Iodine stains starch because of the chemical reaction between it and the polysaccharides in starch molecules. This reaction turns the starch a deep blue hue. This phenomenon is called Lugol's reaction for starch. Iodine is useful in laboratories and medical offices because it also reacts with many other compounds.
One of the most common medical uses for iodine is identifying possibly cancerous tissue in the vagina and cervix. This is known as the Schiller test. According to the British Society for Colposcopy and Cervical Pathology, iodine turns healthy vaginal and cervical cells brown because of their high glycogen content. Cells that do not turn brown require further screening for cancer.
The soft mucous membranes in the mouth and throat contain much more glycogen than the tissue covering the hard palate and gums. Iodine staining helps dentists and laboratory analysts visualize the areas where these two tissue types intersect. This border is called the mucogingival junction.
Another of iodine's most important applications is staining cells on microscope slides. Iodine also enhances and fixes pigments on bacterial samples undergoing Gram testing.
Iodine has many other applications besides cell staining. It is crucial to the health of captive marine animals and plants. Corals reap immense benefits from iodine supplementation. The iodine also kills harmful aquarium bacteria and promotes vibrant coral coloration under artificial aquarium lights.