[Kenichi]

Alright, so each year for the holidays I color pinecones with various chemicals, cupric chloride, barium, strontium chlorides etc. as gifts to friends and family and I normally dry them out on paper towels.  For the first time, I spread out some Aluminum foil and went to dry them on it instead.  The pinecones in barium chloride (prepared from barium carbonate and HCl, the solution was quite acidic as I miscalculated the molarity of my acid) went on without anything odd happening. But when I placed a dripping pinecone that had been sitting in the cupric chloride solution onto the foil, a reaction ensued which left me without any foil....A bit exothermic, slight bit of gas generation. It ate right through the foil in a matter of seconds.  I can't smell anything as I have a bit of a cold right now, so I can't describe the odor.  Any ideas as to what might have happened?
Thanks

[Borek]

Strange, acidic solution (BaCl₂) should eat through the foil.

[billnotgatez]

This is pure speculation on my part.
The solution that has the copper and chlorine compound is very acidic due to excess hydrochloric acid. That favors an aluminum and chlorine compound instead. So the copper is replaced in the former compound and the hydrogen is driven off in the acid. If the temperature is high enough you might even get the hydrogen to react with the air to form water. One may also suspect with appropriate temperatures that any excess aluminum might also react with the air, if there is not enough chlorine. Hydrogen and water vapor are odorless gasses.

How is that for a guess?

[Kenichi]

Thanks for the suggestions.

Given time I knew the HCL in the BaCl2 solution would slowly eat away at the foil, giving of hydrogen gas.  But the cupric chloride solution was just a mix of Copper (II) Chloride dihydrate dissolved in water. I am positive that it was not acidic.  I could hear the hissing of the reaction as soon as the wet pinecone hit the foil.  Within just a second or two, the foil underneat the cone was gone.  I didn't smell H2 gas, but that might have been formed anyway.  The remains of the foil look a bit like it could be aluminum chloride.  Which makes me think that it was HCl that was reacting with it. But like i said before, the cupric chloride solution was not acidic. 

[Borek]

You won't smell H₂ as it is odorless. Copper chloride solutions are always acidic due to the Cu²⁺ hydrolysis - 0.1M solution should have pH in the 4.3 range.

Can you estimate what excess of HCl have you used preparing BaCl₂ solution? For me fact that copper chloride solution reacted with aluminum foil is easier to understand than fact that acidic solution was ineffective.

Have you seen reddish/black traces of the metallic copper in the alufoil remains?

[Kenichi]

Went back today to examine the remains, and indeed there are some reddish and dark grey remains of the foil left. 

[woelen]

A combination of copper (II) ions and chloride ions is remarkably corrosive towards aluminium. It really is this combination. Chloride alone, or copper (II) alone does not react like the combination. See my webpage below:

http://woelen.scheikunde.net/science/chem/exps/cu+al/index.html