Introduction: Turn Copper Pennies Into Silver and Gold [Chemistry Trick]
In a display of alchemy we turn copper pennies into silver and finally to gold. Obviously it's a chemistry trick but still impressive.
First we get 30g of zinc sulfate and dissolve it into 100mL of water.
Zinc sulfate was made back in our video on making a copper sulfate and
zinc battery: http://www.youtube.com/watch?v=Id3tL2iI0Vw
If you don't have zinc sulfate or can't make it, you can also use zinc chloride. This can be made by simply mixing hydrochloric acid with zinc metal and waiting until the fizzing stops.
Then we drop in several cut strips of zinc metal.
Zinc metal was obtained from our video on getting useful materials from batteries: http://www.youtube.com/watch#%21v=knc1lSupAwQ
The solution is heated to a boil and copper pennies, that have been thoroughly washed/cleaned, are dropped in. They must touch the zinc in order for this reaction to work. Leave it in for five to ten minutes.
The zinc metal dissolves and releases electrons that go into the copper and give it a negative charge. The zinc ions in solution now redeposit form a thin layer of zinc metal. Giving the coin a silvery color.
Now the smart physical chemist might wonder how this can possibly work. Overall we're going from zinc metal to zinc metal and at first it seems like we're missing something about thermodynamics. What is the driving force? I've searched the literature and there doesn't seem to be a consensus among scientists, some claim it's a surface free energy issue with zinc having a lower potential on copper than in free solution (at high concentration). Others state that it's an alloy of zinc metal and copper on the surface. The silvery color you're seeing on the penny is actually a type of white brass, not pure zinc. This is thermodynamically more favorable than pure separate metals and thus serves as the driving force.
Anyway, whatever the specific chemical reason, we've now deposited zinc onto copper.
The most amazing trick is to turn it into gold. Simply wash the coin and place it on a hotplate heated to about 300 Celsius. As it heats the zinc will diffuse into the copper, forming brass on the surface that looks like gold.
Eventually it will wear off but the results are rather impressive.