Next there is a substance, generally dextrose, included, followed lastly by potassium iodide. There is usually an anti-caking agent, often sodium silicoaluminate. Salt is the first ingredient as expected. Next, we pick up the box of iodized salt and discuss the ingredients listed on the box. The iodine color disappears and a starch test is negative.
To show this, we add some Vitamin C to the iodine solution we prepared previously. The reduction of iodine to iodide is also possible, by the addition of an appropriate reducing agent. Next we explain that iodide salts are very easily oxidized by oxygen in air to afford elemental iodine, writing the equation for the reaction on the board and noting that the reaction not only requires oxygen but also hydrogen ions: 2 I - (aq) + 2 H + (aq) + O 2 (g) → 2 I 2 (aq) + 2 H 2O (l) We show this by marking a slice of white bread with the iodine solution, using a glass stirring rod to write the formula “I 2” on the bread (see figure 3). We remind them that any starch source will do, and conversely the test for starch is to add iodine. We demonstrate this by adding a drop of the iodine solution we just prepared to some starch solution in a large test tube, and note the deep blue color. 3įigure 3: Iodine reacts with starch in a slice of bread Chemical Propertiesįirst, we remind the students that the customary test for iodine is the starch test, which they will often remember from an earlier biology class. This reaction, along with the use of ethyl alcohol as a co-solvent, is used to dissolve iodine in the preparation of the disinfectant tincture of iodine. On the board, we briefly explain the following reaction: NaI (aq) + I 2 (s) → NaI 3 (aq) We question the class as to why this may be and guide them to the idea that a chemical reaction must have occurred. Next, we add some potassium iodide or sodium iodide to the beaker containing the water and observe that suddenly the iodine is now dissolving. 2 If desired, a third beaker containing some mineral spirits may also be used. We add a few iodine crystals to a beaker of water and to a beaker of ethyl alcohol and note the colors produced. This behavior is generally unfamiliar to students, whose experience with dissolution has been largely limited to water soluble ionic solids and polar substances such as sugar. 1 Iodine is insoluble in water, but dissolves in other liquids like alcohols and hydrocarbon solvents.
The flask will quickly fill with a beautiful violet vapor (see figure 2). We show this by sprinkling a few crystals of iodine into a 1 L or larger flask and briefly heating the flask over a flame or heat gun. When heated, iodine typically sublimes from the solid phase to the gas phase rather than melting and passing through a liquid phase. We show the class a sample of elemental iodine sealed in a glass tube (figure 1). In fact, its name comes from the Greek word for “violet”. We note that iodine is a non – metallic, very dark violet crystalline solid. Nevertheless, it is found in the human body in the hormone thyroxin, which is synthesized by the thyroid gland and is critical for metabolic control.įigure 2: Iodine vapor in a round bottom flask Physical Properties It is found in seawater at low concentrations, in seaweeds and kelp, in marine organisms, and in some salt deposits. Iodine is a scarce element and does not generally occur in mineral deposits. Despite this, by now the students are generally quite enthusiastic about the Element of the Month program and the posters produced for iodine are some of the most colorful and well executed that we have seen. Beyond its occasional use as a disinfectant, the students will have little familiarity with iodine. The inclusion of iodine as the Element of the Month allows for some color and fun to be injected into the classroom at this time. In many areas, the winter weather brings rain, snow, overcast skies, or unwelcome cold. The fall and winter holidays are over, and the school year seems to stretch on interminably. January is usually a no-fun month for high school students. Iodine is the Element of the Month for January in our program. Read The Element of the Month - An Introduction for an overview of the project and links to the other articles in the series." - Editor Folger (MRF), chemistry teacher (now retired) at Lyme – Old Lyme High School in Connecticut. Wright (SWW), Associate Research Fellow at Pfizer Inc., and Marsha R.
"In honor of the International Year of the Periodic Table this series of articles details the Element of the Month project developed by Stephen W.