Prove experimentally that CO 2 (carbon dioxide) is essential for photosynthesis.

Aim : To prove experimentally that CO 2 (carbon dioxide) Is essential for photosynthesis. Experimental material used: Two healthy potted plants of nearly the same size. Instruments and Chemicals: Two glass plates, two bell-jars, Petri dish, beaker, 70% alcohol, iodine solution, Potassium hydroxide (KOH). Method: Keep the two healthy potted plant A and B in a dark room for 3 days. Now place each plant on separate glass plates. Place a watch glass containing potassium hydroxide (KOH) by the side of one (A) of the plants. Cover both plants (A) and (B) with separate bell-jars. Use Vaseline to seal the bottom of the jars to the glass plates so that the set-up is airtight. Now keep the plants in sunlight for about two hours. Pluck a leaf from each plant and boil hem in 70% alcohol solution separately. Now check the presence of starch. Observation: The leaf of the potted plant (A) turns colourless while the leaf of the potted plant (B) turns blue. Conclusion: In the potted plant (A), CO 2 present in the bell-jar is completely absorbed by KOH and turns into KHCO 3 . Therefore, the leaf further was unable to absorb CO 2 and hence photosynthesis, as well as starch synthesis, does not occur. In potted plant (B), leaf absorbed CO 2 easily and exhibits photosynthesis. Therefore, the colour of the leaf is blue-black. Thus, we can conclude that CO 2 is essential for photosynthesis.