To show that oxygen is given off during photosynthesis.
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.
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.
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.