Written by: Science Made Fun! on September 13, 2012 @ 2:06 pm
Autumn is one of the most beautiful times of the year in many places around the world. But have you ever thought about why some trees turn yellow and others turn orange?
Perhaps you’ve noticed that some years the trees seem more brilliantly colored than the year before? Grab a rake and let’s head out into the woods as we explore the science behind autumn’s falling colors.
Have you ever thought about…
- Why does a tree have leaves?
- Why do leaves change color and fall off trees as winter approaches?
- What effect does the weather have on leaf colors?
Did you know?
Before we learn why leaves change color in autumn, let’s talk about what leaves do during the rest of the year. Each leaf on a tree is like a tiny solar panel, gathering sunlight the tree uses to make food. Sunlight helps turn water and carbon dioxide into oxygen and glucose, a sugar that the tree uses for food (energy) to grow. This process of converting water and carbon dioxide into oxygen and glucose is called “photosynthesis.” A chemical called “chlorophyll” helps the process of photosynthesis occur. Chlorophyll is also what gives plants their green color. As summer ends and autumn begins, the days get shorter and shorter. With fewer daylight hours, leaves are not able to make as much chlorophyll as they can during the long daylight hours of spring and summer. As the chlorophyll fades, we are able to see other colors, such as orange and yellow, emerge.
Many people mistakenly believe that weather makes leaves change color. While this is not true, weather can affect how vibrantly the colors appear. If the weather is too hot or cold, the leaves will not be as bright as they begin to change. The best weather for brilliant autumn foliage is sunny, warm days and cool nights.
Water also plays an important role in autumn leaf colors. If a tree doesn’t receive enough water, the leaves will die faster and fall to the ground. If there is too much rain, the tree won’t receive enough sunlight, and the leaves will not be brightly colored. You may be surprised to learn that each leaf has small amounts of other colors in it year-round, even if we can’t see them. During the spring and summer, chlorophyll overpowers the other colors, and all we see is green. With less chlorophyll to give the leaf its vibrant green color, we begin to see the other colors, such as orange, yellow, and red, which have been there all along.
Try it out!
Autumn is the perfect time to get hands-on & investigate the beauty of the environment around you!
Separate Colors in a Green Leaf using Chromatography -
What you need:
- leaves, small jars (baby food jars work well)
- covers for jars or aluminum foil or plastic wrap
- rubbing alcohol, paper coffee filters
- shallow pan, hot tap water, tape, pen
- plastic knife or spoon, clock or timer.
What you do:
- Collect 2-3 large leaves from several different trees. Tear or chop the leaves into very small pieces and put them into small jars labeled with the name or location of the tree.
- Add enough rubbing alcohol to each jar to cover the leaves. Using a plastic knife or spoon, carefully chop and grind the leaves in the alcohol.
SAFETY NOTE: Isopropyl rubbing alcohol can be harmful if mishandled or misused. Read and carefully follow all warnings on the alcohol bottle.
- Cover the jars very loosely with lids or plastic wrap or aluminum foil. Place the jars carefully into a shallow tray containing 1 inch of hot tap water.
SAFETY NOTE: Hot water above 150 F can quickly cause severe burns. Experts recommend setting your water heater thermostat no higher than 125 F.
- Keep the jars in the water for at least a half-hour, longer if needed, until the alcohol has become colored (the darker the better). Twirl each jar gently about every five minutes. Replace the hot water if it cools off.
- Cut a long thin strip of coffee filter paper for each of the jars and label it.
- Remove jars from water and uncover. Place a strip of filter paper into each jar so that one end is in the alcohol. Bend the other end over the top of the jar and secure it with tape.
- The alcohol will travel up the paper, bringing the colors with it. After 30-90 minutes (or longer), the colors will travel different distances up the paper as the alcohol evaporates. You should be able to see different shades of green, and possibly some yellow, orange or red, depending on the type of leaf.
- Remove the strips of paper, let them dry and then tape them to a piece of plain paper. Save them for the next project.
Separate Colors in a Fall Leaf using Chromatography -
This FUN experiment uses the same items used in the experiment above except you will need to switch out the green leaves for leaves that have already changed color. You may have to wait much longer in steps (4) and (7). There is normally much less of the other colors in the leaves compared to the green chlorophyll.
Observe How Light Affects Color Development -
What you need:
- a tree with leaves that turn red in autumn
- aluminum foil or heavy paper and masking tape.
What you do:
- Before the leaves turn colors in the fall, find a maple tree, flowering dogwood, sweet gum, or other tree or shrub that you know will turn bright red or purple.
- Find several leaves that receive bright sunlight, and cover part of them with foil or heavy paper and tape.
- After the leaves have changed color, remove the covering and observe the different colors underneath. These are the colors that were in the leaf all summer. The bright reds and purples are only made in the fall, with exposure to light.
The next time you take a nature walk around your block, pluck a few green souvenirs along the way and try this colorful activity! Don’t worry if you live where the leaves have already fallen from the trees, you can substitute with some leaves from fake trees from your local craft store. Leaf prints are a unique gift from you — and from nature!
Think About it Some More!
Want to learn more? Check out these additional resources to fall in love with science this Autumn!
- Nature Detectives: Leaf Identification Sheet
- Design Your Own Fall Leaf Book
- State by State Guide to Fall Colors
- Weather Channel: U.S. Fall Foliage Maps
- Learn About LeafSnap - The Original Leaf ID App