Letter 13443

References to help explain why citric acids clean pennies best. 

++

My ten year old daughter and I read some of your letters about "What cleans pennies the best". She has 5 different juices: lemon, lime, pineapple, orange and grapefruit, she has actually found that lime has cleaned the best. However, with all of your letters, there is still not a research site or references given, so that she can back up her findings. She knows that the citric acid is reacting with the copper, but why or how is this reacting in terms that she can understand and then write her report on. Thank you for any help.

Paticia H [name deleted for privacy due to age of posting]
- Cleveland Heights, Ohio, United States

We applaud her for actually doing the experiment instead of just asking for the answer, Patricia! It's also good that she is looking for high quality references, rather than the tossed salad of information, misinformation, and disinformation that is the internet. However, I do not fully agree with the structure & nature that you and perhaps the teacher seem to be trying to give to this investigation.

What is important for a ten year old student to get from a project of this nature (in my opinion) is the concept of "the scientific method" and the mechanics of a science project. She should be trying to learn the vocabulary of hypothesis, variable, observation, and conclusion. She should be learning that her efforts do not become a scientific experiment unless she follows a particular protocol. She should be learning what "junk science" is, why it is so poisonous, and how to avoid it. Please start with our FAQ on Cleaning Pennies.

Your daughter could have "cleaned" the penny by applying sandpaper to it, scraping away the copper oxide tarnish and exposing the underlying copper metal. Citric acid is an acid, and acids have the power to "clean" pennies, by dissolving this copper oxide tarnish. The tarnish dissolves into the acid and is washed away, exposing the "clean" non oxidized metal lying under it. Personally, I think that is enough depth for a ten year old. But the reaction is roughly:

Acid + copper oxide --> copper salt + water

balancing the equation before proceeding:

2 acid + 1 copper oxide --> 1 copper salt + 1 water

2 H⁺ [citrate anion]^-- + Cu⁺⁺ O^-- --> Cu⁺⁺[citrate anion]^-- + H₂0

Acid has a 'sour' taste. So whichever fruit tastes the most sour may do the best job of cleaning. I agree with you that lime juice is quite good at removing tarnish from a penny. I'm not sure that it's better than lemon juice, though. It may depend on season, ripeness of individual piece of fruit, country where it was grown, etc. There is enough random variation in technique, tarnish depth, dirt on the pennies, etc., that the experiment may not have enough precision to resolve whether limes are better than lemons. But if repeated experiments demonstrate it, then we can perhaps assert it as true.

Citric acid is not just an acid, but also a chelating agent. Even after college chemistry I don't fully understand chelation, and I doubt that you or the teacher do, and I don't believe that a 10 year old should be expected to. But it does aid in dissolving of the metal. There ought to be leads to references in the teacher's guide; or you might write to the publisher's website, since they should have the answer to the question they wrote.

Shake a little salt in with the lemon or lime juice and it will turn the penny salmon pink in a heartbeat. Why? Because the chloride from the salt is a very powerful copper corroder; but it is difficult to understand why this is so, too -- let alone explain it to a 10-year old.

I hope this reply may get you to slightly rethink what this experiment is supposed to be teaching your daughter. Not that I claim special insight, just that I have faced this question about 300 times now and I think I'm starting to understand the issues. Good luck!

Ted Mooney, P.E.  finishing.com Brick, NJ

Anion, Cation, Salt, Confusion...

I'm doing my 7th grade science project on what is the best way to clean a penny. For my research report to accompany it, i need to write about ions and salt and how it reacts with the copper/copper oxide. Please don't simplify... I may not be able to totally comprehend it but i guess i could ask my mom for help... Please respond ASAP! My teacher keeps telling me to explain more and more. I'm on my fourth draft. Please help me!
~sofia

Sofia F
student - Berkeley, Ca, United States

Hi, Sofia. As an aside, think about this: there are 3000-page books in the library that talk about ions; so how do you decide what portion of such books to leave out from your report? Simple, your research must be written in terms that explain or comment upon your own results. But you haven't told us your results! This may be the source of the confusion. Try this:

"My hypothesis was . . . In order to try to prove that hypothesis, I conducted the following experiment . . . The results of my experiment tended to confirm my hypothesis but . . . I believe that the reason that this happened is because . . . This reasoning is supported by these excerpts from . . . which explain that . . ."

Ions are charged particles that are dissolved in a water-based solution. Positively charged ions are called cations, negatively charged ions are called anions.

Let's look at vinegar for an example of cations and anions. Vinegar is dilute acetic acid, and the formula for acetic acid can be written as HC₂H₃O₂. Vinegar will ionize as H⁺ + C₂H₃O₂^- , and the H⁺ are cations and the C₂H₃O₂^- are anions. As another example, citric acid from citrus fruits can be written as HCOOH, which will ionize as H⁺ + COOH^- , where the H⁺ are cations and the COOH^- are anions.

In either case the cations can react with copper to form copper salts as explained in the formulas in my previous response to Patricia. Good luck.

Regards,

Ted Mooney, P.E. 
finishing.com
Brick, NJ

I want to answer or follow-up on this question.

 

I want to post a question or inquiry of my own on a different subject.