Starch turns blue when exposed to iodine, which is iodine element.

Elemental iodine is a purple-black crystal that is easy to sublime and easy to condense after sublimation. Toxic and corrosive. Iodine turns blue-purple when exposed to starch. Mainly used in pharmaceuticals, dyes, iodine, test paper and iodine compounds.

Iodine is extremely important to animal and plant life. Iodide and iodate in seawater enter the metabolism of most marine organisms. In higher mammals, iodine is concentrated in the thyroid gland in the form of iodinated amino acids, and a lack of iodine can cause goiter. About 2/3 of iodine and its compounds are used to prepare antiseptics, disinfectants and medicines, such as iodine tincture and iodoform CHI3. Sodium iodate is used as a food additive to supplement insufficient iodine intake. The radioisotope iodine-131 is used in radiotherapy and radiotracer technology. Iodine is also used in the manufacture of dyes and photographic film.

In addition to being used as a photosensitive agent for photographic films, silver iodide (AgI) can also be used as a seed crystal for cloud formation during artificial rainfall. The alcohol solution of I2 and KI, namely iodine, is a commonly used disinfectant; iodoform (CHI3) is used as a preservative.

Iodine-containing preparations such as iodine tincture, compound iodine solution, iodine throat lozenges, iodine glycerin, etc. are widely used drugs in medical treatment. Iodine tincture is a common disinfectant drug in the home.

Reaction of iodine and water:

(1) Type of reaction between halogen and water: its own oxidation-reduction reaction will occur in water.

(2) Reaction between iodine and water: Iodine has the smallest solubility in water and is only slightly soluble in water. The solubility is 0.029g/100g water. I2 and water cannot undergo the redox reaction that occurs between F2 and water. When oxygen is introduced into the hydrogen iodide solution, iodine will precipitate:

4HI + O2 → 2I2 + 2H2O

I2 can undergo its own oxidation-reduction reaction under alkaline conditions, Generation of iodate and iodide ions:

3I2 + 6OH- → 5I- + IO3-+ 3H2O

This is because there is no hypoiodite IO- in the solution. At any temperature, IO- rapidly undergoes its own redox reaction to generate I- and IO3-.

Reference: Baidu Encyclopedia Iodine