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Chemicals, Chromatography, and Crime! (8 Favorites)

LAB in Observations, Solubility, Chemical Change, Physical Change, Precipitate, Identifying an Unknown, Solute & Solvent, Graphing, Mixtures, Error Analysis. Last updated May 24, 2021.


Summary

In this lab, students will test “evidence” that has been collected from a crime scene. In order to determine if the victim was poisoned, students will perform a solubility and crystallization test on an unknown powder. Then, students will attempt to identify the culprit by using paper chromatography to analyze the lipstick from the potential criminals.

Grade Level

Middle and High School

NGSS Alignment

This lab 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.
  • HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data
    • Engaging in Argument from Evidence

Objectives

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

  • Accurately determine the dissolving point and crystallization point of a substance.
  • Interpret data from a solubility curve.
  • Create and interpret a chromatogram, by separating the mixture of lipstick, to identify unknowns.
  • Calculate Retention Factors or Rate of Flow numbers for the chromatogram.
  • Describe both solubility and crystallization as either physical or chemical changes.
  • Describe the outcome in terms of the structure of and attractive forces of matter.

Chemistry Topics

This lab supports students’ understanding of

  • Solutions
  • Solubility
  • Chemical Change
  • Physical Change
  • Identifying Unknowns
  • Mixtures
  • Observations
  • Precipitates
  • Separating Mixtures
  • Graphing

Time

Teacher Preparation: ~30 minutes
Lesson: 90 minutes

Materials

Part 1: Solubility and Crystallization (per pair of students)

2.50 3.50 g 4.50 g
2.75 g 3.75 g 4.75 g
3.00 g 4.00 g 5.00 g
3.25 g 4.25 g
  • Massing paper
  • Balance
  • Scoop
  • Large test tube (greater than 10 mL)
  • Distilled water
  • Two 250 mL beakers
  • Hot plate
  • Test tube holders/tongs
  • Ice
  • Two thermometers

Part 2: Chromatography (per pair of students)

  • Scissors
  • Rulers
  • Strips of chromatography paper (four strips per pair of students)
  • 3 Lipstick samples of similar shades
  • Well Plate
  • Two 400 mL beaker
  • Pencils/wooden sticks
  • Toothpicks
  • Chromatography Solvent
  • Fume hood

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.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.
  • An operational fire extinguisher should be in the classroom.
  • Refer to the SDS for Potassium Chloride for additional safety information.
  • If creating a chromatography solution with ethanol, acetone, ammonium hydroxide refer to each SDS for additional safety information.

Teacher Notes

Part 1: Solubility and Crystallization

  • This portion of the lab was originally written to include the use of potassium nitrate crystals, however due to the potential risks associated with their use, potassium chloride was chosen as a substitute. Alternatively, other crystals might be used if they have similar dissolving and crystallization points.
  • Depending on class time available the teacher may want to organize the samples of potassium chloride by mass ahead of time. This is an “unknown” sample to the students, so be sure to not reveal the identity of the crystals. Make sure that students get different amounts of sample. I suggest assigning masses at 2.50 grams and increase each sample by 0.25 grams. Do not exceed 5.00 grams of potassium chloride per pair of students. If another substance is used instead of potassium chloride, refer to a solubility chart to determine the best range of mass for the substance to provide to each group.
  • Do not use tap water for this experiment since the dissolved salts will affect the outcome.
  • The goal of this activity is to find the identity of crystals that might have “poisoned” the victim through use of the collected lab data and the provided solubility curve.
  • Depending on the prior knowledge and ability of students, this short Fuse School video about Solubility Curves may be helpful to share.

Part 2: Chromatography

  • Each pair of students will need a small sample of lipstick “A”, “B”, “C”, as well as the “Crime Scene” sample.
  • Placing small samples in well plates ahead of time is suggested—remember to reuse one of the samples, A, B or C as the “Crime Scene” sample. Or, you may choose to use an entirely different sample as the “Crime Scene” sample, which would mean all three of the suspects would be innocent. You can also change the culprit between groups, so there are multiple correct answers in the class.
  • Alternatively, depending on class time and the supplies available it might be easier for the instructor to place the lipstick samples on the chromatography paper and label each paper as “A”, “B”, “C”, and “Crime Scene” in advance of the lab.
  • Remember to keep a log of which sample belongs to which culprit and which sample was from the crime scene.
  • Lipstick samples should be similar colors so that students cannot determine which suspect’s sample matches that of the crime scene by color alone. 
  • Cheap lipsticks that are great for these labs can be found at the dollar store, drugstores, convenience stores, department stores, or other discount stores.
  • To create the chromatography solution, I suggest making a solution of 5 parts ethanol to 5 parts acetone to 2 parts household ammonia (or 0.100 M ammonium hydroxide) and 1 part water should work to dissolve the lipstick (other recipes use n-butyl alcohol instead).
  • Make enough solvent so that each pair has at least 50 mL to use (600 mL for a class of 24).
  • Add solvent to the beaker before the chromatography paper samples are hanging—this will prevent accidental splashing onto the lipstick samples and contamination of the solvent.
  • Make sure that students hang their chromatograms evenly and that all just touch the bottom of the beaker. See the diagram on the student handout for more guidance. Often 2-3 papers can be attached along side of each other, in the same beaker. Students can also share beaker space between groups/as needed.
  • All beakers should be placed in the fume hood for this part of the lab. If a fume hood is not available the lab can still be done using small vials with screw top lids. The chromatography paper will need to be cut to fit into the vial.
  • If lipstick samples are not available, the scenario can be changed to a note left at the scene of the crime and chromatograms of ink can be made instead. The lab, Investigating Black Ink is a good reference.  
  • Students should be encouraged to begin working on the Calculations and Analysis sections of the lab while waiting for the chromatography results, as they can take 15-30 minutes to complete.

For the Student

Lesson

Scenario

Mr. Frost was a very cold and conniving individual. He was an unpopular certified public accountant who had managed to make enemies of everyone he knew, including the organized crime bosses he worked for! Even his own family found him to be a jerk and a bully who lied, cheated, and stole from his estranged wife and grown daughter. He lived a lonely life after being exiled from his home a year ago and put into the witness protection program. No one ever visited him in his cramped one-bedroom apartment. He hated house cleaning more than he hated people and his apartment was always a mess. 

On the day that his body was found by the building superintendent, one scene in the apartment was notable because it was so unusual. The dining room table was laid out with a fresh tablecloth and fancy candleholders. Two wine goblets, a bottle of unfinished champagne, two cloth napkins, and a partly eaten Spanische Windtorte (the “fanciest cake of Vienna”) were on the table. One of the napkins and a goblet contained a smear of lipstick. Mr. Frost’s wine glass had the dried remains of a white crystal on the rim. Poison, perhaps? In his hand was the remnant of a torn note with the words “we” and “meet” written in fine tip pen.

According to records stored on his phone, Mr. Frost had scheduled a meeting with his new attorney, Ms. Lawler, on the afternoon of his death. This was preceded by a meeting the day before with this wife, Olga Frost to sign divorce papers, along with a visit by his daughter, Crystal Frost, the night before (as witnessed by the superintendent).

Prelab Questions

Most substances have a specific solubility (amount of solute that dissolves) in water at a given temperature. Solubility is defined as the ability of a substance to dissolve in another substance. Solutions are mixtures containing a solute (substance being dissolved) and a solvent (material that dissolves another substance). Solutes and solvents can be solids, liquids, or gases. The most common solutions, however, have liquid solvents, particularly water. 

Other substances, in a mixture, can be separated by use of paper chromatography.  In this technique, a drop of the mixture is placed on a piece of filter paper; the filter paper is then dipped into a solvent, a liquid in which the substances in the mixture will dissolve. When the solvent, moving along the filter paper, reaches the mixture, each of the substances in the mixture dissolves and moves along the filter paper. However, some substances are more soluble than others. This difference, in combination with other factors, means that the dissolved substances move along the filter paper at different rates. If the substances are colored, then each separate color represents a separate substance. The rate at which a substance travels (its rate of flow, or Rf) is a property of the substance and can be used in identifying it.

  1. Is solubility a physical or chemical property?  Explain your answer.
  2. Based on the reading and your previous interactions with it, is lipstick a mixture or a pure substance?  Explain your answer.

Objective

  • You will first use the properties of solubility to identify the unknown crystal. This will be done by developing solubility curves based on the dissolving point and crystallization point of a compound. 
  • You will analyze the lipstick sample left on the napkin left at the scene of the crime and compare it with the lipstick samples found in the possession of the only three women who were known to have visited Mr. Stone:
    • His estranged wife, Mrs. Olga Frost
    • His daughter, Crystal Frost
    • His attorney, Ms. Lucille Lawler

Materials

Part 1: Solubility and Crystallization
  • A sample of unknown white powder
  • Massing paper
  • Balance
  • Scoop
  • Large test tube (greater than 10 mL)
  • Test tube holder/tongs
  • Distilled water
  • Two 250 mL beakers
  • Hot plate
  • Ice
  • Two Thermometers (standard or electronic)
Part 2: Chromatography
  • Scissors
  • Rulers
  • Strips of chromatography paper
  • Lipstick samples in well trays
  • Toothpicks
  • Several 400 mL beakers
  • Pencil(s) or wooden sticks
  • Solvent
  • Fume hood

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Exercise caution when using a heat source. Hot plates should be turned off and unplugged as soon as they are no longer needed.

Procedure: Part 1

  1. Determine the mass of the white crystals for your experiment. Note: Each group will be assigned a specific amount (2.00 through 7.00 +/- 0.01 grams).
    *This step might have already been done for you.
  2. Record this data in the data table below. 
  3. Fill the first 250 beaker about halfway full with tap water. Place the beaker on a hot plate. Heat the water (it doesn’t need to be boiling). This is called making a "hot water bath". 
  4. Fill up the second 250 mL beaker halfway with tap water and place a few ice cubes in the beaker to cool the water (this is called making a "cold water bath").
  5. In order to make the solution, accurately measure out 10.00 ml of distilled water in a graduated cylinder. Add it to a test tube. 
  6. Add the massed white crystal to the test tube. Agitate the mixture by stirring it gently with a glass stirring rod. Continue until the mixture is dissolved. Do not allow the undissolved solid to stick to the sides of the test tube.
  7. The purpose of the next procedure is to completely dissolve the solute. Place the test tube containing the solution into the hot water bath. Insert the first thermometer into the test tube and carefully stir the solution until all of the solute has dissolved. Record this temperature. This temperature is known as the dissolving point.
  8. As soon as the chemical compound has completely dissolved, use the test tube holders/tongs to carefully transfer the test tube and the second thermometer in the cold water bath. 
  9. While cooling, shake the test tube frequently to prevent supersaturation. As soon as the first crystals appear in the test tube (they will look like little snowflakes), record the temperature in the data table. This temperature is the crystallization point.
  10. Pour the solution out in the correct container as stated by the instructor (NOT DOWN THE SINK). Then wash the test tube in the sink with lots of running water.

Data: Part 1

Amount of Chemical (g) Temperature Chemical Dissolved (oC)Temperature Chemical Crystallized (oC)

Procedure: Part 2

  1. Obtain four pieces of chromatography paper. Make sure each piece is the same length.
  2. Using a PENCIL label the first strip A; the second B; the third C; and the fourth CS at the top of the strip.
  3. Measure roughly 1.50 cm from the bottom and draw a line across the paper in PENCIL (see sketch). This is the starting point for the lipstick samples.
  4. Using a toothpick carefully make a spot of lipstick (located in the well tray provided by your teacher) from Mrs. Frost (suspect A) in the middle of the pencil line that you just drew. Make sure the sample of lipstick is about 1.00 cm in diameter.
  5. Repeat steps 2-3 for the lipstick sample from Crystal Frost (B), Ms. Lawler (C), and the sample from the Crime Scene (CS).
  6. Tape each strip onto a pencil so that the samples are hanging. You may be able to attach more than one paper to a pencil (place them next to each other), just be sure that they will not touch each other!
  1. Take your beaker to the fume hood, where the instructor will pour enough solvent into the beaker so that the end of each strip is just touching the solvent. The solvent must not get on the lipstick sample itself. You may have to adjust the paper strips.
  2. Move the uncovered beakers into the fume hood.
  3. After about 15 minutes (longer if stated your teacher) remove the samples from the fume hood. Record all observations regarding the colors on the samples (take a picture if possible). 
  4. Measure the distance the solvent moved up each strip of the paper from the pencil line. Quickly draw a line, in PENCIL, at highest point the solvent traveled. Record this in the data table.
  5. Measure the distance each component of the lipstick moved up the paper. Pick the highest point for the first color, the highest point for the second color, etc. Record this in the data table. See the example below. Note that some same may not separate into multiple colors.

  1. Determine the Rf for each dye component of all the lipstick samples. For example, is using the image above, four Rf calculations would be needed.
  2. Enter the Rf values on the data table as well. The Rf is calculated by dividing the distance traveled by each ink component (solute) by the distance traveled by the solvent.

  1. The chromatograms may be thrown away in the trash when finished

Data: Part 2

Colors Observed in Lipstick Samples
Distance Solvent Moved (cm)
Distance Each Component Moved (cm)
Rf
A
B
C
D
CS

Calculations

  1. Convert the mass of white crystals in 10.0 ml of water to the mass of white crystals in 100.0 ml of water. This means that the original masses must be multiplied by ten. This is done so that graph is larger. So, 10.00 grams of white crystal in 10.0 mL of water becomes 100.00 grams of white crystals in 100.0 mL of water.
  2. Share out your data with the rest of the class. The data must be taken from EVERY group. Record this data below in the space provided.

Mass of Chemical (g)
Temperature Dissolved (oC)
Temperature Crystallized (oC)
Mass of Chemical (g)
Temperature Dissolved (oC)
Temperature Crystallized (oC)
  1. Record the mass of the chemical and the temperature that it crystallized on the graph below and see which curve the data comes closest to.
    1. Identity of crystals based on solubility data:
    2. Based on the true identity of the white crystals, determine a percent error for this lab. To do this, take the mass of chemical that you had and determine the actual temperature of crystallization. Use your data as the experimental value and the one on the chart as the actual number when determining percent error.

Analysis

  1. What information must be specified when a chemist describes the solubility of a solute in water (or what information is essential in order to make a solubility curve on a graph)? 
  2. Please describe the relationship between each pair of words. Don’t just define the terms.
    1. Compound vs. mixture
    2. Heterogeneous mixture vs. Homogeneous mixture
    3. Physical property vs. Chemical property
    4. Physical change vs. Chemical change
    5. Solute vs. Solvent.
  3. Use the Chemical Solubility vs. Temperature graph (used previously) to answer the following questions:
    1. Based on the graph, at what temperature would the following amounts of KNO3 crystallize? (If it cannot be determined by this graph, answer "N/A").
      1. 10.0 grams?
      2. 100.00 grams?
      3. 50.00 grams?
      4. 130.00 grams?
    2. Based on the graph, what amount of KNO3 would crystallize at the following temperatures?  (If it cannot be determined by this graph, answer "N/A").   
      1. 40.0 oC?
      2. 100.00 oC?
      3.  10.00 oC?
      4. 25.0 oC?
  4. When a stick of butter is left on the kitchen table, it softens gradually rather than melting at a specific temperature, or melting point. Based on this information is butter a pure substance or a mixture? Why?
  5. Recrystallization doesn’t only happen in a lab. Stalactites and stalagmites in caves are another example of recrystallization. Stalactites form at the ceiling of the cave and point down. Stalagmites form on the floor of the cave directly beneath stalactites and point up. Do some outside research and simply describe how stalactites and stalagmites form. 
  6. Thinking of the attractive forces found in atoms, explain the results of this experiment.

Conclusion: Who Done It?

Be sure to answer all of the following in your conclusion:

  • Based on the information collected and analyzed who was drinking wine with Mr. Frost at the time of his demise? Why do you say that?
  • Do you think the police have enough evidence to be certain? Why are why not?
  • What other pieces of evidence would you like to have in order to be certain that you were correct regarding the identity of the criminal?