A metal plate has an unwanted coating of $\ce{CaCO3(s)}$. You must find a way to clean this metal plate without causing any scratches or abrasion. How might you dissolve the coating?

I know I should find a substance that will dissolve the coating, and then soak the plate in it. How do I determine an appropriate substance?

  • $\begingroup$ Acetic acid, aka vinegar $\endgroup$ – Gimelist Jan 12 '15 at 6:31

It might take some time, but I would use vinegar.

The acid in vinegar is strong enough to dissolve the calcium carbonate, but it is not strong enough to dissolve metals (unless the metal reacts with water).

  • $\begingroup$ Do you mind explaining the chemistry behind that? $\endgroup$ – McB Jan 12 '15 at 1:28
  • $\begingroup$ @McB I think it's because if a metal doesn't have a negative redox potential, it won't spontaneously oxidize (my redox chem is extremely rusty). I'm not sure what the difference between acetic acid or (what I imagine would work better) hydrochloric acid would be though. $\endgroup$ – Nick T Jan 12 '15 at 2:43
  • $\begingroup$ @NickT Depending on what the metal is, hydrochloric acid could dissolve the metal (especially zinc and copper). $\endgroup$ – LDC3 Jan 12 '15 at 3:16
  • $\begingroup$ @LDC3 acetic acid will happily attack zinc, and HCl anecdotally won't (e.g. the "hollow penny" demo); what determines it though? More concentrated HCl might, as it would shift whatever equilibrium and thus (I think) the redox potential. $\endgroup$ – Nick T Jan 12 '15 at 8:01

I recommend sulfamic acid with formula $\ce{H3NSO3}$. The reaction is irreversible, no dangerous compound:

$$\ce{2 H3NSO3 + CaCO3 -> Ca(H2NSO3)2 + CO2 + H2O}$$


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