Iodine is a solid grey-black halogen that is chemically similar to fluorine, chlorine and bromine (though not quite as reactive). It forms orthorhombic crystals that evolve a rich purple vapour when heated. This vapour does not become liquid upon cooling, but instead recrystallizes. The scientific term for the process of converting from a solid to a gas, without transitioning through a liquid phase, is "sublimation." Introductory science courses often demonstrate sublimation by heating iodine.
- Skill level:
- Moderately Challenging
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Things you need
- Laboratory hood (if you're working indoors)
- Lab gloves
- Iodine crystals
- Stainless steel spoon
- Erlenmeyer flask (125ml or 250ml)
- Heating stand
- Heating square (wire mesh with asbestos-substitute centre)
- Bunsen burner or alcohol lamp
Place the heating square atop the heating stand in the laboratory hood. Center a Bunsen burner or alcohol lamp beneath the heating stand.
Use the stainless steel spoon to scoop out a small amount of iodine crystals---e.g., a quarter of a teaspoon---and put them in the Erlenmeyer flask.
Center the flask over the heating square. Place the watchglass over the top of the flask, so that it totally covers the opening.
Ignite the burner, and adjust the flame until it's as low as is practical. (Iodine melts at just over 45 degrees C Centigrade, so this experiment doesn't require much heat.)
Heat the iodine until the air inside the flask turns a rich purple, and most or all of the crystals on the bottom of the flask disappear.
Turn off the burner and allow the flask to cool. The sublimed iodine vapour will form delicate crystals on the surface of the watchglass.
Consider the reasons that the iodine vaporised instead of melting. Iodine forms a heavy diatomic molecule that bonds to itself by means of weak Van der Waals forces. Heating quickly provides enough energy to overpower these forces, producing vapour. If the Van der Waals forces were stronger, liquid would form.
Heated iodine crystals can form a liquid given sufficient pressure. For example, if you heat the bottom of a thick, solid layer of iodine, the solid layer above puts pressure on the heated layer below, which consequently liquefies instead of subliming.
Tips and warnings
- If you are performing this experiment for informational purposes, or as part of a school experiment, you can avoid wasting the sublimed iodine by scraping the crystals back into the bottle. This also prevents needless contamination of the environment.
- Use solid elemental iodine, rather than a solution such as iodine tincture.
- Iodine vapours are toxic. Do not heat iodine indoors without using a laboratory hood.
- Use laboratory gloves and goggles for skin and eye protection.
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