Design an experiment to prove that Carbon dioxide is necessary for photosynthesis

Here's an experiment to prove that carbon dioxide is necessary for photosynthesis:

Objective: To demonstrate that carbon dioxide is essential for photosynthesis.

Materials:

• Two potted plants of the same type and size (e.g., Coleus or Geranium)
• Two bell jars
• Two glass plates
• Petroleum jelly
• Potassium hydroxide (KOH) pellets or solution (to absorb CO$_2$)
• Sodium bicarbonate (NaHCO$_3$) solution (to provide CO$_2$, optional for control)
• Iodine solution
• Alcohol (ethanol)
• Beakers, test tubes, Bunsen burner/hot plate

Procedure:

1. Destarching: Place both potted plants in a dark room for 2-3 days. This ensures that any pre-existing starch in the leaves is used up, so a positive starch test later will indicate newly synthesized starch.
2. Setup:
   • Experimental Setup (without CO$_2$): Place one potted plant on a glass plate. Next to the plant, place a watch glass containing KOH pellets or a beaker with KOH solution. Cover the plant and the KOH with a bell jar. Seal the base of the bell jar to the glass plate with petroleum jelly to make it airtight. The KOH will absorb all the carbon dioxide from the air inside the bell jar.
   • Control Setup (with CO$_2$): Place the second potted plant on another glass plate. Next to this plant, place a watch glass with plain water or a beaker with sodium bicarbonate solution (to ensure ample CO$_2$). Cover the plant and the water/solution with a bell jar. Seal the base with petroleum jelly to make it airtight. This setup serves as a control, providing normal atmospheric CO$_2$ (or enhanced CO$_2$ if using NaHCO$_3$).
3. Exposure to Light: Place both setups in bright sunlight for 4-6 hours.
4. Starch Test:
   • After exposure, pluck one leaf from each plant.
   • Boil both leaves in water for a few minutes to kill the cells.
   • Transfer the leaves to separate test tubes containing alcohol and heat them in a water bath until the chlorophyll is removed (leaves turn pale white). This step is crucial for observing the color change with iodine.
   • Wash the destarched leaves with cold water and spread them on a white tile.
   • Add a few drops of iodine solution to each leaf.

Expected Results:

• Experimental Setup (without CO$_2$): The leaf from this plant will show no change in color or turn yellowish-brown when iodine is added. This indicates the absence of starch.
• Control Setup (with CO$_2$): The leaf from this plant will turn blue-black when iodine is added. This indicates the presence of starch.

Conclusion:

The control plant, which had access to carbon dioxide, produced starch, indicating that photosynthesis occurred. The experimental plant, which was deprived of carbon dioxide, did not produce starch, indicating that photosynthesis did not occur. Therefore, this experiment proves that carbon dioxide is necessary for photosynthesis.