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Clarifying compounds and mixtures – the reaction of aluminium and iodine

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.