Aim: To prove that Carbon dioxide is essential for photosynthesis.
Apparatus: A potted healthy plant with long and narrow leaves, a wide mouthed glass bottle, split cork.
Chemicals: Potassium Hydroxide solution (KOH), vaseline, Iodine solution.
Procedure :
1) Select a healthy potted plant with long and narrow leaves and keep it in darkness for two or three days so that leaves are free from starch. Take a wide mouthed glass bottle, a cork.
2) Take 5-10ml of potassium hydroxide solution into the bottle.
3) Close the mouth of the bottle with split cork.
4) Early in the morning insert one selected half of the leaf without cutting it from the plant, into the bottle through the split cork.
5) The other half of the leaf is outside the cork.
6) The bottle is kept air tight by applying vaseline around the rim of the mouth of the bottle.
7) Keep this apparatus in sun light for 3-4 hours. After the completion of time, detach the leaf from the plant and take it out of the bottle.
8) Test the detached leaf for starch test with iodine solution.
Observation : The part of the leaf which is inside the bottle does not turn deep blue, while the basal part of the leaf that is exposed to surroundings prepares starch and turns deep blue.
Inference : The potassium Hydroxide absorbs the carbon dioxide present inside the bottle hence the leaf inside the bottle does not turn deep blue, while the basal part of the leaf that is exposed to surroundings prepares starch and turns deep blue.

Conclusion : Green leaves can perform photosynthesis only when Carbon dioxide is available. Hence carbon dioxide is essential for photosynthesis.
Precautions :
1. The bottle should be kept airtight by applying vaseline around the rim of the mouth of the bottle.
2. Make sure that potassium hydroxide does not come in contact with leaf inside the bottle.
3. The leaf should not be detached from the plant till the experiment is completed.