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Melting Pennies

Tom Kuntzleman's picture
Tue, 08/26/2014 - 13:34 -- Tom Kuntzleman
melted penny

I am fascinated by the chemistry of pennies.  Dissolving a penny in nitric acid is one of the most visually stimulating chemistry reactions I have ever seen.  The combination of a bubbling green solution and noxious brown gas that changes the color of an acid base indicator enlivens any discussion on redox chemistry.  Likewise, the golden penny experiment is pretty cool, too.  In this experiment, a penny is plated a penny with a silvery zinc coat that subsequently turns a golden color when heated.  There are a large number of other chemistry experiments that use pennies.  Erica Posthuma-Adams recently described a boat building experiment in which students design and build boats; pennies are used as weights to test each boat’s seaworthiness.  Finding the density of pennies is a classic chemistry experiment. 

My favorite penny experiment is melting pennies.  It is simple to conduct and it can be used to discuss a variety of chemistry topics.  Check out the video below.

I would love to learn more about the chemistry of pennies and other coins.  If you have any experiments to share, I would enjoy hearing from you. 

You can find out more about the composition of pennies as well as more experiments to do with pennies in the Journal of Chemical Education.  A small sample of articles on pennies is listed below.

http://pubs.acs.org/doi/abs/10.1021/ed085p817

http://pubs.acs.org/doi/abs/10.1021/ed074p952

http://pubs.acs.org/doi/pdf/10.1021/ed075p1362

http://pubs.acs.org/doi/pdf/10.1021/ed052p102.2

http://pubs.acs.org/doi/pdf/10.1021/ed060p142

Chemical compound: 

Comments

Andres Tretiakov's picture
Submitted by Andres Tretiakov on

Hi Tom,

here are a couple of more ideas. I'm sure you must be aware or tried them before.

HETEROGENEOUS CATALYSIS

Copper pennies as catalyst for the oxidation of propanone (acetone)

British copper pennies are used instead (pre-1980) or the modern ones which are made of steel and copper plated.

http://www.rsc.org/education/eic/issues/2012September/catalytic-copper-heterogeneous-catalysis-demonstration.asp

Copper pennies as catalyst for the oxidation of propanone (acetone)

British copper pennies are used instead (pre-1980) or the modern ones which are made of steel and copper plated.

 

Easy to prepare and you do not need an oxygen gas supply. You do need on the other hand a dark room to be able to see the bright glow on the penny. 

Next one is how to turn copper pennies black using calcium or sodium polysulfide

Calcium or sodium polysulfide can be prepared by heating a dilute solution of calcium or sodium hydroxide and adding elemental flower of sulfur powder little by little until the solution turns red in colour. Then simply add the penny and leave for a couple of minutes. I believe the penny gets a coat of copper sulfide which is black; you can confirm this by scratching the surface and the copper should be visible underneath. I'm not sure if this is what happens and other tests should be carried out to confirm this.

I have a photo with the different pennies (silver, gold and black) but I cannot insert it here.

Hope this helps,

Andres

 

 

 

 

 

 

 

 

Andres Tretiakov BSc MRSC RSci
Science Technician / Demonstrator
St. Paul's School
Lonsdale Road
London
SW13 9JT
UK

Tom Kuntzleman's picture
Submitted by Tom Kuntzleman on

Andres,Thank your for alerting me to these experiments.  The video is quite helpful.  I will be getting in the lab to try these out...

Tom Kuntzleman

Bob Worley's picture
Submitted by Bob Worley on

More from the across the pond where we do like to spend a penny!

There is an experiment carried out on pennies which involves coating the surface with zinc. In many protocols this involves boiling 6M sodium hydroxide solution containing zinc powder and then adding a clean penny. This is a very hazardous process as you can imagine. Well it works with 1M sodium hydroxide but it is still not pleasant.

There is a less hazardous process to making copper appear “silver”. This involves electroplating the copper coin with zinc. The anode of zinc and electrolyte is and alkaline solution of zinc sulfate(VI), ie sodium zincate, at room temperature. This is extremely rapid and can watched on CLEAPSS video, https://www.youtube.com/watch?v=qG6pnr1Ywc8&list=UU2ZRkBjqUw5-CW7vB-g0Xcw.

If you pass the zinc-coated copper coin through a Bunsen flame, the zinc melts into the copper to make a brass surface and looks like gold; the chemist’s dream.

Bob Worley in London

Tom Kuntzleman's picture
Submitted by Tom Kuntzleman on

Great experiment, Bob!  I will be soon trying out this experiment you have shared. 

In the video you shared, it looks as though the 6V DC power source is capable of generating a pretty high current.  What current is necessary to generate a good zinc plate on the copper?  I ask because I'm wondering if batteries purchased from a grocery store could be used as the DC power source to plate the zinc.

Thanks for sharing this with us!

Tom Kuntzleman

Andres Tretiakov's picture
Submitted by Andres Tretiakov on

Hi Bob and Tom,

I found this even less hazardous procedure for the alchemy pennies experiment. Instead of using highly corrosive 6 M sodium hydroxide and zinc powder/dust or mossy zinc, you can use 1 M zinc chloride solution and zinc granules. You can quickly clean the pennies with vinegar and salt first to facilitate proper plating of the brass alloys.

Over a hot plate heat 25 ml of 1 M zinc chloride with 1 g of zinc granules. When it starts boiling, and using tongs immerse two pennies in the mixture until they appear silver. Use the tongs to remove the pennies and dip them into a beaker filled with distilled water. Rinse and shine with paper towels. Different brass alloys have different colours according to the content of zinc and copper metals. The resulting silver colour of the brass alloy pennies contains more than 45 % zinc. When the zinc-coated penny is heated, it turns gold in colour due to another change occuring in the brass alloy.

Here ia a photo:

https://twitter.com/Andrestrujado/status/504560444045295616

Hope this helps,

 

Andres

Andres Tretiakov BSc MRSC RSci
Science Technician / Demonstrator
St. Paul's School
Lonsdale Road
London
SW13 9JT
UK

Submitted by David Allan on

When discussing the changes in penny composition in the US, I like to use a pair of scissors to cut a new penny in half. The zinc core is readily seen a a bright, silvery center. Then, I like to dissolve the zinc with some HCl (doesn't have to be very concentrated), leaving behind the very thin copper veneer. The scissors I use can be purchased at a hardware store. They have serrated blades that hold the penny in place and keep it from sliding down the scissor blades.Here is a link to suitable scissors at Amazon.com: http://www.amazon.com/SE-Stainless-Steel-Utility-Scissors/dp/B003N2J0JY/....

Tom Kuntzleman's picture
Submitted by Tom Kuntzleman on

Sorry for the late reply Dave.  This is a great idea.  My classes have been doing penny density experiments this week, and we've been fiddling around with the chemistry of pennies as well.

Tom Kuntzleman

Bob Worley's picture
Submitted by Bob Worley on

The secret of electroplating is to have a small concentration of cations available for plating but good conductivity. This is achieved by complexing the metal with a ligand but with the equilibrium constant well the complex side of the equation. I must admit to my surprise when this worked first time. You will see a lot of bubbling at the cathode as well. This is hydrogen. I used a 9v battery for this. (Gold plating is achieved with the cyanide ion as the ligand but other methods are preferred to bypass the toxic nature of hydrogen cyanide – please don’t try this.)

Brassificaton is a little hit and miss but the message is to pass the metal steadily through the hot flame, not too slowly and not too fast.

Our group at CLEAPSS (www.cleapss.org.uk) are primarily involved in safety in school science and DT but our aim is to enable practical work to be carried out not to go about banning it. With chemicals, we take the MSDS and make it relevant for school teaching. Our work is encouraged by the Health & Safety Executive, www.hse.gov.uk

Many schools had done nickel electroplating but Information from Europe was now demanding some pretty nasty hazard warnings on nickel compounds. The main concern was that it can cause cancer by inhalation and we had to be careful of inhaling aerosols. The risk is very, very small and is only found in badly organised nickel plating factories but when people see these warnings the shutters go up and chemophobia sets in. Our safety regulations suggest we look for less harmful alternatives and despite the solution being quite caustic, teachers understand that hazard better. Nickel compounds are involved in allergic reactions as well. So I looked for a simple complexing reaction and for once, hit the button first time.

 

Tom Kuntzleman's picture
Submitted by Tom Kuntzleman on

Bob, thanks for your safety insight on this experiment.  It would be nice for many of us that teach chemistry to be more connected with those of you that know a lot about the hazards associated with certain experiments.  Thank you so much for your insight into this particular experiment.  I look forward to hearing more from you in the future.

Tom Kuntzleman

Submitted by joanne gervais on

comment to Bob...Brassification. Are you electroplating the copper with a zinc solution, heating it in an open flame and then throwing it in water? This may be  an alternative to coating pennies with zinc dust in 3 M sodium hydroxide. What have you tried? Why are the results hit and miss?

Dave Gervais

Bob Worley's picture
Submitted by Bob Worley on

Dave

I will try and answer the question in reverse order. The hit and miss reference is to passing the zinc plated copper through a flame. Go though too quickly and the temperature is not hot enough to melt the zinc into the layer of copper to form brass on the surface; this gives the “gold” effect. Heat it for a long time in the flame and oxidation takes place of the metals.

The original method of adding the copper to zinc powder in boiling 1 to 6 M sodium hydroxide solution (I have seen and used all concentrations) is rather not very pleasant. In fact the experiment will even work in acid solutions but the zinc powder forms a ball and the result on the copper is patchy. The important thing is to have the zinc powder in contact with the copper.

If you read the previous notes in this thread, I was looking for a replacement for nickel plating for school students in the UK to do, discovered that I could electroplate with zinc and then remembered the gold coin experiment and just put them together.

I hope that helps. There is a video on https://www.youtube.com/watch?v=qG6pnr1Ywc8 and some of my ideas are on www.microchemuk.weebly.com.

All the best and be careful of Halloween over there!

Bob