Do coins corrode more in salt or fresh water?

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!

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!

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!

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.

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.

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,

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

Regards,

Thanks, 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,

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,