To prove that CO2 is necessary for photosynthesis the following experiment can be formed:

1. A potted plant is kept in dark for three days so that the leaves become free from starch.

2. A healthy leaf is selected for experiment.

3. a wide mouth bottle is taken and a little KOH (it absorbs CO₂₎ is added to it. 

4.Half part of the leaf is kept in the bottle with the help of split cork.

5. The apparatus in kept in bright sunlight for few hours.

6. The leaf is plucked and boil in 70% alcohol. It is stained with iodine solution.

Observation : It was observed that the portion of leaf which was out of the bottle turned blue showing the presence of starch. The portion on leaf which was enclosed within the bottle will not show the presence of starch .

Conclusion : This shows that CO₂ is essential for photosynthesis, because the portion inside the bottle did not get the CO₂ as it was absorbed by KOH solution. Thus CO₂ is necessary for photosynthesis.