Summary

In this lesson, students will perform a flame test on a sample of vitreous humor (liquid found in the eyeball) in a forensic investigation. They will determine which element from the sample is used to determine the time of death. Then they will engineer a simple spectrophotometer to quantify that element. Evaluating a fake sample of vitreous humor in their spectrophotometer will help them determine the time of death for a hypothetical cadaver.

Grade Level

High School

NGSS Alignment

This lesson will help prepare your students to meet the performance expectations in the following standards:

• HS-PS4-3: Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described either by a wave model or a particle model, and that for some situations one model is more useful than the other.
• HS-PS4-4: Evaluate the validity and reliability of claims in published materials of the effects that different frequencies of electromagnetic radiation have when absorbed by matter.
• HS-ETS1-2: Design a solution to a complex real-world problem by breaking it down into small, more manageable problems that can be solved through engineering.
• Scientific and Engineering Practices:
  • Using Mathematics and Computational Thinking
  • Developing and Using Models
  • Analyzing and Interpreting Data
  • Planning and Carrying Out Investigations
  • Engaging in Argument from Evidence

Objectives

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

• Interpret results of a Flame Test.
• Explain how movement of electrons results in the absorption of light.
• Create standard solutions.
• Design, evaluate and revise a simple chemistry tool.
• Analyze a line of best fit to compare two variables.

Chemistry Topics

This lesson supports students’ understanding of:

• Energy
• Electromagnetic Spectrum
• Solutions
• Concentration
• Data Analysis
• Graphing
• Engineering

Time

Teacher Preparation: 1-2 hours
Lesson: 4-5 hours

Materials (for a class of 30)

Flame tests:

• 20 Beakers, any size
• Wooden splints
• Bunsen burner
• Striker
• 50 mL of 1.0M Barium Chloride (BaCl₂) – Note: Barium chloride is highly toxic. Do not ingest the salt or solution.
• 50 mL of 1.0M Calcium Chloride (CaCl₂)
• 50 mL of 1.0M Copper Chloride (CuCl₂)
• 50 mL of 1.0M Lithium Chloride (LiCl)
• 50 mL of 1.0M Potassium Chloride (KCl)
• 50 mL of 1.0M Sodium Chloride (NaCl)
• 50 mL of 1.0M Strontium Chloride (SrCl₂)
• Deionized water
• 250 mL beaker or plastic or cardboard cup half full of water
• Class copies of the visible light spectrum chart showing wavelength and frequency values (example)

Spectrophotometer Engineering:

• Vernier LabQuests and light meters or other devices that can measure light intensity (cell phones, iPads)
• Flashlights or lab lights
• A wide assortment of cardboard boxes (cereal boxes, butter boxes, cracker boxes, etc.)
• Black paper, black tape
• 20mL vials (80)
• Purple food coloring (purchase or make your own using 3 parts blue to 16 parts red food coloring)

Data Analysis:

• Graphing Calculators or Computers with Microsoft Excel (optional)

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.
• Always use caution around open flames. Keep flames away from flammable substances.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure loose clothing.
• Open flames can cause burns.
• Exercise caution when using a heat source. When lighting the match and wooden splint, be cautious with the flame.
• An operational fire extinguisher should be in the classroom.
• Do not consume lab solutions, even if they’re otherwise edible products.
• Food in the lab should be considered a chemical not for consumption.

Teacher Notes

Day 1: Introduction to Vitreous Humor and Quantum Mechanics:

• Hook: This is a great real-world application of quantum chemistry. You can start by reading to students from the book "Stiff" by Mary Roach. Chapter 3 contains a section that describes research at the University of Tennessee where chemist Arpad Vass took 700 samples from 18 bodies to see what chemicals were most effective for determining the time of death. The reading is witty, gross and non-fiction. It’s a great hook, and later you’ll be using this data to figure out the time of death of your class’ hypothetical cadaver.
• Engage: Students will conduct Flame Tests. This will help them to identify which element found in vitreous humor (eyeball liquid) is used by forensic scientists to determine time of death.
  • Note: I recommend doing this as a lab instead of a demo but otherwise following the directions as described in the AACT Flame test (Rainbow Demo) for this portion of the lesson.   
  • Call the “unknown” solution the vitreous humor and make it out of potassium chloride because potassium is the ion that steadily rises in the vitreous humor after death. It will show up as violet in the Flame test.

Day 2: Visualizing Quantum Mechanics and the Body Farm Data:

• Expand: Use the Flinn POGIL Activity: Electron Energy and Light to allow students to explore the relationship between light and the behavior of electrons. This will help students better understand what is happening in the flame tests on a particle level. Students learn the fundamentals of quantum mechanics.
• This activity should be completed in cooperative student groups and be done before direct instruction on the topic. Students should uncover core concepts together using the models in the worksheet.
• It is helpful to point out that the 2^nd model is basically like a flame test except that the element is hydrogen and it is excited using electricity instead of using a Bunsen burner.  The light the hydrogen gives off is then passed through a prism in order to determine its specific wavelengths.  
• Additional helpful background information can be found from the Khan Academy if you need it.
• Next give students a copy of the Body Farm Graph (this is on page 1 of the student handout). It shows the relationship between time of death and the concentration of Potassium ion. Each point on this graph is a sample taken from one of the bodies at the body farm.
  • Students will add a trendline and write a caption that describes the relationship between the two variables.
  • Have students discuss in small groups why the points are not all right on the line. What other variables might impact how fast bodies decay? Examples could include size, age, temperature, location…
  • Have students find two bodies/samples that have the same concentration of potassium ion but have been dead for different amounts of time. 
  • Have students find two bodies/samples that have been dead the same amount of time but have different concentrations of vitreous potassium.

Days 3 and 4: Engineering and using a Spectrophotometer:

• Teacher Prep: Since potassium absorbs violet light, make a fake vial of vitreous humor from our “dead body” using purple food coloring and water:
  • A solution concentration of 0.33 drops in 20mL of water works well, and can be made by adding 1 drop to 60mL of water. 
  • Mix thoroughly and divide it into three 20mL vials. Label as “vitreous humor”. 
  • These are the samples students will use in their homemade spectrophotometer after they have made and calibrated their spectrophotometer.
  • Note: vitreous humor would not actually look purple like this, but it would absorb violet light since potassium ion absorbs violet light so we use purple food coloring in this simulation.
• Make several standard solutions to show the students as well. For example:
  • 2 drops/20mL
  • 1 drop/20mL
    
  • 0.5 drops/20mL
  • 0.25 drops/20mL
  • 0.125 drops/20mL
  • An easy way to make differing concentrations is to make your most concentrated solution and then serial diluting. For example, make a solution of 2 drops of food coloring in 20 ml of solution. Then, take half of this solution and double the volume. You now have the equivalent of 1 drop in 20 ml. Keep doing this serial dilution until you have all of your standards. Faster and more precise. If you have enough time, I recommend not teaching this method to students and instead letting them figure out their own methods because it can lead to a great conversation.
• Engineering: Have students read and complete “Part 1: Engineering a Spectrophotometer” pre-lab questions.
  • The students should spend a class period making homemade spectrophotometers and making their own standard solutions with known concentrations of purple food coloring. (This is outlined in the student handout).
  • They check the engineering of their spectrophotometer by graphing light over concentration and must refine their spectrophotometers until they get a consistent trend. 
  • Then they test it in their spectrophotometer and use real data from the Body Farm to calculate the time of death.
• As time allows, or at the beginning of day 4 facilitate a brief discussion of the calculations and interpretations of the graphs. 
• You could show a few different hypothetical calibration curves: one that shows a more clear trend and one that is more scattered and ask students which spectrophotometer they think is better. 
• You can also ask students to determine how many standard solutions the makers of the hypothetical calibration curves used (each dot on the calibration curve is a different standard solution). 
• Also show students how to draw results from the calibration curve: I like to draw a red box that touches the trendline (this is shown in the Answer Key document).
  

Additional Tips:

• It is highly recommended that you accept a wide variety of answers from the students. You don’t need to ever tell them the concentration of the vitreous humor that you made. Nor is it important for different groups to get the same answer or for their answer to be close to the “true” answer. The real learning here is in engineering, revising, and evaluating their level of certainty using evidence. 
• Make sure that student answers are consistent with their findings (ex: if they found low lux level when they tested their vitreous humor, that means it was a relatively higher concentration and relatively “older” dead body). 
• If a group’s results fall outside of the range of the Body Farm graph or their calibration curve that is a great chance to talk about “extrapolation” and “interpolation”. I don’t suggest that students be required to redo their work, instead have them talk about how extrapolating beyond the data set impacts their level of confidence and implies what additional data would be useful to collect (i.e. “the body farm should test older bodies!” or “we should run standard solutions that have a lower concentration”). 
• If a group is just really struggling to get consistent data with their spectrophotometer there are a couple great ways to help:
  • Let them use the standard solutions that you made.
  • Show them that their eyes are built-in spectrophotometers. Can you estimate the vitreous humor concentration by comparing it visually to the standard solutions?

Differentiation:

• This activity works very well in heterogeneous cooperative groups. Try pairing students from different backgrounds and abilities together. Help create equitable conversation by having them draw straws for different roles. The roles of recorder, engineer, and facilitator work well on days 3 and 4.
  • Recorder: records data and records the group’s answers to pre and post lab questions.
  • Engineer: leads building efforts while gathering input from the rest of the group
  • Facilitator: Reads directions, asks others for inputs, and makes sure everyone is involved.
• Sentence starters (such “what do you think about ____?”, “how could we do ___ better?”) can help students understand their roles.
• Inclusive conversation should be modeled by the teacher, as well as celebrated and pointed out when it is overheard.

Extensions:

• There are so many great ways to make this activity more challenging, especially mathematically. Read through the basics first. If you can spend more class time on the lesson, consider doing some of the following: 
  • Instead of having students just sketch a trendline for the Body Farm graph, have them make sure their trendline goes through at least two data points. Then require them to use those points to calculate the y=mx + b (a chart of the data labels for the Body Farm graph is included in the Answer Key document and you would need to give that information to students.) 
    • They can compare the equation for their “line of good fit” to the “line of best fit” that a program like Microsoft Excel can find by mathematically minimizing the distances to each point on the scatterplot. The equation for the line of best fit for the Body Farm graph is y=0.174x + 6.6 
    • Compare the slope: is their line steeper or more shallow? 
    • Compare the y-intercept: how much potassium ion is generally in people’s eyeball juice right when they die? 
    • Was your y- intercept similar to the one generate by Microsoft Excel?
  • If students work with equations, they could plug in their values in both the Body Farm graph and the Calibration Curve graph and calculate numbers that are more accurate.
    • Note: Sometimes students will want to keep lots of significant digits if they do this. But because the trendlines do not perfectly match the data points, an estimate is actually probably still more accurate.
  • The common core for mathematics teaches students about R2 in 9th grade. R2 is a statistic that tells you how well a trend line fits a data set. If all the points are right on the line the R2 is 1.00. If the points are so scattered that there is no trend what-so-ever, the R2 is 0.00. This activity is a great way to apply the concept and show how often it is used in science. The R2 for the Body Farm graph is 0.84 which tells you that the two variables are highly correlated, but that there’s variation among individuals. You could have students visualize this by measuring the distance from each of the points to the line. 
  • A correctly working spectrophotometer would give a calibration curve with an R2 above 0.95. Students can use Microsoft Excel or a graphing calculator to find possible equations for their calibration curve (compare a linear trendline to an exponential trendline to a power trendline). The one with the highest R2 is the best fit. 
    • Note: the non-linear equations can be really tricky to solve so you might still want to use the red box method shown on the graph. I like to tell students at the start of the engineering that I won’t give them the vitreous humor sample until they build a spectrophotometer with an R2 above 0.95, but then I usually relent and help out groups who are struggling to do that.