Iodine reacts with the starch present in onion cells, producing a coloration that makes the cells easily visible under a microscope. Onion cells are naturally transparent, so it is difficult to properly visualize them without using a solution to increase contrast.
To obtain a clear image of onion cells, a wet mount microscope specimen must be prepared first. The person preparing the specimen must place a drop of water on a clean microscope slide, then place a piece of onion skin on it. To ensure contrast, a drop of iodine solution is added. The specimen is then protected with a cover slip and observed under the microscope. If the specimen contains a higher amount of starch, adding iodine creates a blue coloration.