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.
Middle and High School
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
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.
This lesson supports students’ understanding of:
- Physical Properties of Matter
- Refractive Index
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.
- 10mL Graduated cylinders
- Microscope slides or well plate
- 50 mL beaker
- Mortar and pestle
- 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.
- 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.
- Show a video clip about a hit and run case that was solved by glass analysis.
- 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.
- 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.
For the Student
Forensic Science often uses physical properties of matter to determine a match of an unknown item left at a crime scene to a known standard. Commonly in hit and run cases, small parts of the suspect’s car can be found at the scene, items such as paint fragments or small pieces of glass are found on the ground or on the victim. These tiny pieces of evidence have specific physical properties that make them unique enough to be compared to known standards from a suspect’s car in order to place the vehicle at the scene of the crime.
When determining physical properties of glass there are two common physical properties that can distinguish different types of glass:
- Density: the amount of mass per unit volume.
- Refractive index: how light bends as it passes from one medium (liquid) to another (glass).
Density of an irregular shaped object can be determined by placing the object in water and, using Archimedes principle, measuring how much the water rises to determine volume.
Refractive index can be determined by placing small pieces of glass in liquids of the same refractive index. Refraction happens when light changes speed between different objects and bends as a result of the change in speed. When a piece of glass and liquid have similar refractive indexes, the glass will seem to disappear in the liquid because the light is moving through them both at the same speed and therefore does not bend.
- Density is a measure of ____ per ______.
- Calculate the density of an object with a mass of 2.0g and a volume of 1.5mL.
- Refraction is the change in ______ of light when it _______ from one medium to another.
- A forensic scientist submerged a small glass sample in three different liquids. Using the data observations below, which refractive index is the closest to the refractive index of the piece of glass?
|Water (Rf 1.33)||Isopropyl Alcohol (Rf 1.38)||Vegetable oil (Rf 1.47)|
|Glass sample||All glass is still visible along the outside edge, but center appears clear.||Only a small part of the edge that not submerged in liquid is visible.||All of the glass in the liquid is visible.|
A car has smashed into the sign of a local business and completely destroyed the sign. A small piece of glass, which appears to be from a car was found at the scene near the wooden business sign. You are asked to examine the glass evidence along with refence glass pieces to determine which suspect may have been involved in the crash. Five suspects were seen driving down the road on the day of the incident:
- Suspect car 1 has a broken windshield
- Suspect car 2 has a broken headlight
- Suspect car 3 has a broken rearview mirror.
- Suspect car 4 has a broken side window.
- Suspect car 5 has a broken side mirror.
Using the information you obtain from your unknown evidence, determine which suspect may have been present at the crime scene.
Correctly identify the unknown piece of glass left at the crime scene by measuring the density and refractive index of the glass and comparing the results to known standards of glass taken from different cars.
- Various types of glass from automobiles (windshield, side window, headlight, rearview mirror, side mirror)
- Various liquids (water, isopropyl alcohol, coconut oil, vegetable oil, clove oil, cinnamon oil, wintergreen oil) with known Rf values
- 10 mL graduated cylinder
- Microscope slides or well plate
- 50mL beaker
- Always wear safety goggles when handling chemicals in the lab.
- Always use forceps when handling glass pieces.
- Wash your hands thoroughly before leaving the lab.
- Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
- Obtain glass standards from your teacher for testing.
- Using forceps to handle the glass, measure the mass of each type of glass on a scale and record the mass in the data table below.
- Using a 10mL graduated cylinder, place about 3mL of water into the cylinder and record the volume in the data table below.
- Using forceps, place one of the glass standards into the graduated cylinder and record the new volume of water in the data table below.
- Repeat steps for the remaining glass standards.
- Obtain a well plate or microscope slides.
- Place a few drops (2-3) of each of the different oils in each well/slide.
- Obtain a small 50mL beaker and fill 2/3 with water for rinsing.
- Place the plate/slide onto the stereoscope.
- Using forceps, place each glass sample into the first well. Observe if you can see the glass and how well. Record your observations in the data table below. Be specific in your description of what you see, these observations will help you determine which Rf value is closest to your piece of glass.
- Using the forceps, remove the glass, rinse in water beaker and place into the next well/slide and record observations.
- Continue placing the glass into all the liquids and recording observations.
- Repeat steps with each type of glass.
Unknown Evidence Sample:
- Record the evidence item number assigned to your group.
- Perform density and refractive index procedures for your unknown glass sample as describe above. Record information in the data table below
- Compare your data of the standard glass pieces with the data collected for the unknown glass sample. Determine which type of glass was found at the scene.
|Glass Type||Mass (g)||Initial Volume (mL)||Final Volume (mL)||Calculate Density (g/mL)
(Mass/Difference in Volume)
Isopropyl Alcohol (1.38)
Coconut Oil (1.43)
Vegetable Oil (1.47)
Clove Oil (1.49)
- Complete the table with the results obtained from your data. Explain your results in the justification column (explain how you determined the Rf value).
|Sample||Density(g/mL)||Estimated Rf Value||Justification|
|Identify the unknown sample:|
- What other types of physical properties of glass could have been observed/tested to assist in the identification of the glass evidence?
- How can physical properties of matter be used in other forensic cases?
Based on information gathered from your standards and unknown evidence sample, which suspect could have been present at the crime scene and involved in the hit and run case? Please discuss your results in your answer.