|MadSci Network: Chemistry|
Good question. The answer is that a nail doesn't rust in vinegar in the first place, so water would likely produce the most "rust". To explain this, I need to refer to a bit of acid/base and "redox" chemistry. You may have heard of the latter if you have done any experiments involving batteries or electrochemical cells. Vinegar is an acidic substance with a pH that is dependent upon the concentration used. Typically, vinegars have a pH of between 2.4 and 3.4. The acidic component of vinegar is provided by acetic acid which is a weak acid (pKa 4.76) which means that it is not very dissociated. (Another way to think of this is that you need an awful lot of acetic acid to get a pH of 2.4!) At the pH of vinegar, iron is converted not to rust but to iron ions in solution. Specifically, the iron is oxidized to iron(II) (Fe2+). This is illustrated in the "Pourbaix diagram" for iron (there is a copy available at: http://www.metallographic.com/Newsletter/Chemical- Etching.PDF) which shows that below a pH of about 6, iron converts directly to a water soluble iron(II) ion. It is only above pH 6 that iron converts to iron hydroxide and then undergoes the myriad of reactions that eventually result in the formation of Fe2O3 (ferric oxide or "rust"). So, to split hairs (or nails), iron can't rust in vinegar! (Or, at least, not until all of the acidic component is used up and the vinegar is neutralized to just "water".) Instead, it reacts with the acetic acid to give hydrogen and iron acetate. What will eventually happen in the vinegar is that the nail will dissolve completely. However, because vinegar is a weak acid, this process will be very slow. It will take a long time for any visible signs to occur. Water, which has a pH of greater than 6, in combination with dissolved oxygen, will react with the iron in the nail much more rapidly. The result is that you get rust formation - which results in the formation of the ferric oxide deposits which show up as brown deposits on the surface of the nail. Some of the iron will dissolve into the water but most of it will be left on the surface in deposits. Water/oxygen will not dissolve a nail. It will transform it to "rust". Thus, in vinegar we have: iron + acetic acid ---> iron(II) ions + hydrogen and in water, we have: iron + water/oxygen ---> iron oxide ("rust") The latter reaction is much more visible and faster than the former. In soda, the pH is much higher than in water. This has the effect of slightly passivating the surface with the result that rust doesn't form as fast. It is one of the quirks of nature that the pH of water is such that it promotes the formation of iron oxides and results in the rusting of nails and such. By the way, if you have a hardware store that carries "muriatic acid", you can use it to dissolve a nail much more quickly. "Muriatic acid" is hydrochloric acid and fairly strong, so care must be taken if you are using it. However, because it is both a concentrated and strong acid, it will rapidly dissolve the nail forming not "rust" but a solution of iron (II) chloride. If you try this, extreme care should be exercised! Hope this helps with your science fair project. Good luck with the results!
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