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topic 11013

Do coins corrode more in salt or fresh water?


(2001)

Q. I need to know if coins corrode more in salt or in fresh water.

It's an emergency. I appreciate if you could answer me as fast as you can.

Thank you.

Andrea V [last name deleted for privacy by Editor]
- Miami, Florida, USA


(2001)

A. It's an emergency, Andrea? Is someone choking on a coin and you want to know which kind of water to pour down their throat to dissolve the coin?   :-)

Seriously, this is a public forum and a good reply may not come until next month, or after you graduate. Sorry! But please explain your science project or patiently apply the search engine. Most of all, do the experiment first and tell us what result you got. Good luck!

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

(2006)

Q. How can I find research on coins rust faster in salt water or fresh water? For my daughter's project she's in 6th grade.

Thanks,

Tina S [last name deleted for privacy by Editor]
- Jacksonville, Florida


(2006)

A. Hi Tina. Check the town library (maybe the reference section) for a book on "Corrosion"; the issue of salt water vs. fresh water corrosion will be covered, although maybe at a level beyond her grade. The librarian can help with age-appropriate stuff.

Salt water conducts electricity because it has salt ions dissolved in it that can transport electrical charge. This means that larger galvanic corrosion currents can flow. If a coin is made of two metals, like post-1982 pennies (which are copper plated zinc), the chemical reactions between copper and zinc will go faster. Also chlorides (salt is sodium chloride) are corrosive. You've probably heard that cars corrode faster in the industrial belt where salt is applied to icy roads.

But please have her conduct the experiment before she looks for the answer because otherwise she will probably be unable to resist the temptation to dismiss contrary observations as experimental errors, and to assign far too much weight to results which support the answer she thinks she's "supposed to get". And then she is being taught junk science instead of real science :-(

Good luck!

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


(2007)

Q. My mom and I have been working on a corrosion project. We have the cups set up with salt and coins in them and we have been documenting them for the past 2 weeks. And now the teacher is asking for websites on where we are getting my information. Funny thing is I have been doing this experiment with no computer. And now that I need websites that have more information on this we cannot find them. I was hoping someone out there would have an insight on some websites that we could look at? Thanks for anyone's help in guiding us to more knowledge.

CHARLES [last name deleted for privacy by Editor]
6TH GRADER - WPB Florida


Award Winning Science Fair Projects

(2007)

A. There are dozens of threads on the subject on this site, Charles. We hope they'll help. But what were your results? What "hypothesis" did they support or tend to disprove? Because the real purpose of this exercise is not to prove that salt water corrodes a coin faster or not, but to get you to practice the methods of conducting an experiment.

Now here's the thing. If your results were like mine -- that is, that the salt water did NOT corrode the coin faster -- yet your research says that salt water is more corrosive, what is your project going to say? Being in sixth grade I don't think you are prepared to explain the science behind that conflict; rather, I think you will say that it is widely reported that salt water is more corrosive than fresh water but your experiment did not demonstrate it. That is an interesting result because you can justifiably say "prove it! -- my results don't support that claim". Good luck!

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


(2007)

Q. Mr. Me and a couple of friends are doing a 9th grade science project and we want to know, where to get information on this question: "Do coins rust faster in saltwater or freshwater?" We need as much help as possible; this is counted for 5 grades and we need help!

Desera M [last name deleted for privacy by Editor]
Student - Dallas Texas United States


Everything Kids Science

(2007)

A. Here are the two different approaches you could take, Desera:

1. Do the research first or
2. Do the experiment first.

The first unfortunately can lead to "junk science" (deciding what you want the result to be, then doing the experiment again and again, as wacky as necessary until it finally comes out the way you were led to expect. Then you cross your fingers and hope it would come out the same way next time.)

The second approach is, I think, a better route to 'the scientific method' for school kids who will find themselves very tempted to "fix" their observations so things come out the way they think it's supposed to. Please tell us the results you got and we'll try to help you explain them. Good luck.

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


(2007)

Q. I am conducting an experiment concerning the rate of corrosion versus the amount of salt in water. I did my research and concluded that metal should rust faster in relation to the salt content of the water. However, the opposite is true. I have several jars set up. the first being pure distilled water, then a series with 2 tsp, 4 tsp, and 8 tsp of salt in 4 oz of water. While trying to answer why my experiment came out opposite of what I thought, I found several confusing answers all written for more advanced students. Can someone offer an easy to understand reason. I am going to give my experiment two more days, then repeat it to see if it comes out the same.

Thanks,

Courtney J [last name deleted for privacy by Editor]
5th grade - Fort Bliss, Texas


January 10, 2008

A. Your results are your results, Courtney; if they do not support the theory that more salt leads to faster rusting, that's fine.

Science is based on experiment, not consensus! If you read that "The majority of scientists believe . . .", what that really means is everyone is still just guessing.

But I think that once you have 2 tsp of salt in 4 ounces of water, it's plenty salty. I would not expect more salt to have any effect. In fact, when you see any salt "sludge" in the bottom of your bowl, you've passed the "solubility limit" and any extra salt isn't dissolving anyway.

If you were in high school and the laboratory had an analytical balance (a very precise scale), you might be able to weigh the metal after immersion to determine which piece had the greatest weight loss, but just going by eye on the amount of rusting, my results were the same as yours. It is true that salt water is more corrosive than fresh water, but the experiment apparently needs to be conducted differently to prove it, and I don't know how a 5th-grader can prove it. I'd suggest that when the test is done you rub all the rust off the metal onto the inside of a coffee filter, then pour the remaining water through the coffee filter and display these rusty coffee filters as your evidence of which solution caused the most rusting. Good luck.

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


January 27, 2008

Q. I'm doing a science experiment for 6th grade and I'm trying to figure out the corrosive effects of fresh versus salt water over time on various steel surfaces, from untreated, to brass plated, zinc plated, painted, etc., to see what happens to such common building materials with exposure to elements over time.

So far, my results completely contradict the information I've read in books. My materials are completely under either fresh or salt water kept at a constant temperature. I didn't use a cover. They are exposed to air, like regular building materials would be. They seem to be corroding at the same rate yet I know that's not possible.

Can someone please give me some advice on what I'm doing wrong?

Francesca L [last name deleted for privacy by Editor]
Student - New York, New York


November 2013

A. Hi Francesca. Your results are not necessarily wrong. I and some other people got the same results.

However, what could be the case is that your experiment, fully submerging metals in water and leaving them there, may not be fully representative of using the metals on a building. Maybe you could take some samples and leave them out in the sun and spritz them twice a day with fresh water and salt water?

Regards,

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


February 25, 2008

Q. Please help me find which a coin will corrode faster in, fresh or salt water! Please today!

Jazmine S [last name deleted for privacy by Editor]
student - Houston, Texas


March 9, 2008

Q. I am in 6th grade with a science fair that counts for much of my grade. I am required to have 2 research sources (1 print based) and am wondering how to get them. My project is about corroding coins in freshwater, salt water, and chlorinated water. This is the only site I could find that really helps me. Please post your response soon. Thanks!

Ray B.
6th Grade Student - Boone, North Carolina



A. Hi Ray. As I said earlier, get to the library for the print-based research.

Regards,

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



December 7, 2008

!! Just in case anybody was doing an experiment and wasn't getting accurate results, here is the scientific method (sort of an outline for your experiment):

1. Ask a question
2. Research topic
3. Make a hypothesis
4. Design and perform an experiment
5. Analyze data
6. Form a conclusion

Lingshon Mai
- Sacramento, California, USA

October 12, 2010

thumbs up signThanks, Lingshon. I am not a teacher, but I have reviewed hundreds and hundreds of these postings from schoolchildren, and here's how they interpret those steps:

1. Ask a question: Get an assigned question from the teacher
2. Research topic: Ask someone what the conclusion is "supposed to be", i.e., what answer will make their teacher "proud of them".
3. Make a hypothesis: State the answer that they now believe the teacher wants.
4. Design and perform an experiment: Take careful note of any experimental results that agree with what they think the answer is supposed to be, while dismissing as an error and making themselves blind to anything they are seeing which does not support it.
5. Analyze data: Massage the data in whatever way is necessary to not contradict the answer they feel they are "supposed to get".
6. Form a conclusion: State the "conventional answer" that they learned back in step two.
7. Reinforce "junk science" instead of learning how to conduct an honest experiment :-)

That's why I think the "scientific method" for first-time grade-school children should be modified to be:

1. Ask a question
2. Make a hypothesis
3. Design and perform an experiment while record all data, in ink, in a lab book with dated and numbered pages
4. Analyze data 5. Research topic
6. Form a conclusion that best explains how the recorded data correlates with the research
7. Suggest a hypothesis for a followup experiment next semester that will hopefully reconcile any discrepancies between the recorded experimental data and the research.

Regards,

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



November 4, 2013

Q. But why do coins corrode FASTER in Salt Water then Fresh Water?

Aleah [last name deleted for privacy by Editor]
- North Carolina


November 5, 2013

A. Hi Aleah. Please tell us your procedure, and the results you got which cause you to make this claim. Thanks!

Personally, I'm not going to try to explain why something supposedly happens when an experiment has indicated that it doesn't. Newton did not waste his time trying to explain why an apple would shoot up from his head and reattach to the tree :-)

Regards,

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



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