Is an acid a salt or not? In our office, we discussed today what the exact definition of a salt is and whether an acid itself can be classified as a salt.

Our first problem was that we couldn't get a definitive definition for what exactly a salt is. Wikipedia states:


  In chemistry, a salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base.


Let's take, for example, carbonic acid: it is a molecule comprised of the ions $\ce{H+}$ and $\ce{HCO3-}$. We could say that this molecule forms due to the reaction of a base ($\ce{HCO3-}$) and an acid ($\ce{H_3O+}$):

$$\ce{H3O+} + \ce{HCO3-} \rightarrow \ce{H2CO3} + \ce{H2O}$$

Thus, it seems that the acid fulfills the definition above. We are pretty sure that acids are not considered salts. So where is the error in our reasoning?
It is a semantics question with an open ended discussion. If you recall the old problem "How many angels can dance on a pinhead? Medieval problem, this issue here is similar.

Basically in chemistry, all the terminology is controlled or endorsed by the International Union of Pure and Applied Chemistry (IUPAC). They have a very open ended definition of a salt: "A chemical compound consisting of an assembly of cations and anions."

If we follow some really old chemistry texts (>1850s), yes an acid was defined as a salt of hydrogen ion, i.e., an acid is a salt whose electropositive element is hydrogen Definition of an acid.

Did this definition by a few chemists become popular? The simple answer is no. Not in a general sense. 

The modern interpretation of an acid, again from IUPAC is, "A molecular entity or chemical species capable of donating a hydron (proton) or capable of forming a covalent bond with an electron pair."
If you have heard of ionic liquids, they consist of a large organic cation and an anion, this is a salt as well.

Then you have so-called acid salts, such as sodium hydrogen sulfate e.g.,  $\ce{NaHSO4}$. Their pH is very acidic and these "salts" will neutralize bases like a typical acid.

Thus one may loosely say, that all acids may be considered as salts but not all salts are acids.