I suspended about 15g of iodine crystals over a couple of nails to see how quickly they would begin to rust.

After a week, these droplets had formed. When prodded they broke apart like water drops.

What are these? My guess is that the iodine sublimated and then combined with water vapor.

The original question that prompted this experiment.

Mystery droplets

The setup (~one week prior)

Experiment Setup


Iodine vapors combine with iron to form iron(II) iodide:$$\ce{I2 + Fe->FeI2}$$ In an anhydrous form the compound is dark red-purple. But iron(II) iodide is hygroscopic and deliquescent which means that it dissolves in moisture it absorbs from the air, and forms those droplets. This complicates a bit the chemistry of the droplets - the $\ce{Fe^2+}$ ion exists most likely in a form of various aquo-, hydroxo- and iodo- complexes. Also, when an iodide anion and elementary iodine are present together they form triiodide anions:$$\ce{I- + I2->I3-}$$ Therefore, the droplets are a solution of a rather complicated mixture of iron and iodine compounds.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.