In order to test for starch in a leaf, it is first boiled in water, then treated with hot alcohol, washed with warm water and finally treated with iodine solution. The hot water kills the leaf and opens the cells. The hot alcohol breaks down the chlorophyll, taking the green color out of the leaf, making it yellowish. Now the color changes are more clearly visible. Finally, the leaf is treated with iodine solution (which contains some iodide ions too), the reddish brown color of iodine solution turns blue-black. The amylose units formed by degrading starch molecules form a complex compound with the triiodide ion (`I_2+I^(-) rarr I_3^-` ) of iodine solution to produce the characteristic blue-black complex.
This experiment demonstrates that the leaf has been performing photosynthesis and producing starch.