A page from the "Causes of Color" exhibit...
If chlorophyll is green, how is it possible that leaves turn red, gold, and orange in the fall? With a few simple materials, you can easily discover the answer to this question.
- Green leaves (spinach works best)
- Isopropyl alcohol (rubbing alcohol)
- Coffee filter
- Small glass jar (mayonnaise, pickle, or jelly size)
- Mixing bowl
What To Do
- Pour about half an inch of rubbing alcohol into the bottom of a clean, empty jar. (Note: Alcohol is flammable and should be used with care.)
- Tear up the green leaves into small pieces.
- Put enough green leaf pieces in the jar with alcohol to cover the bottom of the jar.
- Cover the jar and place it in a bowl of hot tap water for about one hour.
- Stir the leaves vigorously from time to time. If the water becomes tepid, replace it with hot water.
- Prepare the coffee filter by opening it up and cutting a 1” x 6” rectangular strip from the middle of the paper.
- When the liquid in the jar is a dark green color, place the filter strip into the jar so that the bottom edge of the strip is in the green liquid.
- Watch as the liquid seeps up the paper. Remove the paper strip once the alcohol reaches the top. Let it dry.
- You will see a yellow band of color and an orange band of color.
How It Works
Green leaves contain a mixture of two or more of the following pigments: chlorophyll a, chlorophyll b, xanthophylls, carotene, and phaeophytin. Chlorophyll is a compound that is known as a chelate. A chelate consists of a central metal ion bonded to a large organic molecule, composed of carbon, hydrogen, and other elements such as oxygen and nitrogen.
Chlorophyll has magnesium as its central metal ion, and the large organic molecule to which it bonds is known as a porphyrin. The porphyrin contains four nitrogen atoms bonded to the magnesium ion in a square planar arrangement. Chlorophyll occurs in a variety of forms. Read more about the molecular structure of chlorophyll.
Chlorophyll absorbs certain wavelengths of light within the visible light spectrum. As shown in detail in the absorption spectra, chlorophyll absorbs light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum. Green light is not absorbed but reflected, making the plant appear green.
Your filter paper strip is a chromatogram. The chromatogram shows the different pigments. Chlorophyll a is blue-green, chlorophyll b is yellow-green, carotene appears bright yellow, and xanthophyll is pale yellow-green. (You may only see two of these pigments.) The colors of leaves in Fall can be red from anthocyanin pigments, which are potent antioxidants found in many plants such as beets, purple grapes, violets, and hyacinths. The orange colors come from carotene and the yellow colors from xanthophylls.
There are other types of chromatography. For example, a thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with the a thin layer of adsorbent material, usually silica gel, aluminum oxide, or cellulose. Gas chromatography is a common type of chromatography used in organic chemistry for separating and analyzing compounds that can be vaporized without decomposition. Column chromatography and high-pressure liquid chromatography are two other types.
Use a mortar and pestle to grind the spinach leaves with one tablespoon of alcohol. When you have a dark green liquid, use a medicine dropper to transfer the chlorophyll to the bottom of the coffee filter strip about an inch from the end of the strip. As the drop dries, repeat this three of four times, and you will have a dark sample of chlorophyll. Place the bottom of the strip in a glass that has about a half-inch of acetone nail polish remover. Be sure the green dot is above the liquid. This method is takes more effort but sometimes produces better results.
Ink chromatography is also fun. You need three strips of coffee paper filter and a black marking pen and three liquid solvents, such as water, alcohol, and acetone. With the marker, draw a line about one inch from the end of each strip. Dip the end of one strip in water, one in alcohol, and one in acetone. One the liquid has inched up the strip, remove and let dry. You will see a rainbow of colors.