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When aluminum (foil) is placed in an aqueous solution of copper(II) chloride, the aluminum atoms lose electrons to the copper(II) ions and replace them to form aluminum chloride, and to free up metallic copper.

The metallic copper has a tendancy to float on the surface of the solution, being buoyed by gas bubbles. The bubbles appear to evolve from the interface of the aluminum foil with the solution. What is the source and nature of the gas bubbles?

I have two hypotheses:

  1. The bubbles are coming from dissolved $\ce{CO2}$ that is coming out of solution due to an increase in temperature close to the site of reactions.

    enter image description here

  2. The bubbles are $\ce{O2}$ bubbles coming from the layer of aluminum oxide on the surface of the aluminum foil.

If this is not the case, what happens to the portion of the portion of the foil that was in oxide form prior to the reaction. How would the aluminum oxide react with copper(II) chloride?

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The gas bubbles are very likely just air that was dissolved in the water previously and now gets released. I would expect most of the bubbles to actually be nitrogen, followed by oxygen, argon and then some carbon dioxide — that is assuming you used deionised water which should not have significant amounts of carbonate and hydrogencarbonate dissolved.

The copper particles merely supply suitable nucleation sites for the gas.

The theory on aluminium oxide being oxidised to pure oxygen is not correct. Oxygen is a very oxidative element meaning that you require strong oxidants to oxidise oxide anions back to oxygen. With ‘strong’, we’re talking the oxidative strength of potassium permanganate. Aluminium’s oxidative powers are nowhere near that strong.

Copper(II) chloride and aluminium oxide do not react with each other; only metallic aluminium beneath the passivisation layer is able to react with copper(II) ions.

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    $\begingroup$ I was thinking that given the temperature it could also be water vapor. $\endgroup$ – Joseph Hirsch Dec 20 '16 at 21:19
  • $\begingroup$ @JosephHirsch Water vapour does not form bubbles in water at room temperature. You didn’t mention that you were boiling your solution. $\endgroup$ – Jan Dec 20 '16 at 21:27
  • $\begingroup$ Not boiling. The reaction is exothermic. Not violently but I suspect that it gets warm enough at the reaction sites. $\endgroup$ – Joseph Hirsch Dec 20 '16 at 21:34
  • $\begingroup$ @JosephHirsch No. $\endgroup$ – Jan Dec 20 '16 at 21:40
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    $\begingroup$ I object. This is most probably hydrogen. $\endgroup$ – Ivan Neretin Jan 22 at 9:24
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The currently accepted answer does not consider the possibility discussed here, where magnesium is the metal and again there are bubbles. Transition metal salts of strong acids are weakly acidic in water, and that weak acid enables a strongly electropositive metal which reacts only slowly or not at all with pure water to begin displacing hydrogen more readily. Thus the aluminum in this case is displacing hydrogen (from the acid produced by hydrolysis) as well as copper.

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