then make combinations to create more samples, giving me a total of seven different
samples for the lab.
For the second part of the lab, I prepare enough samples so that every student gets his or her
own victim sample to test. This way, students can work together to share observations, but
everyone has to come up with his or her own final answer.
Prepare Once and Assign for Several Years
The preparation can be a bit time
consuming, but I only have to do it once every few years, as students only use a small amount of
the victim samples for testing. I use old 35 mm film canisters, which I' ve labeled with numbers
one through 28. I fill each canister with different samples and create a master list of which
sample is in which canister. Some of the " pure" samples are fairly obvious, visually, so I use
mostly mixed samples for this portion.
Distributing Samples to Students
I always make sure to have all students complete part one
before I give out samples for part two, just to ensure that students work off their own notes and
not a direct side-by-side comparison. I find that students make more thorough, complete
observations if they know the exemplars won' t be available.
Store Everything in a Box for Next Year
One of the reasons I enjoy this lab is because it' s only a lot of work the first time – after that I
keep everything in a box and it' s ready to go for the next year. The information the students need
is all contained in the lab, so I can plan it for anywhere in the unit without worrying about time to
introduce it or doing notes ahead of time. It' s self-contained enough that they walk away with a
lot of vocabulary that I never had to directly teach them.
Students Are Surprised by the Acid and UV Tests
Here are the handouts used in my classroom. Feel free to print and use them with your students.
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This lab has been a hit with students, too. They don' t expect sand to effervesce in acid, and
when portions of some of the samples glow under UV light students respond with the
requisite " ooh" s and " aah" s.
They' re always more engaged when they have a mystery to solve,
so I have no problems keeping them on task.
Most importantly, my students leave the lab
knowing that dirt isn' t just dirt and sand isn' t just sand, and that both can be valuable pieces of
evidence. This lab is a must-have in any trace evidence unit.
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| Sand is found in many different "environments". The color, composition and properties of the sand will be different for each environment. Sand from a tropical beach might be made up almost exclusively of sand-size shell and coral debris. Sand from a scrubby desert environment might have frosted grains and be contaminated with particles of plant debris. Sand from a stream might consist of rounded grains and its compositiion will reflect the soils and bedrock from upstream locations in the drainage basin. Manufactured sand will be very different. It is made by crushing rocks and the particles will be very angular. Images © iStockphoto and clockwise from top left: Alberto Pomares, Vlynder, AdShooter, snokid. |
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The Forensic Teacher Magazine
This lesson on "The Sands of Crime" was published in the Summer, 2009 issue of The Forensic Teacher Magazine. Visit their website for more ideas about teaching forensic science. |
| When students begin to examine sand the first difference between samples that they might notice is color. Then without using a microscope they can make observations of texture, grain size, and grain shape. Under the microscope they will detect differences in mineral composition, the presence of organic materials and the luster or diaphaneity of mineral materials. Images © iStockphoto and clockwise from top left: Rusm, Don Wilkie, Olena Pantiukh, Vivian Seefeld. |
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