Rusting Nails student experiment & research (cont'd)

A. Hi, Chelsea. Although this has been explained a dozen times on other threads on this site, it is probably hard to find, so I'll bore some readers by repeating the explanation here :-)

Students (and some teachers) assume that rust is the best indication that corrosion is occurring, but it isn't! In fact, in industry, acids are used not to create rust but to remove it. Although acids can dissolve metal, they will dissolve rust faster, leaving the surface rust-free.

So what has happened is --

With your vinegar and orange juice, any rust has been dissolved into the acid. In addition, a very small amount of iron has probably also been dissolved. Steel nails are made of iron plus a small amount of carbon. As the iron dissolved, it left behind the very tiny specs of carbon because carbon is not soluble in mild acids. You will probably find out that this black "smut" can be wiped off with a tissue or paper towel. But I'd suggest wiping it off onto a coffee filter instead, so all of your corrosion products are "mounted" on coffee filters.

In the salt water, the iron from the nails combined with oxygen dissolved in the water to form iron oxide (rust). I'd suggest rubbing any rust off onto a coffee filter and pouring the liquid through the coffee filter.

The amount of rust and smut on the coffee filters should be a good measure of what happened. The ideal way to measure corrosion is by the reduction of weight of the nail between start and finish, but I doubt that you have scales of sufficient accuracy, so you may have to exhibit your coffee filters as your measurements. You can do a rough graph of how much smut and rust you see on each coffee filter. Good luck.

Regards,

A. I can't be anybody's tutor or personal advisor, Chelsea, and this is my 4th response to you -- but hopefully this dialog will be helpful to other students.

Let's take it from the top --

You don't just start an experiment with no purpose. As I noted in my response to Zoe in about the middle of this thread, Darwin himself said that this would be like going into a gravel pit every day and pointlessly writing down the size and color of each piece of gravel. There needs to be a point to your experiment, something that you are trying to prove or disprove. That "something" is your hypothesis.

The "hypothesis" that you now suggest might have been okay, but you were supposed to form your hypothesis before you began your experiment, and the purpose of the experiment was to see whether the evidence supported or tended to refute your hypothesis. The position you are in now is that you want to use your "conclusion" that salt water rusts things fastest as your hypothesis. See the problem, and why that's no good?

So let's look at it this way: when you began your experiment, you suspected that different contaminants would affect the rusting rate. You didn't know it to be a fact, you just suspected it based on intuition, previous experience, or rumors you heard, etc., right? What you wanted to do was to conduct an experiment to support or refute that suspicion. So, how about this as your hypothesis: "When nails are immersed in water-based liquids, the amount of rust that is formed will vary depending upon what contaminants are in the water they are immersed in."

So you picked a good variety of liquids (salt water, sugar water, vinegar, orange juice, and Dr. Pepper) and ran an experiment to prove this hypothesis -- and the conclusion was pretty clear: the amount of visible rust on the nails in salt water was the greatest. You also discovered that immersion in vinegar or orange juice produced a black smut on the nails. You believe that this black smut is carbon that did not dissolve in these mild acids, but you do not know that for a fact, so you suggest a good hypothesis for next year's science experiment might be: "Immersing nails in vinegar or orange juice dissolves some iron from their surface, leaving behind a carbon smut".

Regards,

A. Hi, Naomi. Are you in 2nd grade and this is your first-ever science project, or are you a senior in high school, with a wealth of such projects behind you, and ready to start your career or head off to a university?

I'm not trying to give you a hard time, but please understand that students are expected to learn an awful lot in those nine or ten intervening years, so a senior project bears no resemblance to a 2nd or 3rd grade project and nobody can help you plan a project until they have some sense of how deep your knowledge of chemistry is supposed to be at this point.

Step 1: tell us what grade you're in :-)

Regards,

A. Hi, Ella. If you desperately need help, I feel you are probably not properly interfacing with your teacher because it was most certainly not your teacher's intent to force you to contact strangers on the internet to help you :-)

The ideal way to measure rust is to weigh the nail before you start the experiment, let it rust in the solution for your experiment, then use special chemicals to dissolve away the rust, and then weigh the nail again -- determining the amount of corrosion by how much weight of metal the nail has lost.

But depending on what grade you are in, what chemicals the school lets you use, and what equipment your school has, you probably have to compromise rather than doing it the ideal way. A compromise which is probably suitable for younger grades is to rub off any rust you can onto a coffee filter, and pour the rusty water through the same filter.  You can't actually "measure" rust this way, but I think you can "judge" the relative amount of rusting by looking at how much rust is on the filter.

I am only guessing that the black smut is carbon specs; neither of us has done an experiment that proves it. But, no, carbon is not rust. "Rust" is iron oxide, the chemical combination of iron from the nail with oxygen from the air or dissolved in the liquid.

I believe the right approach to your report is to understand the principles that we've talked about here, and describe what you've found and the limitations on what you've been able to do based on a student's limited access to chemicals and exotic testing equipment. Good luck!

Regards,

A. Hi, Casey. Sorry but I can't even understand what you are trying to relate -- you have said you expected it to be black and it was, both on the 10th day and the 14th, but it seems that what you said isn't what you mean. So I think the most important thing you need to do is work on a careful description of exactly what you did and exactly what you saw. The methodology of your project and report is far more important than getting the "right" answer. Explain what happened very carefully, then advance a theory that might explain it, and research whether the theory makes sense. Good luck.

Regards,

A. Hi, Casey. I know you don't quite believe me, but you continue to not carefully define for yourself what the words you are using mean. I can see you tossing the word "rust" around in an inconsistent fashion, where you mean visible rust one moment, total dissolved iron the next time, amount of weight loss the third time, and back and forth. Until the words you use have a fixed & stable meaning to you, you will continue to flail around in circles of confusion. Look up the definition or decide for yourself exactly what that word 'rust' means to you, and never use it in a different way.

Of the solutions that you are using, vinegar is the most corrosive. That is because it is an acid, and acids directly dissolve iron into solution. But as mentioned several times on this thread already, acids are more commonly used to dissolve rust than to create rust; that is because they will dissolve rust faster than they will dissolve metal, leaving a rust-free surface.

Here is what may be happening. The vinegar initially dissolved any small amount of rust that was on the nail, then it began to slowly dissolve the iron itself (as proven by your weight loss measurement). So now the vinegar/water solution has iron dissolved into it. The acetic acid of the vinegar, over time, leaves the solution as a fume or vapor (that's why you smell it). At the same time, the water is also evaporating. At some point, as the acid and water go missing, and the iron that was dissolved remains behind, it can't stay in solution because it's too concentrated for the remaining acid and water to keep it dissolved. This should not be a surprise, sweat is salt water and it leaves salt stains behind as it evaporates too. The dissolved iron reacts with the air and water and forms rust. Since vinegar dissolves the most metal (iron), the residue will have the most iron, and when that residue eventually dries and converts to rust, there will be more of it.

Again, sticking with consistent meanings for words: nobody said salt water is more corrosive than vinegar; it isn't! What they said was salt water will produce visible rust whereas vinegar, at least initially, does not create rust but dissolves it instead; but when it's capacity for holding the metal in solution dissipates, the metal is left behind as rust, and because it dissolves more iron than water does, you'll see more when everything is dried up.

Regards,

A. Hi, Kim.

50 note cards?! Does your teacher prefer Edward Everett's style to Abe Lincoln's? :-)

If I had my choice I would not use 6 different brands of soda, all of which probably rust stuff rather similarly (because they have the same basic ingredients of water, sugar, and phosphoric acid). The actual ingredient lists are secret, so if do find any difference in corrosiveness you'll never know why.

There would be little of extensible value that you can learn from such an experiment. So what is "extensible value"? Can you add 201 plus 103 right in your head right on the spot? Sure you can. Even though your teacher never taught you to add those two specific numbers together, you can add them because your teacher did show you the "extensible" method of adding the units column, then the tens column, then the hundreds column. But if you find that soda A is more corrosive than soda B, so what -- what else does it tell you or teach you? Nothing at all, and they could change the formula tomorrow anyway :- (

So please try to redesign your experiment with questions like: does the temperature matter, does more sugar increase the rusting or slow it down or have no effect, does the concentration of salt matter, does the concentration of acid matter (for example, straight vinegar vs. 50:50 vinegar to water). Try to find some property or aspect of your test solution that is measurable in some way, and see whether varying that property or aspect affects the rusting. Good luck.

Regards,

A. Hi, Marie.The answers are already on this thread repeatedly. Please ask your parents to review your understanding the word "procedure" and help you find the procedures that are described on this page. Good luck.

Regards,

A. Hi, Alisha. This is pretty advanced for a 7th-grader, but the chemical formula for vinegar (acetic acid) is vinegar is HC₂H₃O₂, and it will ionize to H⁺ + C₂H₃O₂^-. The chemical formula for an iron or steel nail is basically Fe.

The reaction is approximately Fe⁰ + 2H⁺C₂H₃O₂^- ==> Fe⁺⁺(C₂H₃O₂^-)₂ + H₂^.

The iron goes into solution as ferrous acetate, and bubbles of hydrogen gas are released to the atmosphere.

Regards,

A. Hi, Jackie.

In the long term, in the real world, we know for certain that salt water is more corrosive than fresh water. This is known by every sailor, every engineer, and every homeowner with a beach house; and it can be easily proven by well controlled accelerated corrosion tests. There are at least two reasons: first, chloride ions, as in sodium chloride salt, chemically attack metal and are corrosive to it; secondly, the dissolved salt increases the conductivity of the water, allowing galvanic corrosion currents to flow easily.

But what can we say of short term tests by students? After dealing with reports of hundreds of tests here for many years, many reporting salt water to cause more rust, and many reporting fresh water to cause more rust, I ran the test myself and was unconvinced that there is any significant difference. If we then try to answer the question of WHY one is more corrosive than the other, we are confronted with the problem that the student actually has insufficient evidence to make such a claim in the first place because countless students got the opposite result :-(

One of our readers (Poppy Anne, letter 19024) claimed that the reason that fresh water is reported as more corrosive is that in the short term it is, but it builds a film of corrosion products on the nail that tend to slow further corrosion, whereas salt water penetrates this film and continues to corrode. I don't fully believe this, but it is one possible explanation. Perhaps another explanation is that as iron dissolves into these smallish dishes we quickly approach an equilibrium condition where the dissolved iron slows further dissolution, and the results would be entirely different in flowing water.

In short, I think your son should do the experiment as conclusively as he can, then try to advance an explanation, but introduce this information about the uncertainty of the results, because there are simply too many contrary results to dismiss them all based on testing two or three nails :-)

Your question about the significance of learning that bleach causes a lot of rust is a good one. But the internet is a giant one-room schoolhouse and while 4th graders don't know enough chemistry to learn an extensible lesson from this, high schoolers should. Another issue is that 4th graders can't use "real" chemicals for safety reasons (even high schoolers sometimes aren't allowed to these days, although for that age group I think it is shameful paranoia rather than a legitimate safety concern); this limits the cause-and-effect lessons they can learn. Your son is doing a good experiment for a 4th grader though.

Regards,

A. Hi, Savannah. Rust can be very fluffy stuff like cotton candy, occupying a lot of volume and calling attention to itself when there isn't much substance. Although the rust weighed something, it was probably little enough that it was beyond the resolution of your scales. You did good.

Regards,

Hi, Cristea. Sorry, but faking a project is not something I want to help you with :-)

Regards,

A. Hi, Genesis. We appended your question to a thread that answers it. Please see my response of Feb. 11, 2011, wherein I say that an experiment with a couple of nails under somewhat uncontrolled conditions may not render a conclusive result. But stuff can only dissolve to an equilibrium point, and I suppose it is vaguely possible that something similar to the "common-ion effect" could mean that the sodium and chloride ions in the saltwater could slightly slow down the dissolution of the iron compared to purer distilled water which has nothing dissolved in it yet. Distilled water can be "hungry" to dissolve stuff. Please patiently google "hungry water" to learn more. Good luck!

Regards,

We appreciate the compliment, Roshni, and thank you for taking the time. But as I'm sure you know, your daughter earned a Top 5 because she put in the effort. Thanks again.

Regards,

A. Hi Sahra. Since you want to know the effect of acidity of the solution, you need to know the acidity of the solution. Your teacher will either give you pH paper (litmus paper) to test the acidity, or will tell you the pH of various solutions, or will ask you to research it. That's step 1 in your procedure then: figuring out how you are going to know the acidity of the solutions.

Regards,

A. Hi Adrianna. If you look at Angela B's letter above, she describes the independent variable, the constants, and the dependent variable.

Our FAQ "What juice or liquid cleans pennies best?" explains those terms, and all the terms used in your project. Good luck.

Regards,

A. Hi Angela. If you go nice & slow and don't panic, it's actually very easy :-)

A variable is anything that is changing/changeable as opposed to being constant/fixed. And you already know what independent means and dependent means.

If you put the words together, an independent variable is something we choose to change or vary, and a dependent variable is something that changes as a result of that choice. In everyday life, you can choose how much to eat (independent variable); then how hungry you still are when you get up from the table is a dependent variable, and how much weight you gain or lose is another dependent variable, and how much the dinner cost might be another dependent variable. You can choose to buy a Prius or a Humvee or a Ferrari (independent variable); then the average gas mileage you get is a dependent variable, and how much the car payments will set you back is another dependent variable, and how likely you are to survive a 40-mph crash could be yet another dependent variable.

There might be a thousand dependent variables in theory, but you don't necessarily have to be interested in all of them. For example, in choosing a car, if safety & survivability is extremely important to you, you might not care about gas mileage or cost (or leg room or top speed or trunk capacity or other facts & figures).

When it comes to an experiment about rusting nails, independent variables you might choose include what the nail is made out of, or what coatings you put on the nails, or what solutions you expose the nails to, or what temperature the solutions are, or how long you leave the nails in the solutions. A 4th-grade student will probably only vary one thing, for example what juices to use as their solutions. Generally, the dependent variable that you will be interested in is how much rust you get.

There might be other dependent variables in theory but you might not care about them. For example, if you chose to use vinegar in one of the experiments, you'd have less vinegar left in the bottle (dependent variable) but it's probably not a variable that you care about, and doesn't impact the experiment your child wants to do. So THE dependent variable for your child is the amount of rust. Good luck.

Regards,

THANK YOU, so very much!!! The way you explained it, makes perfectly good sense.

This website rocks!!!!!!!

A. Hi Giancarlo. Your experiment sounds fine to me but there are a couple of things to watch for. First, that the solution in one or more containers may evaporate; I think you should have a fill line on the containers and periodically top them up if they get down to the 3/4 mark or so; or even better might be to cover them (maybe your mom has four matching tupperware containers with lids). Second, you might say that you have attempted to limit the effect of sunlight, humidity, and other factors by covering the containers, and are making the as-yet unproven assumption that those factors have not influenced your results. Have fun.

Regards,

Hi Nikkita. Yes, that sounds like a good science question/experiment. But so far you have done nothing (that you've shown us) except to post your homework assignment :-)

Please tell us what you have planned, or ask questions about what you see on this page, or something, anything, that we can comment on rather than simply doing your homework for you. Thanks.

Regards,

A. Hi Melinda. Please describe exactly what you see, maybe including pictures if possible, and we will try to explain it.

Regards,

A. Hi Joe. Can you attach photos or your interpretation of the results?

While it is true that some oil-based paints are better at preventing corrosion than some latex water-based paints, I don't think the general hypothesis is necessarily true. Whichever paint is most free of tiny pinholes, thickest, and best adhering is probably most effective. Additionally, some paints are specifically designed to deter rust, and may have rust-preventative chemicals in them, whereas others may be intended primarily to be used on wood and would not include the rust preventative.

I would suggest documenting as carefully as possible any notes on the paint cans that may help explain the differences.

Now if you were comparing water paints that preschool children use in coloring books to oil paints that professional artists use, water paint is designed to immediately wash away with water, so they offer no protection at all from water. Good luck!

Regards,

A. Hi Georgia. In our commercial & proprietary world it can be a bit difficult to determine the exact composition of a metal, so it can be tough to come up with good conclusions from exposing several different metals to the same solution. So I think the idea of exposing identical nails to different solutions is more promising.

I'd suggest laying one nail each in four shallow saucers, and pouring just enough liquid over each to wet the nail, and after 3 or 4 days or as many as you can allow, if the saucers have not dried up, put them in the sun or on a warm radiator until they do. But keep everything identical: the same quantity of liquid poured on each, the same time in the sun or on a radiator, etc.

The dry saucers with the nail in them can be your exhibit. Some will be noticeably more rusty than others. 4 possible liquids are tap water, tap water with a little baking soda, vinegar, bleach. Don't mix any liquids together. Good luck.

Regards,

A. Georgia,
Corrosion is a very interesting subject. One experiment you may wish to try is to use some iron (or steel) nails - make sure they are ungalvanized (i.e., not coated in zinc) or coated with anything else, such as paint or lacquer and are not stainless steel. The cheaper the nail, the better!
I suggest you clean them with a good washing up liquid to make sure they are not coated in any grease or oils and then carefully dry and weigh each one with a chemical balance (don't use kitchen scales because these are accurate enough!)
Then put one nail into flasks or bottles containing each of the following liquids: 1) tap water; 2) boiled tap water; 3) Coca Cola/Pepsi Coal (or the Australian equivalent); 4) dilute hydrochloric acid; 5) 5% sodium chloride (salt) in tapwater. You will also need a "control" so put another nail is a flask of boiled tap water and seal it from the air with a layer of candle wax (let a burning candle drip its wax on to the top of the boiled water until it forms a continuous layer about 2-3mm thick). Do not touch your control container until the experiment is over.
I suggest you then put the flasks or bottles in a safe place and give them all, except your "control", a gentle shake for about a minute at least twice a day (say morning and evening). Do this process for as long as you possibly can, but this will depend on how long you can take over your assignment.
At the end of the test period, examine the nails and note what you see. Then take then out of the various liquids and carefully dry them. If any of them have rusted, make sure you also collect the rust, dry it and also weigh it. Add the weight of the rust to the nail it came from. Then compare the weights of the nail(and any rust)at the end of the experiment and compare it with the nail's original weight. Make sure you do not mix the nails up!
Report what you see in regard to the nails and any rust and compare the results for each solution.
As a further clue, the purpose of shaking the containers is to allow air into the solution, whilst the purpose of sealing the "control" in boiled water and under a wax layer is to prevent air getting into the solution.
Have fun!