I recently did the experiment where you drop a piece of iron in an aqueous solution of copper sulfate. After a while, the piece of iron gets coated with a fluffy coating of copper that can be dried and mashed into a powder. Apparently this happens because iron is more reactive than copper, according to the reactivity series.

Can I do the same thing by making an aqueous solution of aluminum sulfate, then dropping a piece of magnesium metal in? It should work because magnesium is above aluminum on the reactivity series, right?

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    $\begingroup$ No, you can't. $\,$ $\endgroup$ – Ivan Neretin Sep 16 at 6:22

Metallic aluminum cannot be obtained from a solution of its salts. It is because pure aluminum reacts with water, a little bit like sodium. Of course a piece or a sheet of aluminum does not usually react with water, because the metal is covered by a thin, colorless and waterproof layer of aluminum oxide, which prevents the contact between aluminum and air. And if you remove this layer with a knife, the oxide layer will immediately be remade by air.

The only way of showing that aluminum reacts with water is to dip the aluminum piece in a solution of Mercury chloride. There will be a surface reaction like $$\ce{2 Al + 3 HgCl2 -> 3 Hg + 2 AlCl3}$$ Then the mercury Hg will make an alloy with aluminum. And this alloy cannot be covered by the alumina shield. So such a piece of amalgamated aluminum will be quickly oxidized in air, and will produce long white filaments of $\ce{Al2O3}$ or $\ce{Al(OH)3}$ if abandoned in air for a couple of minutes due to one of the following reactions $$\ce{4Al + 3 O2 -> 2 Al2O3}$$ $$\ce{4 Al + 3 O2 + 6 H2O -> 4 Al(OH)3}$$ This production of alumina filaments is very interessant to observe. Of course it is not allowed any more today, because of the toxicity of mercury.

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  • $\begingroup$ I did another experiment with copper sulfate where are you stick two copper rods in an aqueous solution of copper sulfate. You apply an electric current and copper metal builds up on the cathode. I noticed that the copper anode dissolved. Does the dissolved copper from the anode just turn into more copper sulfate? $\endgroup$ – ElectronicsNoob Sep 17 at 0:43
  • $\begingroup$ @ Electronic Noob. What you have done is exactly the industrial method for purify impure copper. This impure bloc is at the anode, and pure copper gets deposited at the cathode. The other metallic impurities fall on the ground of the flask, where they form a deposit that can be eliminated later on. $\endgroup$ – Maurice Sep 18 at 9:55

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