http://www.nuffieldfoundation.org/practical-biology/testing-leaves-starch-technique
By Travis , Jia Xu, Kee Chew
Monday, November 16, 2015
Test for starch.
1.Heat a plant leaf in boiling water for 30 seconds
2.Heat it in boiling alchohol for a few minutes
3.Wash with water and spread onto a white tile
4.Add iodine solution from a dropper
5.After a few minutes, the parts of the leaf that contain starch turn blue-black
Why is there a need to boil the leaf in alcohol?
Ans: It removes most of the leaves color so that it will have little chlorophyll left.
Why is the logic behind leaving the plant in the dark for a few days?
Ans: It will make sure that the plant will not produce any starch via photosynthesis during this period of time.
Test For Starch
Steps:
1. Put one of the plants in the dark for 24 hours; leave the other one on a windowsill.
2. Put some ethyl alcohol in a beaker after 24 hours, and place that in a saucepan full of water. Heat the pan until the ethyl alcohol begins to boil. Remove from the heat.
3. Use tweezers to dip each of the leaves in the hot water for 60 seconds. Then place them in the beaker of ethyl alcohol for two minutes or until they turn almost white. Set them each in a shallow dish.
4. Cover the leaves with some iodine solution and see if the iodine solution turns blue-black or remains yellowish-brown.
Why must we boil the leaf before testing it for starch?
The hot water kills the leaf and opens the cells, allowing iodine to penetrate it. The hot alcohol breaks down the chlorophyll, taking the green color out of the leaf, making it yellowish
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