Density lab Rev 2.doc - Experiment 15.1 Physical Properties...

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Experiment 15.1: Physical Properties of Glasses and Plastics, Density and Refractive Index Determinations Introduction The characterization of physical evidence, such as glass and plastic fragments, can often be critical to a successful forensic investigation.   When glass or plastic is broken, such as in an automobile accident or burglary, fragments are frequently carried away from the scene of the crime on the clothing, shoes or hair of a suspect.  Both glass and plastics are so ubiquitous in our society that many crimes involve either their utilization as a weapon or as evidence from the crime scene. These samples may provide either class information, such as the type of glass or plastic involved, or individual evidence, such as that obtained by uniquely reconstructing a shattered item.  These small pieces of evidence may lead to some rather important discoveries, for example; In 1988, a number of jars of Gerber baby food were discovered to contain glass fragments.   While it was possible that the contamination arose from a single event, such as a light bulb bursting over the production line, careful analysis showed these glass fragments arose from a number of quite different sources (headlight, plate glass, light bulb, etc.).  The conclusion by the FBI was that several consumers independently placed the fragments into the food jars themselves in the hope of forcing the company to compensate them for the potentially dangerous contamination. It is often important to determine, if possible, whether a particular sample of glass or plastic found on a suspect matches, at least in terms of chemical composition, a similar material found at the scene of a crime.  While most glasses look very similar by visual inspection, they may be quite different in terms of their physical and chemical properties.   Similarly, while plastics may be colored materials, two colored samples may also be hard to distinguish visually, especially in small fragmentary samples.   A variety of methods have been employed in determining the specific physical and chemical properties of glass and plastic samples including, density, refractive index, ultraviolet-visible and infrared spectroscopy, inductively couple mass spectrometry (IC-MS), and neutron   activation.     Especially   important   among   these   are   density   and   refractive   index measurements for the substance.
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