An Elementary Science Experiment

When I began researching this topic for an elementary school science project, I found out something amazing; none of the adults I talked with knew the answer to this question!  They all knew that it had something to do with a change in temperature, but that was about the extent of their knowledge.  I shouldn't have been surprised because I didn't know either!  Now, I do.

As I learned why, I also came across a fun, hands-on experiment for children to discover the answer to this question.  This experiment works best when students are in small groups.

Materials:

• Leaves - at least three leaves for each small group (the leaves in each group need to be from the same tree, but different groups can have leaves from different trees)
• Rubbing Alcohol - 2-3 tablespoons per group
• Cup or Small Jar to crush leaves in - 1 for each group
• Plastic Wrap to cover each cup/jar
• Rubber Band to secure the plastic wrap - 1 for each group
• Small Bowl to place the cup/jar in - 1 for each group
• Metal Spoon - 1 for each group
• Paper Towel or Coffee Filter - 1 long strip for each group (the rough school paper towels work great for this - don't use regular paper towels)
• Hot Water - about 1 cup for each group (needs to be hotter than tap water - I microwaved mine and it worked fine)

Experiment:    

1.  Using scissors, cut the leaves into small pieces.

2.  Cover the leaves with the rubbing alcohol.  Using the metal spoon, mash up the leaves until the rubbing alcohol turns slightly green.

3.  Cover the cup with plastic wrap.  Secure the wrap with a rubber band.  Pour about two inches of hot water into the bowl and place the cup of leaves in the bowl.  Leave it in the hot water for at least half an hour.

4.  Remove the cup from the bowl and take off the plastic wrap.  Place one end of the paper towel strip into the rubbing alcohol & leaf mixture.  Make sure that the remainder of the strip is hanging over the edge of the cup.

5.  The rubbing alcohol & leaf mixture will begin to creep up the paper strip.  For best results, leave the strip in the cup for about an hour.  When done, you should have something that looks like this:

This experiment is called leaf chromatography.  Chromatography allows scientists to see the spectrum of pigments within the leaf.  In the chromatography process, pigments are separated by passing them through a medium (the paper towel or filter in this case).  Different pigments are absorbed at different rates.  Chromatography works because different substances have different degrees of solubility (solubility refers to how much of a particular substance can dissolve in a particular solvent – ie. various leaf pigments in rubbing alcohol).  If you look closely at the last picture, you will notice a line of yellow pigment above the green pigment.  This indicates the presence of carotenoid (yellow & orange) pigments in this leaf.

The Explanation:    

Green plants have green leaves, and the leaves are green because of a pigment called chlorophyll.  Plants use chlorophyll to photosynthesize sunlight.  Photosynthesis is the process in which leaves absorb sunlight, combine it with water and carbon dioxide, and turn it into glucose (food for the plant) and oxygen.  Leaves have so much chlorophyll that it hides the other pigments in the leaf.

Most leaves contain two other pigments: carotenoids and anthocyanins.  Carotenoids allow the leaf to absorb wavelengths of light that are not absorbed by chlorophyll.  They create yellow and yellow-orange pigments in the leaf.  Anthocyanins occur as pigments for flowers and fruit.  They create red, purple, and blue colors.

When days get shorter and colder, the chlorophyll doesn’t get enough sunlight to photosynthesize.  When it can't get enough sunlight, it begins to break down.  The break down of the chlorophyll causes the leaf to lose its green color.  As the green diminishes, the other pigments in the leaf become visible.  Trees/plants that have higher amounts of carotenoids will develop yellow or orange leaves, while those with more anthocyanins will have red or purple leaves.

Resources to help teach this concept:

Have fun, and happy experimenting!