Biology
Standard 2, Objective 2
Title: Watching Photosynthesis
Description: Students will use bromothymol blue as an indicator of carbon
dioxide absorption by water plants.
Materials: two large test tubes, elodea or other water plant, bromothymol blue
indicator (prepare the solution by adding concentrated bromothymol blue to water
until you get a bright blue but transparent solution), stoppers, light
source(sunlight works best), straw, test tube rack or glass beaker, ruler, student
sheet (see below)
Time needed: 80-90 minutes (can be split into 2 class periods)
Background Needed: Bromothymol blue is blue in a neutral or basic solution. It
turns greenish and then yellow as it is acidified. Photosynthesis in water plants
can be observed by acidifying the bromothymol blue with carbon dioxide from
breath through a straw to change the solution to yellow. Placing a plant in the
solution will remove the CO2 from the water, changing it back to blue. Sunlight
works best as a light source and can make the color change in about 20 minutes.
If artificial light is used it will take at least 30 minutes if not more, depending on
the strength of the bulb.
It is important that the bromothymol blue solution is not too heavily
acidified by the students’ breath. If too much CO2 is added, it will be difficult for
the plant to remove it in a timely fashion. You can create a “standard” color by
acidifying a test tube with bromothymol solution and placing a stopper on it. The
CO2 will diffuse out of the solution without the stopper. Students can use this
solution as a standard to make their own from.
Procedures:
1. Have a student demonstrate bubbling breath through a beaker with the
bromothymol solution.
2. Discuss with the students how CO2 from your breath creates a weak acid in
the solution and changes the pH, altering the color. Since it was respiration that
created the CO2 in the students breath, photosynthesis by a plant should reverse
the color change.
3. Show students where the materials are and read procedures. You may wish
for different student groups to use different light sources to compare results.
4. Allow time for data collection.
5. Have students present their results and answer the analysis questions and
write a conclusion.
title: Watching Photosynthesis
name_______________
purpose: To observe the absorption of carbon dioxide as a evidence of
photosynthesis.
materials: two large test tubes, elodea, bromothymol blue solution, stoppers,
beaker, light source, straw, test tube rack, ruler
procedure:
1. Get the amount of Bromothymol Blue solution suggested by your teacher.
Blow through the straw until the color changes to yellowish-green. It should be
the same shade as the control in the front of the room. Fill a test tube with this
solution.
2. Place the elodea in this “test” tube and stopper.
3. Fill the other tube with the original blue solution and stopper it. It is the control
tube.
3. Place both tubes in a beaker or test tube rack.
4. Place 6 inches from a light. Turn on the light and start timing.
5. Stop timing when the color in the tubes matches.
6. Record the time and the number of watts in your light bulb.
prediction: Which bulb will go the fastest?
data:
time start: ___________
time finish: ____________
watts: __________
analysis:
1. What amount of watts was the fastest?
slowest?
2. What gas makes the bromothymol blue yellow?
3. What makes it turn blue?
4. What gas did elodea make in the light?
5. Write the equation for photosynthesis:
6. What part of it did you prove today?
Conclusion: