In the Heat of the Summer: Ink Chromatography Test
Lesson 4 of 6
Objective: Students will be able to conduct a paper chromatography test and analyze its results.
This is the fourth lesson in a forensics unit called In the Heat of the Summer. In previous lessons students have analyzed glass densities and boiling points and conducted a test for the presence of aspirin as they work to amass a body of evidence that points to a specific culprit in an attempted murder.
In this lesson students will use paper chromatography to compare ink pens found on the suspect to the ink found on a threatening note. Going into this lesson students have already studied chromatography using this lesson. However, this lesson could stand alone by spending a little more time discussing the purpose and technique of paper chromatography.
This lesson aligns to the NGSS Science Practices Analyzing and interpreting data, and Planning and carrying out investigations because it requires students to carry out an investigation of differnet inks using paper chromatography to separate out different compounds of inks. Students must then analyze their data by examining evidence found at the crime scene to the different ink samples. It relates to the cross-cutting concept of Structure and Function because paper chromatography relies on the the the molecular substructures of inks.
The materials needed to do this lab are:
- 4 different pens or markers
- filter paper
- metric ruler
- clear plastic tape
- rubbing alcohol
- plastic cup or 250 ml beaker
- plastic wrap
Note: It is really important that the teacher conduct this lab ahead of time in order to learn how the markers or pens behave on the filter paper. Once you know this, you can fit this information into the script.
Here is the Teacher Guide: Master Data Sheet, which is a document which summarizes the evidence for each suspect for all of the lessons in this unit.
Do Now: To get students immersed in the story line I ask them to read Investigation Log for July 6 and the evidence for July 6th in the Investigation Notes in the In the Heat of the Summer handout (See Resource in previous section). To remind students about how paper chromatography is conducted I ask them to read a document that outlines the procedure (such as Ink Chromatography from The Museum of Science and Industry). I explain that we will review the procedure as a class in five minutes.
Mini-lesson: After students have had a chance to read the document we review the crime that we are investigating today. I ask students what the crime was and they explain that there was death threat found on the windshield of Dr. Rodriguez's car. Students also explain that the investigators found writing instruments in the possession of each of the suspects that can be compared to the ink in the threatening note.
I remind students that inks are made from different compounds, and that these compounds have different chemical compositions. Because of this fact, the inks will separate out of the solution and park on the filter paper at different heights.
We then review the procedure. I ask the first student what the first step in the procedure is, then ask subsequent students to explain what happens next. The class expectation is that everyone is listening and can quickly pick up the conversation when called on.
We discuss modifications to the lab procedure based on the materials we have at hand. I note that the solvent used in the handout is water, but that we will use alcohol, as it does a better job of dissolving inks. I also note that we will use glass stirring rods to hang our chromatograms from the top of the beaker. I remind students that the use of chemicals and glass means that everyone must wear safety glasses. I remind students that they can dispose of their alcohol in the sink and the chromatograms can be kept or disposed of after the data has been recorded.
I then release students to conduct the paper chromatography in pairs. While they are conducting this test I circulate around the room to make sure that students are correctly following the procedure and that they are paying attention to our safety rules.
Because they made chromatograms in the previous lesson, this lab pretty much runs itself. The entire test only takes 20 minutes. After students are done with the test and clean-up they have the option of beginning the write-up for this test. The directions for the lab are found at the end of the In the Heat of the Summer packet.
During this time I ask students to observe their data. Every student in every class gets the same result. In our murder mystery, I have assigned the role of guilty party to Marianna Goldbloom, so the students name her as the suspect who was in possession of the ink pen that matched the ink found on the threatening note. Here is a chromatography debrief video that shows students explaining their results. Here is a chromatography results picture of a sample chromatogram.
In this sample of student work a student has written the paragraph for the report that ties the whole unit together. The definition of accuracy is not quite right because accuracy is a reflection of true value, and students do not know what the real value is. However, one thing that I am quite pleased with is that this and most other students do not simply draw a conclusion based on their own evidence. Rather, they compile data from another group or the whole class as a means for measuring precision, or consistency of data. While precise data is not always accurate, accurate data must be precise. The student recognizes that similar results from other groups increases the likelihood that the data is accurate. The ideas of replication of results is a hallmark of science, and I am pleased that my students model it in their work.