Materials Included In Kit

Additional Materials Required

Prelab Preparation

Make a 50% sucrose solution by stirring the 500 g of sucrose into 1 L of distilled water. Stir thoroughly until all of the sugar dissolves. It will take several minutes to dissolve the sugar. (Mix less volume in the same proportion if 1 L is not needed.)

Safety Precautions

Isopropyl alcohol is highly flammable. Do not use near open flames or other sources of ignition. It is also slightly toxic by ingestion and inhalation. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Isopropyl alcohol may be disposed of according to Flinn Suggested Disposal Method #18a. Other solutions in the lab can be disposed of using Flinn Suggested Disposal Method #26b.

Teacher Tips

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students.
• The simulated glass beads A are only slightly more dense than water. Air bubbles may cause the beads to rise to the surface. Slowly stirring the water–bead mixture will help eliminate air bubbles.
• Possible teaching strategy:

  Day 1—Test all known beads. Assign Flow Chart as homework.
  Day 2—Discuss Flow Chart. Test crime scene beads for identity.

• Students will have to be very precise in order to distinguish beads A and F. If A and F are used as crime scene beads, expect less success than if B or D are used.
• Use any of the bead types as the crime scene unknown. Use one bead type or give different groups different unknowns. Enough of each bead type is provided for any option.

Sample Data

Answers to Questions

Complete this flow chart:

Introduction

Crime scenes often contain broken glass from windows, cars, glass vases and numerous other glass items. Matching glass pieces with other glass pieces can often reveal patterns that will help explain a crime sequence. Numerous glass analysis techniques can be used. One specific technique, comparison of density, will be examined in this laboratory.

Concepts

• Density
• Density gradient
• Flotation

Background

Forensic scientists compare glass pieces in order to associate one glass fragment with another and at the same time eliminate the possibility that other sources may be responsible for the glass fragments. Glass analysis has its greatest impact as evidence when the glass can be identified with only one source. Such a determination can only be made if fragments can be collected and physically pieced together in a perfect fit like a jigsaw puzzle. Since the probability of two pieces of glass from two different sources fitting together perfectly is low, it becomes compelling evidence when a perfect match is found.

In many forensic settings, however, the physical pieces often do not fit together easily. The forensic scientist is then left with other techniques that may not be as conclusive or compelling. One such technique involves determining the density of glass pieces and then comparing the density to those of known glass sources.

The simplest comparative density techniques is known as flotation (not floatation). It is based on the principle that a solid particle will float in a liquid medium of greater density, sink in a liquid of lower density, and remain suspended in a liquid of equal density. In the crime lab, this is usually done in a density gradient. Solutions of decreasing densities are placed in a long column and then the object is placed into the column. By comparing the point of suspension, the density can be identified and different glasses can be compared. As seen in Figure 1, the glass found at the crime scene has the same density as glass type C.

Density is a characteristic property of materials and pure elements or compounds. Materials can often be identified by their density. Density is defined as the mass of a substance per unit of volume. Density is commonly expressed as g/cm³ or g/mL. The density of pure water is 1.00 g/cm³ at 20 °C. Objects with a density greater than 1.00 g/cm³ will sink in pure water. Objects with a density less than 1.00 g/cm will float in pure water. Alcohols have a density less than 1.00 g/cm³ and some objects that float in water will sink in alcohol. Solutions, such as sugar water, have a density greater than 1.00 g/cm³. The density of a sugar solution will increase as the concentration of sugar in the solution increases.

In this experiment, the principle of density will be used to distinguish simulated glass pieces (plastic beads). Once the properties of various beads are established, a bead found at the crime scene will be analyzed.

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Isopropyl alcohol is a flammable liquid. Do not work near any source of flame, heat or sparks. Wash hands thoroughly with soap and water before leaving the laboratory.

Procedure

1. Fill a clean 100-mL graduated cylinder with 20 mL of distilled water.
2. Add five simulated glass beads A to the water in the graduated cylinder.
3. Slowly swirl the water in the cylinder to make sure there are no air bubbles attached to the beads that would cause them to float instead of sink. Establish whether the A beads sink or float in water. Why are five beads used instead of one?
4. Record the floatability of the A beads on the Density Principles Worksheet.
5. If the beads float, go on to step 6. If they sink, go on to step 11.
6. Remove the beads from the water and blot them on a dry paper towel.
7. Pour the water out of the graduated cylinder and dry the inside of the cylinder with a paper towel.
8. Carefully pour exactly 20 mL of isopropyl alcohol into the graduated cylinder.
9. Place the beads in the alcohol. Do they float or sink? Record the result on the Density Principles Worksheet.
10. If the beads sink, use a Beral-type pipet to add distilled water to the graduated cylinder about 1 mL at a time. Swirl the cylinder to mix the water and the alcohol. Keep adding water until all of the sample beads float. When all the beads are floating, stop adding water and note the level of the liquid in the graduated cylinder. How much water did it take to get the beads to float? What concentration (mixture ratio) of alcohol and water caused the beads to float? Record the results on the worksheet.
11. If the beads sank, obtain a Beral-type pipet and some 50% sugar solution.
12. Start adding sugar solution 1 mL at a time, swirling between additions, until the beads are all floating on the top of the solution in the graduated cylinder.
13. Note the level of the liquid in the graduated cylinder when all the beads are floating. How much sugar solution did it take to get the beads to float? What concentration (volume ratio) of sugar water and water caused the beads to float? Record the results on the worksheet.
14. Clean and dry the graduated cylinder between each key step and repeat steps 1–13 for simulated glass samples B, D and F.
15. When all the tests are completed on the four types of beads, complete the flow chart on the Density Principles Worksheet. The flow chart should depict a complete strategy for identifying bead types A, B, D and F.
16. Clean all equipment thoroughly and discuss your flow chart with your instructor or as a class.
17. Test the Unknown Beads from the crime scene. Determine which bead type was found at the crime scene.
18. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

13885_Student1.pdf