Iodine is used to test a leaf for starch. What happens to the iodine if starch is present?  

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Iodine test is used to determine the presence of starch in leaves. This test is very frequently used to demonstrate if the leaves or which part of leaves are photosynthetic. When covered with iodine solution, the portion of leaves that contain starch turns blue-black. The exact changes that take place are not yet fully understood, however it is believed that the iodide ions get stuck in the amylose molecule and that a starch-iodine complex is formed, which gives this blue-black color. 

The iodine test is used with a number of variations, such as for leaves of plants grown in light versus grown in dark; with portions of leaves covered with aluminum foil; with variegated leaves (that have both green and white color), etc. The green leaves contain chlorophyll and, in the presence of sunlight, carry out photosynthesis, thus forming starch. When dipped in iodine solution, the iodine on these green leaves will turn blue-black because of how iodine reacts with amylose in the plant.

Hope this helps. 

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