Name_______________
Crime Scene Challenge
NYPD Forensics Division
Date___________B____
Forensics Challenge: You are a forensic chemist examining evidence that detectives have recovered from a
crime scene. Your job is to determine whether or not the evidence links the suspect to the crime. The suspect
has been accused of a hit and run accident. Forensic investigators were only able to recover a single fragment
of plastic from crash. They believe the fragment may have come from the headlights of the suspect’s Ford
pick-up truck that were broken during the crash. However, the suspect’s lawyer claim the fragment can not
possibly link the suspect to the scene of the crime and can not be allowed as evidence in court. The defense
attorney’s main argument is that the fragment is too small to analyze. Furthermore, the defense attorney
claims that there is no way to tell if the evidence came from a particular vehicle because the properties of the
tiny fragment would be different from the original much heavier headlight. After your forensic analysis you
must provide the detectives, lawyers and judge with a legal report that is scientifically acceptable for use in a
courtroom, but understandable to a jury of citizens.
“n” = repeating
Melting
Point °C
Thermal
Conductivity
W/(m K)
Density
g/cm3
Polyethylene
(C2H4)n
120-130
0.15-0.24
0.94-0.97
Ford
Polystyrene
(C8H8)n
240
0.033
0.97-1.04
Hyundai
Polyvinyl Chloride
(C2H3Cl)n
100-260
0.19
1.3-1.4
Toyota
Polycarbonate
[OC(OC6H4)2CMe2]n
130-171
0.1 – 0.22
1.20-1.22
Car Brand
Name of Plastic Polymer
used in Headlights
Chemical Formula
Mercedes
Materials: water, salt water, graduated cylinders, scale, droppers, rulers, thermometer, beakers, test tubes
Procedure:
1. Fill a vial with _____ mL of salt solution
2. Using tweezers add the evidence fragment
3. Add _____ drops of water using a pipette
4. Carefully mix the solution in test vial. Avoid aerating the solution. (make sure there are no bubbles)
5. Repeat until fragment is suspended and floating in the middle of the solution.
6. Determine the density of the fragment by determining the density of the diluted salt water it is floating in.
Lab Notes:
Data & Calculations:
Name_______________
Crime Scene Challenge
NYPD Forensics Division
Date___________B____
Evidence Report:



Write a concise scientific description (talk like a chemist!) of how the evidence was analyzed (cite procedure)
Explain how the results (include all data and calculations) do or do not link the suspect to the crime.
Evaluate the suspect’s lawyer’s claim that the fragment is too small to be allowed in court (counterclaim, “ratio”)
(add more paper if needed)
I, ________________________ , certify the findings and forensic analysis above as accurate.
PRINT NAME
________________________
SIGNATURE
________________________
DATE