[Araconan]

So today when I placed a piece of aluminum foil in a solution HCl, the solution turned black, before slowing becoming colourless.

What I'm wondering, is why did it turn black before it turned colourless? Is it because of impurities? And if so, what's the chemical reaction that's occuring that's producing the black colour?

[Hunter2]

The black colour comes from very fine aluminium particles generated during the dissolving.

[Araconan]

But isn't aluminum silver coloured?

[Hunter2]

Not if its very fine particles.

[Arkcon]

And commercial aluminum foil does contain significant impurities, specifically silicon and traces of other metals.  They will also appear as you try to attack aluminum foil with HCl.

That was Reynold's "secret" to making foil, that other manufacturer's have caught on.  The impurities are necessary to disrupt the crystal structure, so it can be rolled thinner, and more easily.  Bulk aluminum has a significant "grain" which is very noticeable when you try to cut it.  It seem weird to refer to a metal, which should be ductile, as having a "grain", as if it were wood.  But aluminum's crystal structure is very pronounced.

[vmelkon]

Your entire solution turn black or there were dark spots on the foil?