In this lesson students will analyze the results of using various solvents in paper chromatography to analyze the pigments in leaves. This lesson aligns to NGSS HS-PS1-3 because it helps students think about the structure of substances. It uses the NGSS Science Practice Planning and Carrying Out Investigations by providing students the opportunity to perform a paper chromatography. It aligns with the Science Practice Analyzing and Interpreting Data by giving students the opportunity to analyze and perform simple mathematical analysis of their data.
The reasons why paper chromatography works are based in part on the idea of solubility. Students should have some familiarity with the concepts learned in the solubility lesson in order to understand this lab.
The materials needed for this lab are:
Students read and perform the first two steps in the the procedure for conducting a Paper Chromatography with Leaves lab. This entails cutting leaves into tiny pieces and then soaking them in alcohol.
They need to do this quickly and immediately in order to have enough time to perform the lab in one class period.
I begin by giving a brief overview of chromatography. The first 8 slides from Paper Chromatography* gives a good overview with pictures that help students visualize the process of chromatography. After showing these slides I ask students to use a Think Pair Share Protocol for this question:
"What is paper chromatography and how does it work?"
I walk around and listen to the conversations, and then I ask a few students to share out something they know about paper chromatography. I keep asking questions until students have explained that paper chromatography is a process used to separate compounds from mixtures by dissolving the compounds in a solution, and those compounds are then deposited at different heights on the filter paper.
I then give students time to read the procedure section of the Paper Chromatography with Leaves Lab.
Once students have had a chance to read, I use cold call to make sure that students have reviewed and understood the rest of the procedure.
*From Oconto Falls Public Schools, WI
Cold call is a basic classroom structure that our school uses to keep students on their toes. Here is how it works. On the first day of school every student fills out an index card with their name on it. I have a pile for each class. I call on students randomly using the cards. The cards help me to make sure I call on every student periodically. Students do not know when I will call on them, but for students with social-emotional or processing speed challenges I may give them advance warning. Students who are able to show that they are paying attention earn a positive effort grade. They can do this by either providing a correct answer or by offering some evidence that they have been following the class discussion. Students who cannot earn a negative effort grade.
Closely associated with this concept is no opt out. This means that if a student does not know an answer, another student will be cold-called. Once we find the correct answer, students who were unable to provide the correct answer are either asked to restate the correct answer in their own words, or show an increased understanding with a similar question.
At the beginning of the year it is important to point out to students that this system is designed to increase the accountability and flow of the class, and it is not designed to call someone out. Students can keep the class moving by simply saying I am not sure and I can come back to them at a subsequent date. I am careful not to sound judgmental in front of the whole class when a student does not know an answer or was not paying attention, but I do check in with the student later. If the student is having a bad day (sick, family problems, etc.) I empathize, and if they just were not paying attention, I remind them of the importance of doing so.
While we are waiting for the various leaf pigments to dissolve in the alcohol, I use the time to teach about Rf values and give students time to practice calculating them. Students get practice calculating Rf values using the Calculating Rf values worksheet.
Rf values are used in chromatography to quantify the relationship between the distance a compound travels compared to the distance that the solvent travels up the filter paper. By using Rf values students have a quantified value with which they could then compare to known values or to other chromatograms. This video shows me teaching how to measure and calculate Rf values.
After comparing their answers with a partner, and to the Rf values answer key, the pigments have had time to dissolve in the alcohol. Students now do steps 4-8 of the Paper Chromatography Lab. Students need to ensure that the paper is touching the solvent without getting bunched up at the paper-solvent interface. If their paper is too long they simply have to roll it up on the stirring rod to make it the right height. Here is a paper chromatography photo of the apparatus. Before they hang their chromatograms to dry, they should be sure to mark the point to which the solvent traveled because that is one of the 2 values need to calculate the Rf values.
During this time, I ask students to compare data to answer the original question--do leaves from different tree species create unique chromatograms? I do not have sufficient data from this lab to answer that question because the students ran out of time to do this lab properly, but the chromatograms I made appeared nearly identical.
See my reflection about why this was not necessarily a failure, and what I did at the end of a class when no data was available. Also, I note that students will have another chance to conduct a chromatography test in the chromatography lesson of the forensics unit.
Sadly, I made one crucial mistake in administering this lab. My hope was that students would be able see the different compounds from the leaves nicely separated out on a chromatogram; after all, this is what happened when I conducted the lab. However, there was one thing I did not anticipate, namely that students would take longer to set up the lab than I had, and therefore they would not get the same results. The leaves in the alcohol do require a fair amount of time to soak before the compounds are dissolved in sufficient enough quantity that they will then be visible on the chromatogram. Next year I may shred the leaves in advance in order to buy some more time for creating the solution.
When I was first starting as a teacher I defined a “failed lab” as a lab that did not produce the results that I wanted or expected. This is because I was new, and before the days of the NGSS. It was a time when I did not understand how important it is to let students analyze a procedure to fine-tune it. I did not understand how important it is for students to understand how challenging the scientific endeavor has been for humanity—all the false starts, all the failed experiments, all the wrong paths.
Now a lab that does not produce the results I expected is almost as useful as a lab that does. In this procedure most students did not know why I got a nice chromatogram while they did not, but in this video showing how to handle a "failed" lab I model one way to handle a lab that did not produce the expected results.