In this month's exhibition chemistry,
we're going to be looking at the reaction of aluminium with iodine,
one of a number of demonstrations you could
choose to highlight how the properties of a compound differ from those of a
mixture of its constituent elements.
Working in a fume cupboard, weigh out 0.1 g of aluminium powder and 0.4 g of iodine into separate vessels.
Use a small beaker or a weighing boat rather than paper to measure out the iodine.
Grind the iodine into a fine powder in a pestle and mortar,
but never grind the two powders together.
Notice the difference in the appearance of the elements: the aluminium is a pale
grey powder and the iodine is much darker.
When we mix them, as we would
expect, we see a shade between the two, with visible darker and paler patches.
This is evidence that we still have the original materials there, just mixed together.
Particle diagrams like these can be downloaded from the EiC website
with supporting information on this demonstration. The particles in the two
solids don't have the opportunity to collide very effectively with one
another, but if we introduce a little water and some washing-up liquid to help
with wetting the reaction can get going after a small incubation period.
With the small amounts that we using here it's quite handy to actually gather the
powder together into a little pile, and I often gouge out a little hole in the top
into which I'm going to place the water with washing-up liquid.
The purple vapours we see here given off are iodine. This is obviously not a
chemical reaction because if they're cooled once more we would get our
original iodine back.
The evolved heat that's causing some of the iodine to sublime
is itself however a sign of a chemical reaction,
as is the change in colour of the solid.
We now have a largely uniform and completely different colour
from before, which will range from white through to pale browns according to how
many impurities are present.
The smaller scale of the reaction was necessitated by recent UK legislation. Because of the small quantities, you might find that the
mixture doesn't ignite on the first attempt. As such is handy to have some one molar
sodium thiosulfate solution on standby for cleanup. There's more
information on how to use this in the article on the EiC website.