PROJECT TITLE:

Oxygen and Photosynthesis

Aim:

To show that oxygen is given off during photosynthesis.

Materials Required:

Procedure:

Put some green submerged water plants such as Hydrilla in a large beaker filled with water. Add a little soda bicarbonate as a source of carbon dioxide or a small quantity of soda water. Cover the plants under water with a glass funnel, and invert over the funnel under water a test-tube filled with water. Cut the stems and tie up the shoots into a bundle and the cut ends should be projected upwards into the funnel.

Observation:

When exposed to light, a stream of small gas bubbles is seen to rise upwards through the cut ends of stems and collect at the upper end of the test-tube, displacing the water. If the apparatus be removed to a dark or semi-dark room, or is covered with black paper, no bubbles will be seen to come out of the plants.

Inference:

The gas is oxygen and can be proved in the following way. Close the test-tube with the thumb under water, and invert it over a dish containing a quantity of pyrogallate of potash (5 % pyrogallic acid to which an excess of caustic potash has been added). Then use a bent tube to introduce into the test-tube a quantity of this solution. The pyrogallate solution coming in contact with the gas will absorb it, and will, therefore, rise and completely fill up the test-tube. The pyrogallate solution absorbs oxygen. The gas in the tube is, therefore, oxygen.