Thickness of the black part of the potato represents the distance up to which the iodine solution has diffused into the potato.
Step-by-step explanation:
Iodine solution act as the indicator for starch solution which is present in potato. The amount of iodine solution that is diffused in potato marks the level of decolourisation in the given sample. Iodine react with starch to form a complex which is black in colour. So the potato turns black. The amount of iodine solution that was diffused into the potato marks the the area in which iodine is reacted with starch that is present in the potato . So that reason of potato turns black which is the colour of the starch and iodine complex. The complex that has been formed due to the process of chelation.