Leaf Chromatography

Elise Flynn

Grade 5 / 6

I’m so grateful to be teaching in an outdoor school in Massachusetts during peak foliage season. Our environment is rich with learning opportunities and it is a beautiful time of year to be outside. Looking around the explosion of color in our New England fall landscape, I knew that I wanted to do a lesson on fall foliage. I wondered, why do the leaves change colors in the fall?

I wanted to conduct an experiment to find out. To prepare students for this task we discussed the scientific method for conducting experiments. We wanted to see the different colors in our leaves and we each needed to sample one type of leaf. We talked about having a variety of tree types and colors. We discussed the importance of not cross contaminating samples; someone testing yellow birch leaves would not want to collect any birch leaves that were still green. If green and yellow leaves were mixed, we would not know which kind of leaf the chemical compounds came from. We looked around our environment and identified the types of leaves each of us would collect before we stood up to find them. I had been worried that we may not be able to reach the fresh leaves from the ground, but luckily, there was an intense rain and wind storm the night before and that morning, so there were many fresh leaves on the ground. No one had to climb any trees to retrieve samples. Our class collected yellow birch leaves, green birch leaves, red maples leaves, and green oak leaves. We took our samples to our rain shelter to begin.

A leaf chromatography experiment can be conducted to see these 3 chemicals separated. Each student found a sample of leaves to cut up into small pieces. Then students muddled the leaves into rubbing alcohol. We used mason jars for this process. The jars with muddled leaves and alcohol were put into a dish that we filled with warm water to speed up the process. We placed filter strips in the jar touching the liquid on one end and folded out over the edge of the jar on the other side. After waiting one hour, the chemical compounds traveled up the filter strip and were visible. The different chemicals have different polarities which cause them to stick to the paper differently. Some compounds travel up the strip easily and can be seen at the top of the filter strip. Others stick more and travel a shorter distance up the filter.

In preparation for teaching students about why leaves change color, I did some research. Chlorophyll is the well known chemical that gives leaves their green color and is essential to the photosynthesis process. The chemical compounds that give leaves their yellow and orange color are called carotenoids, like carrot. Carrots also have carotenoids. Leaves have carotenoids and chlorophyll throughout most of the season. During growing season, chlorophyll is being broken down and reproduced. As it gets colder, the cold night air closes off the veins of the leaves. It becomes harder for water and nutrients to move and rebuild chlorophyll. Chlorophyll breaks down and is not reproduced; the green color disappears. Without chlorophyll, the yellow and orange of the carotenoids can be seen. Anthocyanins give leaves their red color. They are actively produced in the fall. Scientists have no definitive answer to why trees produce anthocyanin. Two theories are that they provide a defense against migrating insects and that they provide “sunscreen” for the leaves left on the tree after the top ones fall.

I wanted students to hear and say the names of all of these chemicals but I did not want the complexity of their spelling and pronunciation to interfere with learning. Students were given a color printed guide to the different chemical compounds with the spellings of the names and descriptions. Students were also given an elementary science experiment template. When writing their hypotheses, students identified which color compounds they would see on their filter strips. Red? green? yellow? A combination of 2 or all of these colors?

We conducted this experiment on a rainy day under a tarp shelter. We left our filter strip in our sample to let chromatography happen. Unfortunately, I had not anticipated the wind blowing the filter strips directly into the muddled leaf alcohol sample. I remedied this an hour into our experiment with new filter strips and paper clips. This oops was not the end of our experiment though. It allowed us to discuss the limitations of the results we found. When conducting any experiment there is always a potential for mistake. Students were able to discuss how this could have affected their results when drawing conclusions.