Forensic scientists: Identifying unknown substances

In this lesson, students use the physical properties of three mystery substances to determine their identities. Students discuss how these skills apply to careers in forensic science.

A lesson plan for grade 8 Science

By Tammy Johnson and Martha Tedrow

Provided by CareerStart

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CareerStart lessons: Grade eight

This collection of lessons aligns the eighth grade curriculum in math, science, English language arts, and social studies with potential career opportunities.

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Legal

Essential question: How can knowing the physical properties of a substance help to identify it?

Learning outcomes

Students will become familiar with the use of physical properties in the identification of substances.

Teacher planning

Materials needed

For each student or pair of students:

• 1 balance
• 1 beaker with water (for measuring volume using water displacement)
• 3 unknown solid samples (e.g. wood, lead, aluminum, iron, aluminum, copper) — labeled “Exhibit A,” “Exhibit B,” and “Exhibit C”
• 1 Magnet
• Calculator
• Density table of solid elements

Note: Students should always wear safety equipment when performing classroom laboratory experiments. Have students wash their hands after handling all metals.

Time required for lesson

One class period (45-55 minutes)

Pre-activities

Before the lesson, copy the density chart onto the board.

Activities

1. Introduce the lesson by explaining to students the job of forensic science technicians and crime scene investigators. According to the description of forensic science technicians in the Bureau of Labor Statistics’ Occupational Outlook Handbook:
   

   Forensic science technicians investigate crimes by collecting and analyzing physical evidence. Often, they specialize in areas such as DNA analysis or firearm examination, performing tests on weapons or on substances such as fiber, glass, hair, tissue, and body fluids to determine their significance to the investigation. Proper collection and storage methods are important to protect the evidence. Forensic science technicians also prepare reports to document their findings and the laboratory techniques used, and they may provide information and expert opinions to investigators. When criminal cases come to trial, forensic science technicians often give testimony as expert witnesses on laboratory findings by identifying and classifying substances, materials, and other evidence collected at the scene of a crime.

2. Present three “unknown” samples — labeled “Exhibit A,” “Exhibit B,” and “Exhibit C” — for each lab group to investigate.
3. Instruct students to treat each substance as if it was a piece of evidence found at a crime scene. Discuss how each substance should be handled, proper labeling, and safety precautions.
4. Have students record the visual characteristics (e.g. color and texture) of each substance.
5. Have students record other physical characteristics of each substance, such as malleability and magnetism. Ask the students, “Does it bend?” “Can it be shaped?” “Is it magnetic?”
6. Have students calculate density of each item. (Use the formula D = Mass/Volume. To determine volume use water displacement.)
7. Using the physical properties of the three samples and the density chart, have students determine the identity of the three unknowns.
8. Discuss and review the properties of each substance and how its unique characteristic helps determine its identity.
9. Discuss with students the variety of careers in forensic science. See the career information below for examples.
10. Extension: Have students write a police log that details a crime scene and the location of each sample found. The log should include the analysis of the sample and the possible link to the crime.

Density chart

Magnesium	1.7
Aluminum	2.7
Copper	8.3-9.0
Gold	19.3
Iron	7.8
Lead	11.3
Platinum	21.4
Wood	.8
Ice (0° C)	.92

Career information

Source: Elka Maria Torpey, “Careers in Forensics: Analysis, Evidence, and Law.” Occupational Outlook Quarterly. Retrieved 7 July 2009 from http://www.bls.gov/opub/ooq/2009/spring/art02.pdf.

Computer forensic investigator

Obtains and searches electronic records for evidence related to a case.

Crime scene photographer (digital forensics)

Takes photographs of details related to a crime. Recovers data using photos, and other electronic/digital media.

Firemark and toolmark examiner

Studies guns, bullet striations, ballistics, and spent bullet casings to help determine the type of gun used during the commission of a crime.

Forensic accountant

Investigates and analyzes financial transactions pertaining to a crime

Forensic anthropologist

Specializes in human bones and helps determine age, weight, and sex of victim.

Forensic artist

Helps produce art-related works that help solve a case. Works can be sketches based on witness descriptions or computer-generated age progressions of the victim.

Forensic biologist

Examines organic substances and performs DNA analysis of blood and hair samples.

Forensic chemist

Does chemical analyses of evidence that includes drugs, soil, and shards of glass.

Forensic document examiner

Analyzes handwriting, printing, and ink to verify authenticity of documents.

Forensic pathologist

Medical doctors who perform autopsies and other investigations to determine cause of death.

Forensic engineer

Reconstructs accidents to determine cause.

Forensic toxicologist

Studies bodily fluids to determine if drugs, poisons, or other toxic chemicals were involved in commission of a crime.

Forensic entomologist

Investigates the insects that remain near or in human remains to determine time of death.

Forensic psychologist

Apply their knowledge of human behavior and thought processes in a variety of legal contexts. Examples include determining a defendant’s mental competency, helping to develop a suspect’s psychological profile, or assessing a witness’s credibility.