Question

To test the presence of starch in a leaf, which of the following instructions to conduct the experiment is not necessary?

• A. Boil the leaf first in water and then in alcohol
• B. Wash the bleached leaf
• C. Heat the leaf on a hot flame
• D. Add a few drops of iodine solution to the belached leave

Answer 1

The correct option is C Heat the leaf on a hot flame

A positive test for starch in a leaf provides evidence that photosynthesis has occurred by the formation of violet black colour in the presence of iodine solution.

1. Collection of green leaf and exposed them to sunlight for a few hours
   
2. Heat the 250cm${}^3$water until it boils. Keep the water at boiling point.
   
3. Place the leaf in the boiling water with the help of forceps and boil for 2 minutes.
   
4. Turn off the Bunsen Burner. (If you are using a heat source without a naked flame - electric water bath or hot plate - this step is unnecessary.)
   
5. Place the boiled leaf in a boiling tube containing 90% ethanol.
   
6. Place the boiling tube in hot water and boil for 10 minutes or until the leaf decolorizes. (It may be necessary to replace the ethanol.)
   
7. Gently remove the leaf and wash with a fine trickle of cold tap water.
   
8. Spread the leaf evenly on a white tile.
   
9. Add a few drops of iodine/potassium iodide solution to the leaf and note any observations.