Crime Scene Chemistry: Determine the Identity of an Unknown Chemical Substance
|Areas of Science||
|Time Required||Very Short (≤ 1 day)|
|Material Availability||A kit containing specialty items needed for this project is available from our partner Home Science Tools. See the Materials tab for more details.|
|Cost||Average ($40 - $80)|
|Safety||Wear disposable gloves when working with chemicals.|
AbstractPicture 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.
Perform chemical analysis to identify the nature of an unknown powder. This is an example of forensic chemistry.
David Whyte, Ph.D. and Sandra Slutz, Ph.D., Science Buddies
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Last edit date: 2020-06-23
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 an elderly woman accidentally consumed toxic levels of aspirin. Here is the scenario: 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.
Your role as the forensic chemist is to use four tests to determine if ingredients labeled sugar, salt, and cornstarch in her kitchen are aspirin instead. To do this, you will compare a known sample of aspirin (known samples are called controls in science) to the three unknown samples (presumed to be sugar, salt, and cornstarch) collected from the lady's home. 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). There are five possible outcomes to your testing: 1) None of the unknowns are aspirin 2) the unknown presumed to be sugar is aspirin 3) the unknown presumed to be salt is aspirin 4) the unknown presumed to be cornstarch is aspirin 5) more than one of the unknowns is aspirin. Do a bit of background reading about each test and then diagram out what you expect the test results to look like for each of the five possible outcomes. Do you think you will have enough data from the four tests to determine if any of the unknowns is actually aspirin?
Terms and Concepts
- Forensic chemistry
- Toxic levels
- Physical traits
- Iron nitrate
- Lugol's iodine
- What symptoms does an overdose of aspirin cause?
- What does a forensic chemist do?
- 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.).
- 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?
- American Chemical Society. (n.d.). Forensic Chemists. Retrieved November 16, 2009.
- Wikipedia Contributors. (2009). Forensic Toxicology. Wikipedia: The Free Encyclopedia. Retrieved October 8, 2012.
- Aspirin Foundation. (n.d.). What is Aspirin?. Retrieved January 27, 2014.
- Wikipedia Contributors. (2012, September 10). Aspirin poisoning. Wikipedia: The Free Encyclopedia. Retrieved October 8, 2012.
- General Chemistry Laboratory. (2012, January 10). Iodine Test for Starch and Glycogen. Retrieved January 27, 2014.
This paper is a little advanced, but it has good information about the iron test for aspirin.
- Gil, M.J. and Martínez-Merino, V. (2007). Determination of free salicylic acid by forming Fe +3 complexes. Retrieved April 27, 2010.
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This project uses a kit available for purchase from our partner Home Science Tools.
Crime Scene Chemistry Kit (1). Includes:
- Reaction plates (3)
- Wax pencil
- Powdered aspirin
- Mystery samples (3)
- Distilled water
- Sodium hydroxide, 1M
- Ferric nitrate, 0.2M (also called Iron nitrate)
- Lugol's iodine
You will also need to gather the following items:
- Lab notebook
- Optional: salt, sugar, and cornstarch
- Optional: 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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Recommended Project Supplies
- The kit contains three samples marked unknown. According to the scenario in the Introduction, these were taken from an ill elderly lady's kitchen from the containers where she supposedly keeps her sugar, salt, and cornstarch for baking. But the doctor is concerned that one of them may be aspirin instead. Your task is to determine if any of the unknown samples are aspirin. Based on your background reading, do you think you'll be able to determine this using the tests described below? Why or why not?
- Put on a pair of disposable gloves.
- Place a reaction plate on your work surface.
Use the wax pencil (or a permanent marker) to label each column of wells as follows:
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.
- Su, for sugar
- Sa, for salt
- CS, for cornstarch
- C, for control, which in this case, is aspirin.
Label the first three rows of the reaction plate, as follows:
- The reaction plate should look like Figure 1:
Figure 1. Reaction plate for "crime scene" analysis.
Place a small scoopful of the appropriate substance to be tested in each of the wells for
the labeled column. Use a clean toothpick as a "scoop" for each substance. The sample marked aspirin should go in the aspirin column. The unknown sample presumed to be sugar should go in the sugar (Su) column. The unknown sample presumed to be salt should go in the salt (Sa) column, and the unknown sample presumed to be cornstarch should go in the cornstarch (CS) column.
- The sample marked aspirin should go in the aspirin column.
- The unknown sample presumed to be sugar should go in the sugar (Su) column.
- The unknown sample presumed to be salt should go in the salt (Sa) column.
- The unknown sample presumed to be cornstarch should go in the cornstarch (CS) column.
- 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.
- Do any of the unknown substances have the same physical characteristics as aspirin?
- Add several drops of water to each substance in the row labeled Water. Mix each sample gently with a clean toothpick. Use a different toothpick for each sample so you do not contaminate any of them!
- Record your observations for each substance in your lab notebook. Do any of the unknowns behave like the aspirin sample when mixed with water? If so, which one(s)? Do any act differently? Which one(s) and how so?
- Add two or three drops of NaOH (sodium hydroxide) solution to each sample in the row labeled Iron. Mix each sample gently with a different clean toothpick.
- Wait 5–10 seconds and add two or three drops of iron nitrate solution to each sample in the row labeled Iron.
- 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. 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!
- Record your observations for each substance in your lab notebook. Do any of the unknowns behave like the aspirin sample? If so, which one(s)? Do any samples react differently? If so note which one(s) and how they react.
- Place two or three drops of Lugol's iodine in the row labeled Iodine.
- Record your results in your lab notebook. Do any of the unknowns behave like the aspirin sample? If so, which one(s)? Do any samples react differently? If so, note which one(s) and how they react.
Perform the procedure two more times with clean materials. This will show that your
results are repeatable.
- To dispose of the reaction plates, wipe up each liquid on the plate with a paper towel and place the paper towels and the reaction plates in a regular trash.
- If you would like to re-use the reaction plates (for example, to do the optional experiment in step 17 below) simply wash the wiped up reaction plates with soap and water.
Analyze your results and make a table. What does each test tell you about the unknown substances? Are any of them aspirin?
- You may need to do some research about the tests in order to answer this question.
- Optional: Compare known samples of sugar, salt, and cornstarch from your own kitchen with the unknown samples in the kit. Compare both their physical appearances and how they react to the three tests: water, iron, and Lugol's iodine. Is it possible to know from your observations exactly what each substance is? Why or why not?
If you like this project, you might enjoy exploring these related careers:
- 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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