To demonstrate that plants cannot manufacture its food (photosynthesize) unless carbon dioxide molecules are available.
Plants use sunlight and carbon dioxide present in air to produce food for itself; in the absence of either photosynthesis is affected adversely. Food produced is in the form of starch, the presence of which can be tested chemically using Iodine solution – it turns starch black.
- Plant twig with leaves
- Glass bottle with a wide neck
- Cork with a slit
- Beaker containing water
- Iodine solution
- Caustic potash
Estimated Experiment Time
Approximately 10 minutes to set up the apparatus and 8-12 hours to carry out the observations
- 1. Prepare the twig by placing the bottom end of the twig in the beaker containing water.
- 2. Choose a leaf and position the split cork in the center of the leaf. The leaf must be placed between the two halves of the cork so that one half lies inside the bottle and the other half outside it with the tip of the leaf/twig dipped in water in the beaker or dish.
- 3. Prepare the bottle by pouring a small amount of caustic potash solution and lay the bottle on its side.
- 4. Seal the mouth of the bottle with the split cork inside which the leaf is lodged.
- 5. Expose the apparatus to sunlight for a few hours, preferably until evening.
- 6. Remove the leaf and decolorize it with alcohol
- 7. Test for the presence of starch using iodine solution
- A single leaf will also do instead of the twig with many leaves.
- The setting up of the apparatus must ideally be done in the early morning so that the experiment is conducted with a starch-free leaf.
- Place the bottle in such a manner that the solution does not touch the portion of the leaf inside the bottle.
- The edges of the split cork should be smeared with petroleum jelly so as to make the bottle air-tight.
When tested for starch with the iodine solution, the portion of the leaf outside the bottle turns black, while the portion inside does not indicating the absence of starch.
Plants do not produce starch when carbon dioxide is available; all the carbon dioxide present in the air in the bottle was absorbed by the caustic potash solution causing this.
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