Re: Why does a metal nail rust the most in water as opposed to soda or vinegar?

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!