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
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.