Why Do Leaves Change Color in the Fall?

It’s the perfect time of year to explore why and how leaves change color!  This leaf chromatography experiment makes the answer to those questions a bit more tangible and understandable.

Used with permission from Microsoft
Used with permission from Microsoft

Why DO leaves change color?

So let’s start off by making sure we all understand how leaves change color in the fall.  Basically, leaves are made up of several components that affect their color.  Chlorophyll is the part of the leaf that gives it its green color, and its presence is so strong that it can cover up the color of the other components of the leaf. In the fall, trees sense that the days are becoming shorter and the weather is cooler.  As a result, it stops sending up water and energy to the leaves and so the chlorophyll dies.  Once the chlorophyll is gone, the other colors can shine through.

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Leaf Color Activity

For this experiment we are going to use chromatography to learn more about leaves changing colors.  Chromatography is when you separate the components of a liquid using a piece of filter paper.  Components that are not fully dissolved will be heavier (or more dense) and will only travel a little ways on the filter paper.  Pigments that are able to dissolve more, will be able to travel farther up the filter paper.   So for this experiment, we are going to dissolve bits of leaf in rubbing alcohol, and then use filter paper to see the different colors of the components of the leaves.

Materials:

  • A couple of green leaves from different deciduous trees that are just starting to turn color (try to get trees that will turn different colors (orange, yellow, and red)
  • One leaf from the same trees that have already turned their autumn color
  • Rubbing alcohol
  • Coffee filter paper, cut into long strips
  • Tape
  • Jars (one for each type of tree)
  • A muddler or a pestle (to crush the leaves)
  • Saran wrap
  • A deep pan to place the jar in a hot water bath
  • A pitcher with hot tap water

Procedure:

  1. Start by asking your class to brainstorm things that happen outside in the fall (it gets cooler out, the days get shorter, leaves fall, leaves change color)  Ask students for any guesses on how or why leaves change colors?  Tell students that we are going to do an experiment to help us understand how leaves change color.
  2. Before doing this experiment, take a walk and make note of where you can find some deciduous trees that are just barely starting to change colors.  As a class, walk to these trees and pick a leaf off the tree to show what color the leaves are turning, as well as a couple of green leaves (from a part of the tree that receives plenty of sunshine–this is important!!).  Try to find leaves from trees that turn different colors (some red, some yellow, some orange…).  Make sure you get at least one colored leaf from each tree and at least a couple of green leaves from each tree.  If you don’t have time to collect the leaves as a class, you may collect leaves before class time, but be sure to collect them on the same day as you are going to do the experiment.  If they are picked the day before, the leaves may dry out a little bit, and not reveal their colors.
  3. Bring the leaves indoors and put the colored leaves aside.  Tear up each green leaf into tiny pieces, putting the leaves from each tree into a different jar.  Pour enough rubbing alcohol onto the leaves to completely cover them.
  4. Then, use a pestle or a muddler to further grind up each leaf so that the liquid is starting to turn green.  The better you can mash the leaves, the better your results will be.  Use Saran wrap to cover the lids of the jars.
  5. Next, place the jars into a deep pan and pour hot tap water into the pan, so the jars are in a hot water bath.   Let jars stand for about an hour, replacing the hot water every twenty minutes or so, in order to keep the water hot.  By now, the liquid in the jars should be a dark green color.
  6. Place a strip of coffee filter in each jar, so that just the bottom of the strip of paper is in the liquid.  Use the tape to affix the filter paper to the top of the jar, so the paper doesn’t fall inside. Let jars sit for one more hour, with the filter paper soaking up the liquid.
  7. Check your filter papers.  The a green line of chlorophyll will be at the bottom of the strip of filter paper, while the other colors contained in the leaf will be found higher up the paper.  Compare your filter papers with the colored leaves from each tree.

Colors that may be present on the filter papers:

  • Green–caused by the presence of chlorophyll, which performs photosynthesis to create the energy for the tree.
  • Yellow–carotene is responsible for giving leaves a yellow color in the fall (this is the same pigment that colors carrots and corn)
  • Orange–the presence of anthocyanins will give leaves an orangish color in the fall (this same pigment is found in raspberries and blueberries).
  • Red–in order to turn red, leaves must contain anthocyanins and also have an excess of glucose (sugar) in them when the weather turns cooler.  The presence of glucose, cool nights, and sunlight are all necessary to form a bright red color (if you do this experiment in the summer, you will not see the color red, as there are no cool nights to react with the excess glucose)
  • Brown–brown is produced by tannins in the leaves, and is the final color all leaves will turn

There are a few other factors that will affect the coloring of a leaf as well.

  • Rainfall — a super dry summer will often cause leaves to turn from green directly to brown, and a summer with a lot of rain can cause the anthocyanins to leach away, resulting in a lighter color leaf
  • Available sunlight — if a tree is going to turn red, the leaves that get the most direct sunlight will turn the brightest red; without direct sunlight the leaves would be more orangey
  • Soil acidity — more acidic soil will sometimes yield a yellower leaf, while a more alkaline soil can make the leaves more reddish or purplish.

Other activities you may want to try:

Used with permission from Microsoft
Used with permission from Microsoft

Check out this activity where you test the pH of leaves and soil to predict what color the leaves will turn

Check out this activity where you cover a section of some leaves with masking tape to see how they turn color without the presence of sunlight (scroll all the way to the bottom to find Project 3)

Make connections across your curriculum with these math and language arts activities that are centered around leaves changing colors

Create a web quest for your students using any of these pages full of kid-friendly information and fun activities focusing on leaves changing colors

Here is a web quest someone already created on this subject, designed for fourth graders (although the articles it links to require some pretty high reading levels)

Time lapse video of a tree changing color (I like that you can pause this video to point out how leaves that get the most direct sunlight turn red, while the other leaves turned orange or yellow)

Another time-lapse video, this one is a gorgeous collection of time-lapse autumnal change occurring in Central Park, NYC, both of trees, as well as individual leaves changing colors

Here is a more detailed explanation about why leaves change color

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