Density Determination by Water Displacement and Flotation: An Introductory Experiment in Forensic Chemistry

Abstract

An introductory lab experiment has been developed for a nonscience major course in forensic chemistry, designed to introduce students to basic chemical principles within the context of interesting problem-solving scenarios. Students explore two different methods of density analysis and discover the practical uses and limitations of each. The experiment begins with a simple water-displacement exercise using common objects. Students discover problems involving accuracy and precision when calculating the densities of objects that displace very little water. The concept of flotation is introduced next, in which densities are compared, rather than measured directly, to determine the possibility of a common origin between two pieces of evidence. Students compare the densities of two simulated tire samples in 50 percent ethanol. Next, they perform a two-part analysis of glass and polymer fragments obtained from a simulated hit-and-run accident. Nonglass samples are compared using a saturated sodium chloride solution and water, and glass samples are analyzed in a series of bromoform–bromobenzene solutions. Students are able to successfully match five suspect fragments with the appropriate crime scene fragments using these methods.