How do leaves change colors? Paper Chromatography experiment
A month of thanks giving: I am thankful for the colorful leaves that give autumn its beauty.
This is an adaptation of a lab that I used to do with my biology students, and what better time to share this with children than now. I will try to simplify the explanation, and my apologies to all my fellow molecular botanists out there.
Objective: to separate the pigments within a leaf to show all the pigments that are “hiding” inside the leaf
- spinach leaves (or any soft green leaf)
- white construction paper
- clear plastic cup
- rubbing alcohol
- stapler or tape
1. Cut a piece of construction paper about 4 inches tall and long enough to roll into a circle that fits inside the clear plastic cup.
2. Use the pencil and ruler to mark a line about 2 cm above the edge of the construction paper.
3. Place the spinach leaf on top of the paper, and use the coin to rub the leaf on top of the pencil line. (Parents may have to assist children with this step.) Make sure that the pigment line is heavy.
4. Roll the construction paper into a cylinder, small enough to fit easily inside the cup and stand up on its own. Staple or tape the paper cylinder together.
5. Pour a thin layer of rubbing alcohol into the clear plastic cup, just enough to cover the bottom of the cup and will not touch the pigment line when the paper cylinder is placed inside.
6. Place the paper cylinder inside of the cup.
7. Observe the pigment line as the rubbing alcohol absorbs and draws the pigment up the paper. If the leaf contains different pigments, they will separate out and create multiple color bands along the paper.
8. Before the alcohol soaks all the way up the paper cylinder, remove it from the cup, and allow the paper to dry.
9. Dispose of the alcohol properly by flushing with plenty of water down the sink (or recycle back into the alcohol container if this won’t be reused for medical reasons).
10. Observe the paper cylinder for all the different pigment colors.
The chlorophyll (green pigment) is predominant within a green leaf, but most plants have accessory (secondary) pigments which are not visible in the leaf coloration in the presence of abundant chlorophyll. When the temperature cools in the evenings and warms in the day and as night lengthens, this causes the organelles which contain the chlorophyll pigment to break down and produce less pigments. As they do, the leaf loses its green color. The accessory pigments (carotenoids and anthocyanins) which are typically red, orange, yellow, browns begin to show through and are made in greater proportion during the fall months.
The paper chromatography experiment is a way of showing what pigments are inside of the leaf, “hiding” behind the green chlorophyll. The paper is made of fibers that weave together, and the pigment molecules dissolve in the alcohol. As the alcohol soaks up into the paper, it drags with it the pigments. Some of the color pigments are bigger than others, and the bigger ones cannot squeeze through the paper fibers as easily. Those pigments are “stuck” and leave a color spot behind. The smaller pigments travel with the alcohol further up the paper. A child should be able to observe how many color spots are left on the paper and draw a conclusion about the pigments which are “hiding” inside of the green leaf.
Follow-up experiment: This experiment may be repeated with a black marker in place of the spinach since black ink is made of multiple colors together. Alternatively, kids may use many different colored markers to draw over the pencil line, and follow the same procedure to separate the mixed up colors. If the markers are water-soluble, water may substitute alcohol in the experiment.