Question

electrolysis of water by aluminium electrodes
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Answer 1

Aluminum is a very reactive metal, and is easily oxidized. The white foam almost certainly contains significant amounts of aluminum oxides or hydroxides. Because it is easier to oxidize aluminum to aluminum oxide than to oxidize water to oxygen, there is significantly less oxygen produced (possibly none--you might check to see if it is actually hydrogen gas that is produced at the aluminum anode; this would be from reaction of the aluminum directly with the water, not from electrolysis)