Copper(II) chloride has a unique ability to partially remove the protective oxide layers from aluminium. Copper sulfate, a more common copper compound, does not behave this way with aluminium. Why does copper(II) chloride have this ability? Does copper(II) chloride have the same ability with other reactive metals protected by thin oxide coatings, like beryllium and magnesium (well magnesium is only partially protected)?
Beryllium reacts rapidly with copper(II) chloride solution, even in low concentrations. Hydrogen gas, copper metal, and the metal hydroxide or chloride is set free. Beryllium also reacts with copper sulfate solution. The difference in reactivity is actually questionable here. The first shows a copper and chloride solution; the second a copper and sulfate solution. The beryllium got a soft copper plating on it from the reaction.
Aluminium reacts rapidly with copper(II) chloride solutions, forming similar products to beryllium. If the copper(II) chloride is pure, aluminium hydroxide is produced as a gelatinous white precipitate. If some residual acid is present, the resulting solution of aluminium chloride is clear. Dilute copper sulfate has no effect on aluminium. The difference in reactivity is illustrated most dramatically with this metal.
Magnesium also reacts rapidly with concentrated copper(II) chloride solutions, forming hydrogen, copper, and magnesium chloride. It reacts a little more slowly with copper sulfate solutions (left) than with copper chloride solution (right).
Solutions of copper(II) and chloride ions have the same effect as copper(II) chloride solutions, showing the formation of a copper chloro complex which is responsible for the reactions. Only for aluminium, however, is the difference seen so strongly; the chloro complex has a doubtful effect on the reaction rates of the other two metals.