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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?

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    $\begingroup$ They use a different definition of acid. $\endgroup$ – Ivan Neretin Jul 11 at 14:07
  • $\begingroup$ There's not satisfying def. of "salt" and I don't see it coming. Abandoning such concept would be somewhat reasonable. $\endgroup$ – Mithoron Jul 11 at 21:31
  • $\begingroup$ One might argue that "formed by the neutralization reaction of an acid and a base" is a necessary, but not sufficient, condition for classifying a compound as a salt. You can say that a dog is an animal, but it doesn't follow that all animals are dogs. $\endgroup$ – chepner Jul 12 at 13:11
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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.

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    $\begingroup$ Thanks for the historic perspective, and kind of sad that the high school texts (and wikipedia) can't move on. Oh, I guess I could edit the wikipedia definition... help them move on. $\endgroup$ – Karsten Theis Jul 11 at 14:45
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    $\begingroup$ I am very much disappointed by modern general chemistry texts. It turns away brightest students from chemistry. Some radical changes must be adapted. $\endgroup$ – M. Farooq Jul 11 at 14:49
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    $\begingroup$ Wikipedia has the IUPAC definition: en.wikipedia.org/wiki/Salt_(chemistry) Let's see if it sticks. $\endgroup$ – Karsten Theis Jul 11 at 14:53
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    $\begingroup$ +1 for this answer. IUPAC is the gold standard for definitions. However to me that definition leaves out two important characteristics. First a salt is a solid. Second I would add that a salt should have an appreciable solubility in water. I wouldn't expect $\ce{CuS}$ for example to be classified as a "salt." Yes it has a cation and an anion, but its solubility is next to nothing. If you leave out solubility then every solid inorganic compound is a salt. // Going back to the OP's question, I would consider oxalic acid as being a salt and an acid. $\endgroup$ – MaxW Jul 11 at 15:52
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    $\begingroup$ How will you view ionic liquids? They are "organic" low melting point salts and they don't dissolve in water. They are very hydrophobic. $\endgroup$ – M. Farooq Jul 12 at 2:54
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We are pretty sure that acids are not considered salts

Definitions in chemistry don't have to be exclusive (that is, if X is an acid, it cannot be anything else). It's not very useful to call acids "salts of hydrogen", but it's not fundamentally wrong.

The entire DHMO joke is based on giving water unusual but nevertheless technically correct names.

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You write:

[..] carbonic acid. It is a compound of the ions $\ce{H+}$ and $\ce{HCO^3-}$

This is the error in your reasoning. There is a covalent bond between the hydrogen and the rest of the molecule. When it reacts as an acid, this bond breaks.

I like definition d) of the online MW:

any of various compounds that result from replacement of part or all of the acid hydrogen of an acid by a metal or a group acting like a metal : an ionic crystalline compound

So starting with the acid $\ce{H2CO3}$ (carbonic acid), you would let it react as an acid (in water, perhaps), and replace the lost hydrogen ion by a sodium ion, giving you $\ce{NaHCO3}$, sodium bicarbonate. If you isolate that as a pure solid, you get a typical salt (ionic crystals, dissolve more or less in water).

If you want to get back to your definition, you would add carbonic acid and sodium hydroxide in water. Carbonic acid would act as an acid, sodium hydroxide would act as a base, and depending on the mixing ration (stoichiometry), you would get sodium carbonate or sodium bicarbonate. Removing water would give you the ionic compounds.

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    $\begingroup$ I wouldn't go to a general-purpose dictionary for precise definitions of scientific terms. $\endgroup$ – David Richerby Jul 13 at 8:39
  • $\begingroup$ II edited the Wikipedia article to have the IUPAC definition, I can’t edit MW, but I consider their definition mostly harmless. When you’re discussing chemistry “in the office”, those are the types of references you would check first. @DavidRicherby $\endgroup$ – Karsten Theis Jul 13 at 12:01
  • $\begingroup$ It's great that you are discussing chemistry in your office! Now you can go back to discussing how to [stackoverflow.com/questions/25078285/… large arrays)... $\endgroup$ – Karsten Theis Jul 15 at 19:12

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