This investigation is a visual way to introduce students to photosynthesis. As Elodea uses carbon dioxide from the initial solution, a change in pH occurs, causing a color change in the solution. The change is detectable in as little as 30 minutes to an hour. The investigation may be used as an introductory guided inquiry or a student-led inquiry investigation. In the latter, students devise a way to quantify the CO2 concentration (pH) over a set amount of time.
Bromothymol blue will stain your hands and clothing. Wear gloves and goggles, and practice safe laboratory procedures when performing this activity.
Bromothymol blue is categorized as non-hazardous. Dispose in a manner consistent with federal, state, and local regulations. Let Elodea dry out completely and dispose of it on dry land.
1. With a permanent marker or wax pencil, write your initials or other mark to identify your group on the capped tubes. Toward the top of a tube, write “D” for dark. On the other tube write “L” for light.
2. Obtain two 6-cm sprigs of Elodea and place a spring in each tube.
3. While working over a paper towel, use the dropping pipet to fill both tubes to overflowing with the yellow-green bromothymol blue solution and cap both tightly. Clean up any spills.
4. Quickly make observations of both tubes.
5. Immediately wrap the tube marked “D” with aluminum foil. Cover the entire tube to prevent any light from reaching the Elodea.
6. Place both tubes in the sunlight or under a light bank.
1. There are many items for students to pick up. It may be helpful to place the equipment in a basket or resealable bag.
2. Make sure students keep Elodea in a strand and do not tear off leaves.
3. Make sure the tube is completely covered by aluminum foil. No light should penetrate the tube.
4. The time can vary to fit your schedule, but allow a minimum of 30 to 45 minutes.
Describe and compare the color of the bromothymol blue solution in the tubes.
The solution in both tubes was a yellow-green color. The one exposed to light turned dark green and then blue.
Explain the chemistry behind any color change that has occurred in any tube.
The tube exposed to light turned a darker green to blue depending on the amount of exposure time. This occurred because the Elodea photosynthesized, using up the carbon dioxide in the solution.
Since the reactions exist in equilibrium, more carbonic acid dissociates to form more carbon dioxide and water, replacing the carbon dioxide removed from the solution by the Elodea.
As the carbonic acid dissociates, the solution becomes more basic and turns more green. Over enough time it can turn blue. Bromothymol blue is a pH indicator that turns green and then blue as a solution becomes more basic.
Summarize the process of photosynthesis using chemical formulas and a color coding scheme that matches lab data.
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