AIM: To test a leaf for starch
* bunsen burner, tripod stand, wire gauze(OR 90oC electric water bath OR hot plate) * 250 cm3 beaker
* boiling tube
* anti- bumping granules
* test tube holder
* white tile
* leaf to be tested (hibiscus leaves are excellent)
* 90% ethanol
* iodine/potassium iodide solution
The video above shows the steps in Testing a Leaf for the Presence of Starch. The Procedure outlined below is slightly different. 1. Remove a green leaf fom a plant that has been exposed to sunlight for a few hours 2. Half-fill a 250cm3 beaker with water. Heat the water until it boils. Keep the water at boiling point. 3. Use the forceps to place the leaf in the boiling water. 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 uneccesary.) 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 decolourizes. ( It may be necessary to replace the ethanol) 7. Gently remove the leaf and wash with a fine trickle of cold tap water. 8. Spead the leaf evenly on a white tile.
9. Add a few frops of iodne/potassium iodide solution to the leaf and note any observations.
* The leaf was flaccid (soft) after being boiled in water * The ethanol changed from colourless to green
* The leaf was brittle after being boiled in ethanol
* The leaf become flaccid once more after being rinsed in cold water
| Iodine/Potassium Iodide solution changed from brown to blue- black
| Starch is present
| NEGATIVE TEST
| No colour change
| Starch is not present
DISCUSSION QUESTIONS AND ANSWERS
1. Describe Photosynthesis. Use a symbol equation.
Photosynthesis is the process by which...
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