The experiment inside the working fume cupboard shows the exothermic reaction between the solids aluminium and iodine upon addition of a few drops of water.
Iodine is present in excess in this reaction and the heat generated is enough to sublime the iodine crystals into clouds of purple vapour.
The main reaction taking place is:
2Al (s) + 3I2 (s) → Al2I6 (s)
But some hydrogen iodide is also produced.
Water acts as a catalyst in this reaction, bringing the two solids in very close proximity to each other.
This experiment was adapted from Practical Chemistry