Water-initiated purple vapor
Magnesium and iodine react vigorously with each other, but only in the presence of water. This can be demonstrated with this spectacular experiment.
If this experiment is carried out carefully, then the risks are not really high, but one has to use due care and the experiment should not be scaled up to large quantities!
Procedure for performing the experiment
Take a small amount of iodine and a small amount of powdered magnesium. It might be necessary to crunch the iodine crystals with a glass rod or a hard plastic rod. Do not use metal tools for handling the iodine. It will spoil the metal tools. If crunching of crystals is necessary, do this before mixing.
Mix both solids with an absolutely dry spatula. The spatula must not be a metal spatula.
After this step, a small heap of the magnesium/iodine mixture is obtained. Approximately 1.5 cm long, width of 5 mm and 2 mm high in the middle.
Take a clean glass rod and put the tip in some water. Assure that a drop of water is on the tip of the glass rod. Put the tip of the glass rod on the magnesium/iodine mixture, such that the water falls on the middle of the heap of the mixture. As soon as the water comes in contact with the mixture a violent reaction starts, in which so much heat is produced that a large part of the iodine vaporizes. This produces a nice purple cloud of gaseous iodine, which quickly condenses on anything near the place, where the experiment is conducted and produces a lot of brown stain. During the reaction between the magnesium and the iodine a soft hissing noise is produced. Although quite some heat is produced, no signs of fire can be observed. The following two pictures show the violent reaction, just after putting the drop of water on the mixture and the brown stains, which are formed by condensation of iodine from the purple cloud.
The magnesium/iodine mixture did not react completely. The following close up nicely shows the different colors of the material produced. The colors are due to formation of iodide, which gives red/brown complexes with iodine (e.g. tri-iodide). The grey matter is still unreacted mixture, which can be used for similar experiments, by putting additional drops of water on it.
A little video was made from another run of the experiment, with 115 mg of iodine and 15 mg of magnesium powder. Click here for the video.
Discussion of results
Magnesium and iodine react with each other as follows:
Mg + I2 → MgI2
This reaction is highly exothermic. However, in order to have a reasonable reaction rate, the magnesium and the iodine need to be in intimite contact. The water helps accomplishing this. At first some iodine dissolves in the water, which reacts with magnesium. The magnesium iodide formed dissolves in the water, allowing much more iodine to be dissolved as tri-iodide:
I–(aq) + I2(s) → I3–(aq)
The tri-iodide reacts with further magnesium, by splitting off iodine and forming more magnesium iodide. This reaction is self-accelerating.
The last close up picture nicely shows the red/brown tri-iodide, dissolved in a small amount of water.