Science project: Turning pennies gold color (actually brass)

A. Hi Ms. Lekberg. What is done in industry as a test method for cleaning is to scrub the parts with wet powdered pumice and a brush; then rinse. Adding a few drops of detergent to the pumice to aid the rinsing won't hurt. Most importantly though, the kids have to wear latex gloves so they don't contaminate the surface with finger oils. Good luck.

Regards,

A. Place about 5 g of Zn dust in an evaporating dish. Fill the dish one-third full of NaOH. Heat to near boiling. Clean a penny with 00 steel wool ⇦ this on eBay or Amazon [affil links] and place in the dish. Heat 3-4 minutes until reaction is complete. Wash the penny under running water and gently blot dry. The penny looks silver - it is zinc coated. Hold the penny in a burner flame (use tongs) long enough for the color to change. The penny looks Gold - it is brass coated. Brass is 60 to 82 % Cu and 18 to 40 % Zn.

A. Umm also for the Zinc you could use 3 - 4 nails, and for the NaOH use Drano [affil links] (powdered).

!  Thanks, Kulisia, thanks Alex. People need to know, though, that NaOH or Drano is quite dangerous cold and even more so when near boiling hot. Maybe with goggles ⇦ this on eBay or Amazon [affil links] and gloves this experiment might be appropriate for high schoolers under careful supervision, but certainly not grade schoolers.

For a simple and safe plating experiment, see our FAQ for zinc electroplating. I'm not sure if this will diffuse with the copper of the penny when heated and produce the gold-colored brass alloy -- but finding out what will happen is what experiments are for :-)

A. Boil 20 ml caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] solution (drain cleaner works). Add 1 g zinc filings. Place a clean penny in the mixture. Remove the penny after about 10 mins. It will be silver (I believe plated with zinc). Heat the penny gently with a propane torch. It will turn gold. The zinc and underlying copper combine to form bronze. Needs careful supervision. We prepared a health and safety plan before we started and made it an integral part of the project.

A. Hi Jon. In the "gold penny" experiment, as you know, you heat the penny and the copper of the penny diffuses into the zinc that you have plated on it and produces brass (an alloy of copper and zinc), which is gold-ish colored. Where will the copper come from in your experiment? ...

The cladding on the quarter is cupro-nickel, an alloy of 75% copper and 25% nickel, but I don't know if free copper will be available from the alloy to be able to create the brass. I'd suggest doing it both ways: plating copper and then zinc on the quarter, and just plating the zinc. And then see what happens when you heat them. Simple copper and zinc electroplating methods are described in our FAQ, "How electroplating works". These experiments are for grade school kids, and with your science teacher's supervision you may be able to use stronger chemistry for heavier copper and zinc plating.

Regards,

A. For very simplistic way of copper plating a U.S.A quarter coin you may want to try this method.
Safety equipment (latex gloves, safety eye glasses) small Gerber jar (4 oz jar), very fine #0000 steel wool ⇦ this on eBay or Amazon [affil links] , copper sulphate ⇦ this on eBay or Amazon [affil links] (10 grams) and white vinegar ⇦in bulk on eBay or Amazon [affil links] and mix the solution, it will turn a green color due to the acetic acid ⇦ this on eBay or Amazon [affil links] and let it sit for 10 minutes.
Place the coin inside of the Gerber jar lid, pour some of the copper solution as to immerse the coin, nothing will happen, no plating yet, grab a small portion of the steel wool and start rubbing the liquid and the steel wool on the coin, this will immediately start to copper plate the coin, the more you rub the more copper is plated, repeat the process for the other side of the coin, you can do this for many coins until the liquid has plated out.

The science behind this simple copper plating is the fact that metals are electrically conductive and will displace any of the metals which are lower in the reactivity series when steel/iron comes in contact with a more noble metal, the noble metal adopts iron lower position in the reactivity series and the copper in the solution now being electrically more noble than the coin will start to plate copper directly on the coin, but without any of the downfall of iron(very poor adhesion of the copper layer) with this method you can copper plate any metal with copper; I have done gold, silver, copper, nickel.

Another simple experiment you can try is to 'Silver Plate' a copper penny with Tin, without the dangers of the normal procedures of zinc plating pennies(boiling sodium hydroxide and zinc dust ⇦ this on eBay or Amazon [affil links] )
Safety equipment (latex gloves, safety eye glasses) small Gerber jar(4 oz jar), aluminum foil, Stannous chloride (20 grams).
Add the stannous chloride salts to the glass jar, add about 50 ml of tap water and Mix(it will be very had to dissolve on the water but that is okay), place the penny or pennies inside the lid of the jar, stir the tinning solution and pour some of it inside of the lid to immerse the coins, grab some aluminum foil and compress it to form a small amount to fit the tips of your three fingers, start rubbing/brushing the coin/coins with the aluminum foil, if the aluminum gets too hot(chemical reaction) just add a bit more water to the solution to dilute the chlorine ions on the solution, this will plate the pennies with a strong tin layer that can be polished to a white silver shine.
The science behind this is basically the same as with the copper plating quarter coins, but in this case we use aluminum because of it's lower place in the reactivity series, Aluminum>Iron>Tin, iron is too close to tin in the reactivity series to cause any effect, Tin sulphate will not work unless you add a pinch of salt to the solution enough chloride ions to remove the oxide layer that aluminum forms.

A. Hi Martina,

Zinc is necessary to close the chemical reaction, as copper doesn't oxidize (with this solution), you need some other chemical to oxidize and close the RedOx reaction:

Zn(s) [granular] --> Zn⁺⁺(aq) + 2 e^-
Zn⁺⁺ (aq) + 2 e^- --> Zn(s) [on the penny]

This reaction is happening on the penny because a LITTLE (and very very little of it) copper does oxidize:

Cu(s) [penny] --> Cu⁺⁺(aq) + 2e^-

But this reaction isn't quick and not much copper is dissolved, so it can't push the reaction itself.

Zinc could be omitted if you could have some chemical in the water that makes the last reaction (copper dissolving) more spontaneous (Delta G <0) and doesn't interfere with zinc passing from aqueous solution to solid state. It would be a "selective complexant".

So, I hope you can learn from this experiment (and others to come). Best of luck!!!

A. Let me try to explain the electrochemistry process that takes place here.

Zinc metal + Sodium Hydroxide = Sodium Zincate (electroplaters use it as the first step in electroplating Aluminum, keep that in mind as we will reference this in the future).

So when Sodium Zincate and Zinc metal and Copper Pennies are heated to boiling point (Very dangerous) Zinc is deposited on the copper penny (and to the Zinc metal, but most are unaware of it), but how?

When metals are submerged in a boiling electrolyte their natural electromotive force potential are lowered, in this case Zinc metal becomes more reactive while the Zinc ions on the Zincate solution remain the same, Zinc metal deposits on Zinc, The copper penny in contact with the Zinc metal is electronegativity charged by the Zinc metal becoming the Cathode (as in cathode in electrolytic cell, electroplating) and is coated with Zinc.

Could there be a better and safer way to do this instead of the boiling caustic solution? Sure there can, in this case a non-boiling zincate solution (I have used an electroplating zincate solution from a supplier where you just add water, but there are many formulas for Zincate solutions in the web) and instead of Zinc metal as contact, I have used Aluminum (Aluminum foil will work) wrap Aluminum foil on your penny (or pennies), pinch a few holes to allow the solution to penetrate. then drop them in the Zincate solution, allow 30 seconds to one minute, and decant the Zincate solution back on the storage container. rinse with water and admire the Zinc coated pennies!