According to Wikipedia:
A salt is an ionic compound that can be formed by the neutralization reaction of an acid and a base.
Are all ionic compounds salts? Are all salts ionic compounds?
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1 Answer
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Interestingly, IUPAC states that a "salt" is "a chemical compound consisting of an assembly of cations and anions". Under this definition, all ionic compounds are salts, and all salts are ionic compounds.
Therefore, something like sodium hydroxide ($\ce{Na+OH-}$, definitely an ionic compound) could actually be correctly called a salt. This clashes with the commonly taught high-school level definition of a salt ("the product of an acid-base reaction"), unless you consider very general definitions of acids and bases such as the Usanovich definition, whereby sodium metal $\ce{Na^0}$ is an electron donor (and therefore a base) and water is an electron acceptor (and therefore an acid).
That said, the high-school definition is too simplistic. It is common for compounds to be an acid, a base and a salt all at the same time; consider for example sodium bicarbonate ($\ce{Na+HCO3-}$). It is made of cations and anions, and therefore is definitely a salt. Furthermore, it can act as both a Brønsted–Lowry acid ($\ce{NaHCO3 + OH- -> H2O + Na+ + CO3^2-}$) and as a Brønsted–Lowry base ($\ce{NaHCO3 + H+ -> Na+ + H2CO3}$). Another amusing example is hydrazinium sulfate, a salt, acid and base, where both the cation and anion are also both acids and bases!
answered Nov 26, 2017 at 6:16
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4$\begingroup$ Sodium hydroxide is a potential acid-base product in ammonia solvent, produced from water (acid) and sodium amide (base). Thus the concept of a salt as an acid-base reaction product is a case of "it depends". $\endgroup$ Commented Nov 26, 2017 at 10:22
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1$\begingroup$ I think the better way to produce sodium hydroxide in an acid-base reaction would be to generate sodium oxide and then neutralise that with the acid water. That way, you don’t introduce any additional contaminants. $\endgroup$– JanCommented Nov 26, 2017 at 14:47
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1$\begingroup$ The logic of the first paragraph is flawed. The definition requires anything called a salt to be an ionic compound. The converse doesn't follow - it doesn't require all ionic compounds to be salts. $\endgroup$– Ian BushCommented Feb 23, 2022 at 15:31
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$\begingroup$ @IanBush Indeed, strictly speaking you are correct. That said, I think the "spirit of the law" is to establish bidirectional equivalence between the terms. I can't think of a particularly good reason to carve out some definitional space for "ionic non-salts". $\endgroup$ Commented Feb 23, 2022 at 22:05
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$\begingroup$ What about zwitterionic compounds? $\endgroup$ Commented Jan 5, 2023 at 10:10