Iodine belongs to the halogen family. The atoms of this group all have seven electrons in their outermost shell and are typical non-metallic elements, so they have very active chemical properties and can react with most metals and non-metals. The smallest and most mischievous member of the halogen family is fluorine: its elemental form, fluorine gas, is highly toxic and has the most corrosive properties of all elements, even gold can be corroded by it; "third brother" chlorine is also a toxic gas, and it was the first chemical weapon used by humans in war; "second brother" bromine is a liquid at room temperature, toxic, and can cause burns and intense stinging on contact with the skin, which is difficult to treat; "big brother" iodine is much milder compared to them. It is a purple-black crystal at room temperature, but it has the unique property of sublimation (the solid directly turns into a gas without liquefying), often "disappearing". However, be careful, iodine vapor is also somewhat toxic.
Iodine also has the ability to change color, and it is known as the "King of Color Change". Although iodine crystals are purple-black, they turn yellow-brown when dissolved in water, and turn purple-red when dissolved in ethyl acetate or benzene. If iodine solution meets starch, under different conditions, it will turn blue, purple, red, black, yellow, etc. This is because iodine can react with these substances to form various compounds. Due to the large size of iodine and the instability of its "charge center", the compounds formed have different "charge centers", resulting in different colors.
Let's watch the "color-changing performance" of iodine! Drop a few drops of iodine tincture on white paper, and the paper will leave a brown iodine tincture stain; after placing it on the table for 1-2 minutes, the brown gradually turns gray or black. Place the paper on the hot stainless steel pot lid to remove some water (it can also be baked in an oven or over a flame), and the black will gradually fade to a lighter brown. Cool the paper to room temperature and breathe on the brown stain with your mouth (add some water to the paper), and the light brown will turn gray or gray-black again. Finally, immerse the paper in a small bowl of clear water, and the gray-black will gradually turn blue. Do you know what happened above?
The above color changes are mostly chemical changes, and each color change can be verified by designing corresponding control experiments. For the first color change, you can compare dropping a few drops of iodine tincture on white paper and glass plates respectively. The brown iodine tincture on the glass plate will gradually lighten and disappear, indicating that the blackening on the paper is not a physical change caused by the evaporation of alcohol in the iodine tincture, but a chemical change with the generation of new substances.
Iodine also has many color-changing abilities, which are related to the large size of iodine atoms and the number and arrangement of electrons outside the atomic nucleus. Iodine atoms have up to 53 electrons, corresponding to multiple electron shells, and the outermost shell has 7 electrons, so iodine atoms can gain or lose electrons in chemical reactions and can act as oxidizing agents, reducing agents, or catalysts. Gaining one electron can form iodide ions; combining iodide ions with iodine molecules can form triiodide anions; in addition, various iodine-containing coordination compounds and iodine-containing inclusion compounds can also be formed. Iodine atoms can lose one electron, three electrons, or five electrons, resulting in more products, and correspondingly, more products with various colors will be generated.