Experiment carbon dioxide necessary for photosynthesis

Take two healthy potted plants which are nearly the same size. Keep them in a dark room for three days. Now place each plant on separate glass plates. Place a watch-glass containing potassium hydroxide by the side of one of the plants. The potassium hydroxide is used to absorb carbon dioxide. Cover both plants with separate bell-jars. Use vaseline to seal the bottom of the jars to the glass plates so that the set-up is air-tight. Keep the plants in sunlight for about two hours. Pluck a leaf from each plant.  Remove the green coloured chlorophyll from the leaves by boiling them in alcohol. Wash the decolourised leaves with water and then pour iodine solution over them. Observe the change in colour of the leaves.

We will find that the leaf from the plant (which was inside the bell jar containing potassium hydroxide which absorbed the carbon dioxide), does not turn blue-black on adding iodine solution showing that no starch is present. So we can conclude that the photosynthesis cannot be carried out without carbon dioxide.

The leaf from the plant (which was inside the other bell jar without potassium hydroxide and so having carbon dioxide in it) turns blue-black on adding iodine solution showing that starch is present in this leaf. From this observation we conclude that photosynthesis (to make starch) takes place in the presence of carbon dioxide.

So this shows that carbon dioxide is necessary for the process of photosynthesis to take place.