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Crime Scene Chemistry: Determine the Identity of an Unknown Chemical Substance

Difficulty
Time Required Average (6-10 days)
Material Availability A kit containing specialty items is needed for this science project. See the Materials tab for more details.
Cost Average ($40 - $80)
Safety Wear safety goggles and gloves when working with chemicals.

Abstract

Picture this situation: An elderly woman is rushed to the hospital complaining of severe abdominal pain, tinnitus, and lethargy. Suspecting a drug interaction, the emergency room doctor starts questioning her. The doctor learns that the woman takes no medicines except aspirin for her arthritis. Since she cannot swallow pills well she takes a powdered form of aspirin which she buys in bulk and keeps in a plastic container in her kitchen next to her baking goods. She had just finished a day of baking sweets for her grandchildren when she felt poorly. She admits that she often samples while she bakes and thought that she had just had too many sweets. Immediately the doctor suspects that she has mixed up the aspirin and another ingredient and is suffering from aspirin poisoning. The doctor quickly pumps her stomach. Soon the woman is feeling well again, but the doctor needs to confirm the diagnosis before releasing her. If the doctor is wrong, the woman could be suffering from something far more severe. If the doctor is right, the poisoning could easily occur again unless the aspirin is located and properly labeled. In this chemistry science fair project you will perform a series of tests on the woman's baking goods to help solve the mystery of what happened.

Objective

Perform chemical analysis to identify the nature of an unknown powder. This is an example of forensic chemistry.

Credits

David Whyte, Ph.D. and Sandra Slutz, Ph.D., Science Buddies

Cite This Page

MLA Style

Science Buddies Staff. "Crime Scene Chemistry: Determine the Identity of an Unknown Chemical Substance" Science Buddies. Science Buddies, 28 Jan. 2014. Web. 1 Sep. 2014 <http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p093.shtml>

APA Style

Science Buddies Staff. (2014, January 28). Crime Scene Chemistry: Determine the Identity of an Unknown Chemical Substance. Retrieved September 1, 2014 from http://www.sciencebuddies.org/science-fair-projects/project_ideas/Chem_p093.shtml

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Last edit date: 2014-01-28

Introduction

Chemistry is sometimes referred to as "the central science," because it forms the basis for so many other areas of science, such as medical research, the search for alternative fuels, and nanotechnology, to name just a few. One type of investigation that a chemist performs is chemical analysis. The goal of chemical analysis is to answer the questions: "What is this sample composed of, and how much of each constituent is present?" The analysis can be qualitative or quantitative. If the result of the analysis is a list of ingredients ("this sample contains chemicals X and Y"), that is qualitative analysis. If the result is a list of ingredients and the concentration of each ingredient ("this sample contains 3.2 percent of chemical X and 96.8 percent of chemical Y"), that is quantitative analysis. Quantitative analysis yields numbers.

One goal of chemical analysis is to identify the composition of an unknown sample. For example, a chemist might be asked to determine if several samples of soil contain mercury. To do this analysis, the chemist will run tests on the soil samples that are sensitive and specific for mercury. Sensitivity is important so that small amounts of mercury in the samples will not be missed. And specificity is important so that the test results are due just to the presence of mercury, and are not influenced by other metals, like iron or copper.

The branch of chemical analysis that focuses on crime scenes is called forensic chemistry. A forensic chemist is challenged with questions such as: "Is this white powder an illegal drug, or is it just sugar?"; "Is there poison in the blood of this potential homicide victim?"; and "Is there chemical evidence for arson in this fire investigation?"

In this chemistry science fair project, you will act as a forensic chemist to determine if the elderly woman accidentally consumed toxic levels of aspirin. The Abstract (which can be found on the Summary tab) details the scenario. As a forensic chemist, you will perform a series of tests on four white powders, with the goal of determining which powder (if any) consists of aspirin crystals. The aspirin might have been substituted for salt, sugar, or cornstarch, (or none of these if the woman has another medical problem) so you will need tests that can differentiate between these substances. You will investigate the following properties for the test substances: physical appearance, solubility in water, reaction with iron nitrate (the reaction produces a deep purple color if aspirin is present), and reaction with Lugol's iodine (the reaction produces a blue-black color when starch is present). Based on your analysis, you will be able to identify if one of the "unknowns" is actually aspirin.

Terms and Concepts

  • Chemical analysis
  • Constituent
  • Qualitative analysis
  • Quantitative analysis
  • Sensitivity
  • Specificity
  • Forensic chemistry
  • Toxic level
  • Crystal
  • Solubility

Questions

  • What are some other kinds of analyses that a forensic chemist might perform?
  • If a chemist is asked to determine if a sample of soil has mercury in it, at any level, is that a qualitative or a quantitative question?
  • List some properties of a chemical that can be used to identify it. The list should include both physical properties (color, melting point, etc.) and chemical properties (solubility in water, other chemicals it reacts with, etc.).
  • Based on your research, what is the definition of forensic toxicology?
  • One of the tests you will run used Lugol's iodine reagent. Based on your research, what is the Lugol's iodine reagent test?
  • One of the chemical tests involves mixing the test substances with sodium hydroxide and iron (in the form of iron nitrate). This is a test for aspirin: the sodium hydroxide breaks aspirin down into salicylic acid and acetic acid, and the salicylic acid reacts with iron to form a colored complex. What are the chemical equations for these reactions?

Bibliography

This paper is a little advanced, but it has good information about the iron test for aspirin.

Materials and Equipment Product Kit Available

This project uses a kit available for purchase from the Science Buddies Store.

  • Crime Scene Chemistry kit (1). Includes:
    • Reaction plates (3)
    • Wax pencil
    • Powdered aspirin
    • Mystery samples (3); presumed to be sugar, salt, and cornstarch
    • Distilled water
    • Sodium hydroxide, 1M
    • Ferric nitrate, 0.2M
    • Lugol's iodine
    • Toothpicks
    • Chemical safety goggles
    • Paper towel
    • Lab notebook

    You may also find it useful to have a digital camera to take pictures of the color changes that occur in the chemical tests. These pictures can go on your Science Fair Display Board.

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    Project Kit: $44.95

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Experimental Procedure

In the steps below, you will determine if one of the salt, sugar, or cornstarch containers is mislabeled. Does one of them actually contain aspirin?

  1. Put on your safety goggles and a pair of disposable gloves.
  2. Place a reaction plate on your work surface.
  3. Use the wax pencil (or a permanent marker) to label each column of wells as follows:
    1. C, for control, which in this case, is aspirin.
      • The other samples may be "mislabeled," but this control sample is known for sure to be aspirin.
    2. Su, for sugar
    3. Sa, for salt
    4. CS, for cornstarch
  4. Label the first three rows of the reaction plate, as follows:
    1. Water
    2. Iron
    3. Iodine
  5. The reaction plate should look like Figure 1:
Chemistry Science fair project Reaction plate for crime scene analysis
Figure 1. Reaction plate for "crime scene" analysis.
  1. Place a small scoopful of the appropriate substance to be tested in each of the wells for the labeled columns, using aspirin (C), sugar (Su), salt (Sa), or cornstarch (CS).
    1. Use a clean toothpick as a "scoop" for each substance.
    2. Record your physical observations of each powder in your lab notebook.
      • Record color, whether the substance appears crystalline or not, and any other characteristics you can identify.
  2. Add several drops of water to each substance in the row labeled Water. Mix gently with a clean toothpick.
  3. Record your observations for each substance in your lab notebook.
  4. Add two or three drops of NaOH (sodium hydroxide) solution to each sample labeled Iron.
    1. Note: Sodium hydroxide reacts with aspirin to form salicylic acid and acetic acid. Salicylic acid has the interesting ability to cause iron (Fe III) to turn blue/purple. Aspirin does not change color when mixed with iron. So to test for aspirin in an unknown sample, you first treat the sample with sodium hydroxide, to convert any aspirin in the sample to salicylic acid, then add iron. If a blue/purple color forms, then the test is positive for aspirin!
  5. Wait 5–10 seconds and add two or three drops of iron nitrate solution to each sample labeled Iron.
  6. Record you results in your lab notebook.
  7. Place two or three drops of Lugol's iodine in the row labeled Iodine.
  8. Record your results in your lab notebook. If you are using a camera, take pictures.
  9. Perform the procedure two more times with clean materials. This will show that your results are repeatable.
    1. To dispose of the reaction plates with chemicals, wrap them in paper towels and place them in trash.
  10. Analyze your results. What does each test tell you about the test substances?
    1. You may need to do some research about the tests in order to answer this question.
  11. After reviewing your results, which substance was mislabeled and was not what it was supposed to be? Make a table showing your results. Add a figure with pictures of the chemical tests if you choose.

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Variations

  • Run the tests on salt, sugar, cornstarch, and aspirin from around your house. Do you get the same results? (Since one of the containers was mislabeled, one of the chemicals was not actually tested. This step fills in the missing data).
  • Modify the procedure so that you can estimate how much aspirin is present in a mixture of aspirin and sugar. Hint: The color produced by the iron test is proportional to the aspirin concentration.

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