INTRODUCTION:

In this experiment you will measure the effect of light intensity on the rate of photosynthesis. You will expose Elodea samples to various light intensities, and determine the relative rates of photosynthesis by observing changes in pH.

Elodea is a common aquatic plant used in aquariums. You can obtain it from the pet store. There are more than one species of Elodea used in aquariums. Any of the species will be fine for this lab.

MATERIALS:

pH paper
4, 250-mL beakers
4, 15-cm-long sprigs of Elodea
250-mL graduated cylinder or measuring cup
Sodium bicarbonate (baking soda)
200-watt light source
Distilled water
Stirring rod
Balance (can be omitted)

PROCEDURE:

1. Mark the four beakers with the designations A,B,C, and D. Using a graduated cylinder, poor 200 mL of distilled water into each beaker.

2.Weigh out three 0.50 g sodium bicarbonate samples on weighing papers.
Remember: If you are using a mechanical balance, first find the mass of the weighing paper, then add 0.50 g to the mass of the paper. Set the balance for the mass you calculated, and add sodium bicarbonate until the pointer rests exactly on zero.

If you are not using a balance, you will need to use equal ammounts of sodium bicarbonate as measured with measuring spoons. Try one-half teaspoon first.

3. Add a sodium bicarbonate sample to beakers A, B, and C, and stir the solutions with a glass stirring rod until the chemical is dissolved.
When you add sodium bicarbonate to water, carbon dioxide (CO₂) will form.

Question 1. How could you use the concentration of CO₂ to measure the rate of photosynthesis?

4. Test the pH of the four solutions and record in the DATA TABLE as pH before experiment.

Question 2. What can you infer about the relationship between the pH of a solution and its CO₂ content?

5. Obtain four 15-cm-long sprigs of freshly cut Elodea. Place one sprig of Elodea in each beaker.

6. Place beaker A in darkness, beaker B in room light, and beakers C and D at a distance of 0.5 m from an electric light lit by a 200-watt bulb.

7. After 20 minutes, use pH paper to test the pH of the solution in each beaker. Record your results in the DATA TABLE as pH after experiment.

Question 3. Why is it important to measure the pH of each solution after a set amount of time?

DATA TABLE: pH data.
Sample A With sodium bicarbonate in darkness for 20 min.
	pH before experiment _____
	pH after experiment _____
Sample B With sodium bicarbonate in room light for 20 min.
	pH before experiment _____
	pH after experiment _____
Sample C With sodium bicarbonate 0.5m from a 200-watt light for 20 min.
	pH before experiment _____
	pH after experiment _____
Sample D Without sodium bicarbonate 0.5m from a 200-watt light for 20 min.
	pH before experiment _____
	pH after experiment _____

CONCLUSIONS:

Question 4. How did the pH of the solution in each of the beakers change after the Elodea was added and the 20 minute time elapsed?

Question 5. What is the relationship between change in pH and rate of photosynthesis?

Question 6. What purpose did solution D serve in this experiment? Did its pH change after Elodea was added?

Question 7. According to your experiment, how is photosynthesis affected by light intensity?

INQUIRY:

If beaker C is covered with an airtight lid and kept in bright light, will the pH rise indefinitely? Why or why not?

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