Summary

In this lesson, students will take on the role of a Forensic Scientist to help solve a hit and run investigation. They will learn how physical properties of matter, such as density and refractive index, can be used to help identify evidence samples such as glass.

Grade Level

Middle and High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
• MS-PS1-3: Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.
• HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this lesson, students should be able to:

• Calculate the density of an irregular shaped object.
• Determine refractive index of common types of auto glass.
• Identify an unknown piece of glass using physical properties of matter.

Chemistry Topics

This lesson supports students’ understanding of:

• Physical Properties of Matter
• Density
• Refractive Index

Time

Teacher Preparation: 20 minutes
Lesson: 60 minutes

Materials (per group)

• Copies of student document
• 3-5 different types of glass samples from cars
  • Density testing: ~ 1 cm size of each sample
  • Refractive Index testing: ~ 1 mm size pieces can be used (can be made easily with a mortar and pestle).
  • Glass samples sources: windshield, side mirror, review mirror, window, headlight
  • Note: These can be found at any auto salvage business. But, if automobile glass is not available, any differing types of glass pieces may be used and labeled as needed for the assignment.
• 3-5 different liquids with various known Rf values (at least 1mL of each sample)
  • Many liquids or solutions can be used: water, vegetable oil, isopropyl alcohol, coconut oil, clove oil, cinnamon oil, wintergreen oil. Or create your own solutions using differing concentrations of sugar water.
• Scale
• 10mL Graduated cylinders
• Stereoscope
• Microscope slides or well plate
• Forceps
• 50 mL beaker
• Calculator
• Mortar and pestle

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• Automobile glass is designed to break into pieces that are less sharp than regular glass; however for safety have students handle glass with forceps.

Teacher Notes

• This lesson can be used during an introduction to forensic science unit or during a trace analysis unit. This lesson may also be used during a physical properties unit in a standard chemistry class as an extension lab.

Engage:

• Show a video clip about a hit and run case that was solved by glass analysis.

Explore:

• Ask students to discuss how physical properties could be used to solve a crime. Instruct students to list physical properties and to suggest how they might be used in a criminal case.
• This could be done as a pair/share activity or a whole class discussion.
• Some examples may be color (paint from car, hair, fibers, soil), odor (soil, body fluid), length (fiber, hair), etc.
• Define density and refractive index. Use this video to help illustrate the meaning of refractive index and to help prepare students for what they will observe during the experiment. Alternatively, you may demonstrate this yourself with two Pyrex beakers and vegetable oil.

Experiment:

• Students will complete the pre-lab questions and the lab portion of the lesson.
• For this lab, teachers can obtain many different types of glass samples from cars at an auto salvage business. Try to collect as many different types as possible, windshield glass, side glass, mirror glass, and headlight glass. These are easily stored in paper bags or for ease with students, use small boxes, such as jewelry boxes. Auto glass is designed to break into pieces that are less sharp than regular glass; however for safety have students handle glass with forceps.
• Glass samples will have to be labeled appropriately to match the information on the student handout for suspect’s 1-5 (add or remove information from the student handout as necessary).
• This lab works best in student groups of 2-4 depending on the number of microscopes you have available.
• Students will find that glass from different car manufacturers may have varying density and Rf values.
• The teacher should prepare the glass pieces in advance so students are not crushing the glass. The glass can easily be made into small pieces using a mortar and pestle (~1mm) for refractive index testing. Have larger pieces (~1cm) available for density testing that will fit into a 10mL graduated cylinder.
• Have unknown glass samples ready, you can provide different samples to different groups to determine the suspect. Label these with evidence numbers, such as Evidence Item 1, etc. just be certain to make a key so you know which evidence matches which suspect.
• Different manufacturers may differ in the glass composition, but density measurements should range between 2.5 and 2.8 g/mL.
• Refractive indices may also differ, but here is a list of common values:

• Use as many different types of glass and liquids as are needed, just be sure to have matching liquids and glass samples. Use this list of refractive indices for sugar solutions and this list of common household liquids.
• The pieces of glass may not fully disappear, the students should write their observations and choose the refractive index where most of the glass may not be visible.