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HOW DOES ACID AFFECT DIRTY PENNIES?

An ongoing discussion from 2001 through 2016 . . .

(2001)

Q. WHAT DOES THE ACID DO WHEN IT CLEANS THE PENNY? BECAUSE I AM DOING A SCIENCE EXPERIMENT IN SCHOOL AND ITS TITLE IS " HOW DOES ACID AFFECT AFFECT DIRTY PENNIES AND CLEAN NAILS?" ME AND MY FRIEND ARE DOING THIS AND REALLY NEED HELP PLEASE RESPOND.

THANKS:) FROM A SCIENCE PROJECT GIRL.

CHIP B [last name deleted for privacy by Editor]
- MILLERPLACE, New York


(2002)

Q. How does acid affect dirty pennies?

Billy M. [last name deleted for privacy by Editor]
- Norman, Oklahoma


(2003)

A. Pennies are coated with copper oxide when they are made. The copper oxide makes the pennies look dirty, but they are really not. When the pennies are placed in the acid the copper oxide gets dissolved in the acids, making the pennies clean.

Linsey R [last name deleted for privacy by Editor]
- Boston, New York


A. Hi, Students. Yes, Linsey is right that the tarnish is copper oxide, and the acid dissolves the tarnish. But actually the tarnish is not from when the pennies are made (brand new pennies are shiny); the tarnish is from the copper of the penny reacting with the oxygen in the air over time, people handling them, etc.

There are thousands of students around the world with similar questions, so we've written a Cleaning Pennies FAQ which will hopefully answer many of your questions on this project. Good luck!

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Pennies in apple cider vinegar turned black

November 27, 2016

Q. My son, who is in 8th grade, did a science experiment where he hypothesized that the higher the acidic content of the solution, the shinier the penny would be. Well, that didn't happen. What did happen is that the higher the acidic solution was, the darker the pennies became. In apple cider vinegar the penny was almost black! Additionally, the penny in bleach, our most basic solution, had a white covering on it. We left the pennies in the solutions for two days. Can you explain what happened?

Thanks,

Liz

Liz Folse
Mother of student - New Orleans, Louisiana


November 2016

A. Hi Liz. Bleach is very alkaline, the opposite of acidic, but it does corrode pennies rapidly. That's because bleach is super active chlorine and oxygen with great oxidizing power. The alkalinity is incidental, it probably doesn't cause the corrosion; it's put there because the chlorine would all instantly escape as chlorine gas if the solution was acidic. That's one reason you are warned never to mix other stuff (except highly alkaline laundry detergent) with bleach.

It is true that the greater the acidity, the more power something has towards dissolving metal & tarnish (all other things being equal, which is not necessarily the case). But again that doesn't necessarily mean that all of the other possible ingredients in, and characteristics of, apple cider vinegar are irrelevant to the results. I'm not sure why the pennies turn black, but have you considered trying distilled white vinegar in one cup, and apple cider in another, to see if either gets the same black coloration? It might be interesting to observe whether or not the remains of the cider, with its complex apple proteins, seem to be responsible for the black coloration.

Chemistry is complex, and even most college students understand it only trivially, so I don't think the point of the experiment is for 8th-graders to deeply understand pH and tarnish and all the things that might drive the formation of tarnish or dissolve it -- but to make interesting (but documentable) observations for themselves.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


November 28, 2016

Q. The pennies in lemon juice also became noticeable darker instead of brighter. I just don't understand why and don't know how to help him with his conclusion.

Liz Folse [returning]
- New Orleans, Louisiana


November 2016

A. Hi again. His conclusion would be that acidic liquids turned the pennies darker. You can call the darkest "a darkness of 8" and the lightest "a darkness of 2" and assign intermediate values to other shades, then attempt to graph the pH vs. the darkness, and see if he can conclude that the darkness appeared to be proportional to the pH.

You don't even know what the dark coating is, nor what all of the chemicals in lemon juice and apple cider vinegar are, or their effects, so it might be a stretch to "conclude" much more. But certainly you can conclude that the reported correlation between pH and shininess has been disproven. Good luck.

Regards,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


simultaneous December 2, 2016

Pennies used to be an alloy of 5 +/- % zinc and 95 +/- % copper. Since 1982, they have been copper coated zinc. With that in mind, here is a possible scenario: The copper was dissolved (at least in part) or removed, possibly by the attack of the acid on the underlying zinc. This left a solution of copper acetate (this would be blue) and zinc acetate (colorless). Once the underlying zinc was exposed, the copper (being a more noble metal) was electrolessly deposited on the surface of the zinc as finely divided particles of copper, which (in my experience) look black. This is a POSSIBLE scenario. The thinner the copper plating, the more likely this would be. Experimentation could fill in the details.

tom_rochester
Tom Rochester
Plating Systems & Technologies, Inc.  

Jackson, Michigan, USA



December 2, 2016

Liz,
The general goal of a science project at that level of school is to demonstrate a working knowledge of the scientific method. The student is not necessarily expected to thoroughly explain the results as it may require knowledge above his current education level.

Even in cutting edge science, a lot of papers are published around unexplained results, and somebody else later on figures out what was actually going on. Science history is filled with examples of this.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois


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