1. Elodea & Photosynthesis
Light, Photosynthesis, Carbon Dioxide and Bromothymol Blue
Research Question: Will the amount of light an aquatic plant receives affect the
level of carbon dioxide in the water around it?
Background Info:
• Bromothymol blue (BTB) is an indicator of pH.
•When CO2 is bubbled through water, carbonic acid is formed. CO2 + H2O H2 CO3
•BTB changes color toward yellow in the presence of an acid.
•The more yellow the BTB is, the more carbon dioxide is present.
Pre-lab Questions:
Note: you should read through the procedures on the next page to answer all of these.
1. Bromothymol blue turns ___________________ when _________ is bubbled through it.
2. The formula for photosynthesis is:___________________________________________
3. What color should Bromothymol blue turn if photosynthesis is taking place? _________
4. What is your hypothesis for this lab?___________________________________________________
5. What is the independant variable? __________________________________________________
6. What is the dependant variable? ____________________________________________________
7. List the controlled variables: _________________________________________________________
8. ______________________________________________________________________
9. What is the purpose of setting up test tubes 2 & 4? ____________________________________
10. ______________________________________________________________________

2. Materials
1. (4) test tubes
2. (4) pieces of Parafilm
3. (1) test tube rack (remember you are sharing racks, so make sure your tubes are well labeled)
4. bromothymol blue solution (BTB) (remember: we bubbled CO2 through it to make it green!)
5. (2) 10cm pieces of an aquatic plant
6. tin foil
Procedure :
1. Label the test tubes: #1Pd.__ ; #2 Pd.___ ; #3 Pd.__ ; #4 Pd.__ (Pd. = your class period)
2. Fill each test tube three-fourths full with green BTB.
3. Cut two 10 cm pieces of a water plant.
4. Place one piece of plant into test tube #1, and one piece into test tube #3.
5. Seal each test tube with parafilm by gently stretching it over the opening and wrapping the
remainder around the top of the test tube.
6. Draw an accurate diagram of the experimental set-up (use colored pencils) and all four test tubes.
Make sure you do this on unlined paper and that you date and label everything.
7. Make a table to record your quantitative and qualitative data. Be sure to title the table, date it and
title each column of the table.
8. Hold a piece of white paper behind the test tubes to record the color of the solution that you now
see in all four test tubes.
9. Place test tubes 1 and 2 in the test tube rack under the light until tomorrow.
10. Place test tubes 3 and 4 in the dark, by covering them with foil.
11. Clean, organize and dry your lab table. Please make sure there is no debris in your sink.
12. Answer all analysis questions that you can, then return to your table group where you will work on
your cell study guide quietly.
Day 2:
1. Record all qualitative and quantitative data in your data tables.
2. Please create diagrams of each test tube.
Be sure to hold a white paper behind each tube as you observe the colors.
3. Finish all analysis questions and hand in your lab with all of your data.
Clean- up
Pour the BTB solution down the drain and place the plant in the beaker at your lab table.
Rinse the test tube and place it up-side-down in the test tube drying rack.
Please make sure there is no debris in your sink.
Photosynthesis Lab Analysis Questions:

3. Analysis and Conclusions:
1. What are the three ingredients that a plant must obtain in order to do photosynthesis?
______________________________________________________________________
______________________________________________________________________
2. How was carbon dioxide supplied to the plant? ______________________________
3. How did we observe that carbon dioxide was present in the water?
______________________________________________________________________
4. At the end of the experiment, did any of the test tubes change color? Explain in detail!
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
5. From your observations, can you conclude anything about the gases released by a
plant when it is exposed to the light? or the gases used by a plant when it is in the dark?
(Use the data to support your response.)
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
6. What were some sources of error in this experiment? Select at least one and
describe how it could have affected the outcome.
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
______________________________________________________________________
7. Why did we cover the test tubes with parafilm rather than leave them open?
______________________________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
Please turn in the prelab questions and the analysis questions with your formal lab report.