Name: ________________________________
Using Properties to Identify Materials
Hour: ______ Partners Initials: _____________
Forensic chemists test the physical and chemical properties of materials found at a crime scene.
They also do similar tests on the materials found on a suspect’s skin or clothing. These materials are
often complex mixtures, such as soil, which contains many substances. In this lab, you will compare
the properties of three known materials with two samples of “evidence”. These samples represent
evidence from a crime scene and evidence from a suspect’s shoe. Although your materials and
equipment are less complex than those used by forensic chemists, your overall method will be similar
to the methods they use.
Problem:
Can the properties of materials that appear similar be used to tell them apart.
Materials:
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1 spot plates
Samples 1 - 3
Iodine solution
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Sample from suspect’s shoe
Wash bottle of water
Sample from crime scene
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6 laboratory spatulas
Vinegar
Procedure: Follow the instructions below.
Sample 1:
Use a spatula to place a small amount of Sample 1 in wells A1, A2, and A3. On your data sheet,
record any physical properties of the sample that you observe.
Sample 2:
Use a spatula to place a small amount of Sample 2 in wells A4, A5, and A6. On your data sheet,
record any physical properties of sample you observe.
Sample 3:
Use a spatula to place a small amount of Sample 3 in wells B1, B2, and B3. On your data sheet,
record any physical properties of the sample you observe.
Crime Scene Sample (Sample 4):
Use a spatula to place a small amount of the crime scene sample in wells B4, B5, and B6. On your
data sheet, record any physical properties of the crime scene sample you observe.
Suspect Sample (Sample 5):
Use a spatula to place a small amount of the suspect sample in wells C1, C2, and C3. On your data
sheet, record any physical properties of the suspect sample you observe.
Prediction:
Look at the sample from the crime scene and the sample from the suspect’s shoe. Based on your
observations, predict whether testing will show that the samples are identical or not identical. Do the
crime scene sample and the suspect sample match any of the known materials you observed?
Record your prediction on your data sheet.
Water
Fill wells A1, A4, B1, B4, and C1 with water. Record any changes you observe on your data sheet.
Vinegar
Fill wells A2, A5, B2, B5, and C2 with vinegar. Record any changes you observe on your data sheet.
Iodine Solution
Add one drop of iodine solution to wells A3, A6, B3, B6, and C3. Record any changes you observe.
CAUTION: Iodine solution is corrosive and poisonous. It can stain skin and clothing. Rinse any
iodine spills with water.
Clean Up:
Rinse all materials off the spot plate and flush them down the drain, with at least ten times as much
water. Dispose of your plastic gloves as directed by your teacher. CAUTION: Wash your hands
thoroughly with soap before leaving laboratory.
Analysis and Conclusions:
In the space provided on your data sheet, answering the following questions. Make sure that you use
complete sentences, science vocabulary, and fully explain each answer.
1. Analyzing Data: Were you able to use your observations alone to distinguish between samples?
Were you able to use the ability to dissolve in water to distinguish all three materials? Explain.
2. Drawing Conclusions:
Explain.
Are the samples from the suspect and the crime scene identical?
3. Evaluating and Revising: Did the data you collected support your prediction? Explain your
answer.