Re: Why doesn't a nail rust in Lemonade?

Interesting question. As I haven't seen your experimental apparatus, it 
is a little hard to answer. However, before we go any further, I will 
need to clear up some of your impressions.

The first is that "nails rust" in Coca Cola. They don't. Coca Cola is a 
fairly acidic substance. It has a pH of about 2.5. This comes from the 
phosphoric acid used in making soda pop. Indeed, all of the soda pops 
that I am familiar with employ phosphoric acid and are fairly acidic. 
This is not something that is special to Coca Cola.

The reaction between an iron nail (you are using iron nails and not 
galvanized nails, right?) and the acid in the Coca Cola is an oxidation 
reaction but the products formed are not the oxides of iron that we 
normally associate with rust. Indeed, at the pH of Coca Cola, all that 
happens to the iron is that it is oxidized to iron(II) (Fe2+) and the 
hydrogen is reduced to hydrogen gas. (If you use "flat" Coca Cola - a 
glass that has been allowed to sit for a while to remove all of the 
carbon dioxide - then you should be able to see bubbles forming on the 
surface of the iron.)

When you look at the nails after they have been in the Coca Cola for a 
while, they are likely to be discolored from the sugars and such in the 
Coca Cola but mostly they will be "pitted" with small indentations and 
the like. These are areas where the iron has dissolved away, leaving a 
hole behind. If you leave the nails in the Coca Cola long enough, they 
will completely dissolve. That is, they will disappear as they form iron 
ions in solution.

This is in contrast to "rusting" which results in the formation of iron 
oxides, which are left behind as a browny material. Rusting results in 
making an insoluble compound. The reaction with acid results in forming 
soluble ions.

The pH of water is high enough that the reaction, now, is not the 
formation of iron ions but rather the formation of iron oxides. These 
form because the aqueous iron compound is about as acidic as the water. 
The result is a complex series of reactions that involve making iron 
hydroxide species which then further react to give Fe2O3 (ferric oxide 
or "rust"). Some of the iron will still dissolve into the water but most 
of it will stay behind as a brown patch on the nails. This is "rusting" 
and is the corrosion reaction that occurs on cars and other metal objects 
exposed to water and oxygen. It is aided by electrolytes - such as rock 
salt - which is why they don't salt roads as much as they used to.

So, if in Coca Cola, we have:

             iron + acid  --->  iron ions + hydrogen gas

and in water, we have:

             iron + water/oxygen  ---> iron oxides

then what is happening in the lemonade?

That is a good question. What I think is going on is that the pH of the 
lemonade is likely to be too low for the second reaction to occur. Rust 
formation doesn't occur below a pH of about 6. (By the way, this is 
illustrated by something called a "Pourbaix Diagram". There is one for 
iron at "http://www.metallographic.com/Newsletter/Chemical-Etching.PDF".)

So, the lemonade is too acidic for rust. But the acidic component of 
lemonade is citric acid - a fairly weak acid. It is not really strong 
enough and not dissociated enough to make the acid reaction happen very 
fast as it does in Coca Cola. Phosphoric acid is several orders of 
magnitude stronger than citric acid and will make the reaction go a lot 
faster. The result is that the nails in the Coca Cola will show visible 
pits and markings much sooner than the ones in the lemonade. Indeed, it 
will take about 100 times as long for the lemonade to show the same 
degree of reaction as with the Coca Cola!

Of course, this makes Coca Cola seem like a "bad" drink because it can 
dissolve nails. But any soda pop which has phosphoric acid will do the 
same thing (just read the label to see if it is there). Coca Cola is not 
really "bad". Further, Coca Cola is less harsh than stomach acid. For a 
really (REALLY) gross science experiment, try placing some nails in 
vomit. They dissolve much more quickly than in Coca Cola. Of course, the 
gross part is getting the vomit!!

Hope this helps with your science experiment. Good luck with the results!