3.4 LIGHT MY FIRE
(Level: 10 appl :: Photosynthesis)

Purpose: To see if oxygen is produced during photosynthesis.

What You Need: 2 test tubes :: sodium bicarbonate :: 2 small beakers :: wooden splint :: 2 funnels :: aquatic plant such as Elodea :: rubber stopper

What You Do:

1. Insert freshly cut ends of the Elodea into the stem of one funnel.
2. Fill both test tubes and funnels full of water. You can do this by putting the whole apparatus (beaker, funnel, and test tube) in a sink full of water.
3. Arrange the material as shown in the diagram.
4. Dissolve 2-3 cc (1/2 tsp) sodium bicarbonate in the water of each beaker. This will supply the carbon dioxide needed for photosynthesis.
5. Shine a very bright light on the set-up for at least 48 hours. (More sodium carbonate can be added during the waiting period).
6. Test the gas that collects to see if it is oxygen. Do the test as follows:
   • Fill the beaker full of water
   • Lift the test tube off the funnel, but do not let any air in
   • Put a stopper in the mouth of the test tube
   • Lift the test tube out of the beaker
   • Turn the test tube upright. The gas should now be on top of the water
   • Remove the stopper and quickly put the glowing splint into the gas. (Have the glowing splint ready before you remove the stopper).

Questions:

1. Describe the gas that collected in the experimental test tube.
2. What did this gas do to the glowing splint.
3. How can you be sure that the gas is oxygen and not just air?
4. Why was a control needed?

Click here to see the answers to these questions.
Click here to see the list of activities for this unit.