Semi-quantative analysis of rusting cans

Now this is technically a homework question, but think it will pass muster.

My son (aged 14) has been given a good old-fashioned science experiment to determine how much a can would rust in 10 weeks.

Now I think a pass mark might be earnt for something not much more detailed than "We took two cans, buried one in the garden and left a control in the cupboard. After 10 weeks they looked like this “…”.

But Cam wanted to be more creative and I thought that was something to be encouraged. So we have designed an experiment using various household chemical agents.

  1. water
  2. salt
  3. sodium bicarbonate
  4. vinegar
  5. sodium hydroxide
  6. garden soil
  7. bleach
  8. detergent
  9. washing powder
    plus control

We have also have a second treatment of scratching another set to test if this accelerates the process. We’ve sealed them in plastic bags, and are observing them for the next couple of months.

Now given the limitations of the family’s kitchen equipment this is much more bucket than quantitative chemistry. We’ve tried to have the solutions around 0.1M but they could vary by an order of magnitude.

OK so in 10 weeks time we’ll have about 2 dozen cans in varying stages of rust.
What semi-quantitative test could be done to approximately rank them? We can possibly get permission to use the school science lab. I was thinking something using chromography or light diffusion.

If you can be sure not to lose any rust that falls off the cans, or to pick up too many foreign substances like water or whatever, you could get a good quantitative measure by just weighing the cans before and after. Rust is just oxygen bonding with the iron, after all, and that adds weight.

Very true, but in the time-frame that differential is likely to be measured in micrograms, and may well be less than the variance in weight of the cans themselves.

What every well appointed kitchen really needs is a mass spectrograph!

Why would any of these be better than estimating the percentage surface area that apears rusty?

<scientific pontification>
You’re starting with cans in solutions, where the solutions have a wide uncertainty. Trying to rank the cans with a precise, quantitive test is misleading, IMO, because you could wind up ranking the can in solution X as “rustier” than the one in solutions Y based on a small difference in the measurement result.

It would be better, think, to “match” the precision of your measurement method to the overall test. In this case, because you’ve got only one of each sample and a large uncertainty in the solution percentage, grouping the results into “unrusted,” “lightly rusted,” and “heavily rusted” (with some approximate definition in terms of percentage of coverage) would be a more valid method than an individual numerical ranking.

Now, the scratched v. unscratched (where presumably you have the same solution in each) could be a place where higher precision is justified, but I would expect additional rusting to be confined to the area right around the scratch.
</scientific pontification>

Have you thought about doing a comparison between cans sealed in bags as you have, and cans exposed to air? Of course the bags will have some air in (unless you vac-packed them) but there is a limited supply of oxygen available which imo would limit the amount/rate of oxidation after a certain point.
I understand that this could be viewed as taking it a bit too far but to some minds you could have already crossed that line!
Good luck, may your offspring’s inquisitivity (sp? real word?) never fade.