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Cleaning pennies for science project

Q. Okay, so I did this science experiment on which liquid cleanses pennies most efficiently, and when I did my research, all of the places I went to said that acids cleanse the pennies better, but when I did the experiment, glass-cleaner with ammonia [this product on eBay or Amazon affil links] worked the best, and ammonia is a BASE! So did my experiment go wrong, or am I right and everyone else is wrong? (I wish!)

- Port Orange, Florida

simultaneous replies 2002
Let's take a lesson from Richard Feynman, one of history's most important physicists and a great teacher ...

A. Copper, especially copper salts, react with ammonia solution to form a very deep blue cuprammonium salt complex. Hence I am not surprised you got a reaction; however, I am surprised it was faster than acids. I suppose you learn something every day and you have now seen the wonders of science and the importance of not believing everything you are told. Good luck and keep on experimenting.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

A. I tried ammonia to see for myself, and I didn't find it immediately effective, Melissa. But part of the problem with this question is what do we mean by 'cleansing'? Most students mean removing 'tarnish', the brown color on pennies. But if you mean 'removing dirt, oil & grease, and fingerprints', then you are probably right. ammonia is a good cleanser, it just didn't seem to me to be as good a copper tarnish remover as vinegar [in bulk on eBay or Amazon affil links] & salt.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

A. I'm doing a project on cleaning pennies. I know for a fact that vinegar and salt can clean pennies.

Shewnsey [last name deleted for privacy by Editor]
- Chicago, Illinois, United States

Q. I recently conducted an experiment and found ammonia to clean pennies much more effective than vinegar. Can anyone explain the chemical reaction of copper oxide and ammonia? (CuO + NH3, I think?) and why a weak base was more effective? I'm puzzled.

Beau O. [last name deleted for privacy by Editor]
- Austin, Texas

A. The chemical reaction of Copper Oxide and ammonia (CuO + NH3) can be explained in the equation NH3 + CuO = Cu + N2 + H2O where the Oxygen Molecule (oxide) is stripped from the copper to reform with the Hydrogen in the ammonia creating Water (H2O) and Nitrogen gas (N2) leaving the Copper (Cu) clean and free of Oxide.

The balanced equation is actually 2 NH3 + 3 CuO = 3 Cu + N2 + 3 H2O where 2 molecules of ammonia combine with 3 molecules of Copper Oxide. This reaction happens very quickly but lets look at NH3 (100% ammonia or Anhydrous Ammonia) for a moment. This material is a gas under normal atmospheric conditions and only turns to a liquid state at -27 degrees Fahrenheit or below @ sea level. That's why it makes such a great refrigerant in an enclosed system.

To use it as a cleaner it is combined with Water in a 3% or less solution to stabilize into a liquid (H2O + NH3) under normal atmospheric conditions, leaving trace amounts (1-3%) of the NH3 ammonia molecules undiluted, which is the common ammonia used for household cleaning. Because it is a highly diluted solution, it will take longer to strip the Oxide off the Copper Penny than a higher concentration of ammonia will, say 25% [this product on eBay or Amazon affil links] ), but it does work the same.

When I was cleaning antique clocks for a living, we used to use Industrial ammonia to clean the brass components of clock workings, leaving them gleaming like 24K Gold! As a warning however; Industrial ammonia in concentrations of 25% or higher are highly toxic, lethal and should be used with extreme caution in a controlled environment. Even spilled concentrations of as little as 3% can reach the rate of 500 ppm (considered health hazardous) at breathing height in open air in a room within 3 minutes.

I hope this takes some of the mystery out of your puzzlement.

David Faragher
- Granada Hills, California, USA
December 29, 2011

A. I started in Aeronautical School in Queens New York. I was top rated in Chemistry Major GPA 4.0+.

With combining copper with liquid ammonia, I would let it set till the solution started to turn blue. My Uncle introduced me to all lapidary jewelry. I asked about experimenting with his old pieces. I would soak the jewelry overnight. The solution became a deep Sea Blue. I pulled the jewelry out and to our surprise the jewelry came out clean after running in warm water and with a jeweler cloth. Little to no effort you had a piece that was as pretty as new. Copper chemicals were easy to come by back in the 60's, therefore I used the oldest copper coins I could find. Also I never used plastic to mix the two together. Glass, and I mean clear pyrex glass, is the best -- like a Pudding glass Grandma made Pudding in to. The solution would last in a mason jar for later use. Originator RFS "68"

Robert Sawart Jr.
RFS AND COMPANY - Ormond Beach, Florida
July 6, 2017

Q. CuO + NaCl + CH3COOH --> Cu + CH3COONa + HCl.
Science project for cleaning pennies: where does the oxygen go from the CuO at the start?

Joey forgione
- Raleigh North Carolina USA
November 18, 2018

A. Hi Joey. I think you know this, but I'm not quite sure from the way you worded your question: the equation you wrote is not valid because it's not balanced because of that O that is on the left side but not the right side.

But it also isn't what happens -- The copper in the CuO doesn't become copper metal; rather it dissolves as soluble copper chloride. So the copper you see after removing the tarnish is the solid copper that was always there underneath the tarnish layer.

The equation would probably look better as:
1). CuO + 2NaCl + 2CH3COOH <--> CuO + 2CH3COONa + 2HCl
2). CuO + 2HCl + 2CH3COONa --> CuCl2 + H2O + 2CH3COONa

But a note: The salt and the acetic acid [this product on eBay or Amazon affil links] ionize, but it's not really a simple & complete replacement reaction with all of the sodium actually locking into the acetate and all of the hydrogen locking into the chloride. Then again, all of our chemical knowledge is a simplification, so I don't think this is a bad one for school.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
November 2018

Bonus points, perhaps, for filtering out all the "spectator ions" and listing just the compounds and ions taking part in the reaction:

CuO + 2H+ --> Cu2- + H2O

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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November 29, 2018

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