To understand the role of KOH in experiment to show carbon dioxide is necessary for photosynthesis, the experiment is as follows:
∙ Take a plant with destarched leaves.
∙ Insert a part (half) of one of its leaves (through a split cork) into a conical flask containing potassium hydroxide. (Potassium hydroxide absorbs carbon dioxide).
∙ Leave the plant in sunlight.
∙ After a few hours, test the leaf for starch.
The portion of leaf which was exposed to the atmospheric air becomes blue-black, and the part of the leaf inside the flask containing KOH does not become blue-black, proving that carbon dioxide is essential for photosynthesis.
Thus, role of KOH is to absorb carbon dioxide present inside the conical flask.