Name ___________________________________________ Period ________ Date __________________
Background:
In this lab, you will investigate the process of photosynthesis. More
specifically, you will learn how the amount of light affects
photosynthesis. In order to do this, you will use an Elodea plant, the
chemical Bromothymol Blue (which acts as an indicator to show if
photosynthesis is occurring). Bromothymol blue works because it is
able to detect the presence of CO2 (needed for photosynthesis) and
O2 (released during photosynthesis) in solution.
Write the equation for photosynthesis.
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Hypothesis:
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Pre-Lab Questions:
1. Predict what will happen to the color of the solution when you blow into the straw (causing the
bromothymol blue + water solution to bubble). Explain your prediction.
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2. Predict what color Tube A will be after the experiment. __________________
3. Predict what color Tube B will be after the experiment. __________________
4. Predict what color Tube C will be after the experiment. __________________
Procedure:
***SAFETY GOGGLES MUST BE WORN DURING THIS LABORATORY***
1. Add water to fill half of each test tube labeled A, B, and C.
2. Using a straw, GENTLY blow into the solution causing it to bubble for approximately 1 minute. ***
CAUTION – ONLY BLOW INTO THE STRAW ***
3.
• Tube A will be the control (no Elodea)
• Tube B will be the Elodea in the dark
• Tube C will be the Elodea in the light
4. Pour 25 mL of the bromothymol blue + water solution into each of the three test tubes.
5. Plug Tube A with rubber stopper (there is no Elodea in Tube 1) and place it in the test tube rack
6. Add a piece of Elodea to Tube B. Using your straw; GENTLY push the Elodea to the bottom of the test
tube. Then plug Tube B with a rubber stopper and place it in the test tube rack.
7. Add a piece of Elodea to Tube C. Using your straw; GENTLY push the Elodea to the bottom of the test
tube. Plug the test tube with a rubber stopper. Then place it in the test tube rack.
8. Place the designated test tubes under a light source and keep overnight.
Name ___________________________________________ Period ________ Date __________________
Results: Record your results in the table below.
Test Tube:
Initial color of BTB
solution
Color after
adding CO2
Color after
24 hours
A – Contains Elodea (in the light)
B – Contains Elodea (in the dark)
C – NO Elodea (in the dark)
Conclusions: Write 2 to 3 sentences based on your results.
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Post-Lab Analysis:
5. What color does the bromothymol blue solution turn after you exhale into it? _____________
Explain your answer.
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6. What happened to the color in Test Tube A? Explain why you obtained the results that you did.
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7. What happened to the color in Test Tube B? Explain why you obtained the results that you did.
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8. What happened to the color in Test Tube C? Explain why you obtained the results that you did.
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9. What is the purpose of the control?
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10. Compare the Elodea in the dark to the Elodea in the light. How do you account for any differences in
color? ________________________________________________________________________________
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11. What gas (or gases) can bromothymol blue serve as an indicator for? _________________________
12. List three things that a plant needs to undergo photosynthesis.
a. ________________________
b. ________________________
c. ________________________
13. Write the overall equation for photosynthesis.
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