Identifying Glass and Plastics

In the course of an investigation, you may need to identify whether a sample of glass or plastic came from a specific source. For example, if you found a piece of window glass at a scene, you may be able to determine that it is the same kind of glass that a suspect has in their home. Two properties that vary among types of glass and plastic are density and refractivity. You can use these properties to identify samples of glass.

Forensic scientists usually work with very small samples, and therefore they use accurate and sensitive measuring tools which are very costly. We can simulate these tools by doing the following activity.

Directions:

Obtain a sample of glass or plastic marked "unknown" (this sample is part of an investigation) and other samples marked "drinking glass", "window glass", "plastic cup", and "cd case". Try to determine where your unknown sample came from.

Procedure:

  1. Weigh your sample and record the measurement.
  2. Find the volume by submerging the sample in a known volume of water. The volume of the sample is the volume of water that is displaced when you submerge the sample. Record this measurement.
  3. Now that you know both the mass and volume of the sample, you can find the density using the formula D=M/V
  4. What is the density of your sample?