Question

How would you show that carbon dioxide is necessary for photosynthesis?

Answer 1

Experiment:
Take a wide-mouthed bottle and pour some caustic potash solution into it. Now take a potted plant and insert one half of a leaf into the bottle through the split cork. Leave the plant undisturbed in the sunlight for at least two days. On the third day, pluck the leaf and test it for the presence of starch with iodine solution.

Observation:
The part of the leaf that was outside the bottle turns blue-black, but the other portion of the leaf that was inside the bottle does not turn blue-black. This shows that the part of the leaf inside the bottle could not synthesise food. Although this part of the leaf had chlorophyll and also access to water, minerals and sunlight, it did not have carbon dioxide.

Inference:
Carbon dioxide present in the bottle was absorbed by caustic potash solution. Hence, the leaf was deprived of carbon dioxide and therefore, could not synthesise food.

Conclusion:
Carbon dioxide is necessary for photosynthesis.