# Why do we call salts such as AgCl sparingly soluble?

Though we use the term practically insoluble for salts like $$\ce{AgCl}$$ in inorganic chemistry, we preferably call them "sparingly" soluble salts in physical chemistry seeing their tiny values of Solubility.

But, my thought is that this classification might have arose, because all the ionic compounds should theoretically be soluble in water (no matter how small it is) because the entropy during the dissolution process of salt increases, due to the formation of less restricted hydrated ions.

Is always the dissolution of ionic salts accompanied by entropy increase (so that the dissolution becomes feasible above particular temperature whether it is exothermic or not) and my thought correct?

• If dissolution enthalpy is sufficiently positive ( lattice energy has the upper hand over ion hydration), entropy increase may not help much with decreasing Gibbs energy by dissolution. Aug 13 at 16:06
• All classifications are imperfect. The term sparingly soluble does not have a quantitative connotation. Aug 13 at 16:21
• Why "salt"? Because of lack of better name. All "salts" have both covalent and ionic component in bonding, for AgCl, more so for AgI of Ag2S, covalent component is bigger than ionic - they shouldn't be called ionic compounds. Aug 13 at 16:45
• I haven't heard of the term "sparingly soluble" acid, base or covalent solutions.it seems that this term is specific/ unique for ionic salts. Aug 13 at 17:58
• No sparingly soluble is a universal term. Aug 13 at 19:02

First, yes, the dissolution of ionic salts is always accompanied by entropy increase, as far as I know, because in chemistry is often complicated to say always.

But, as @Poutnik said, it's also important to check the dissolution enthalpy and the hydrophobic efect.^[1]

On the other hand, is just a convention to say that some salts are insoluble or sparingly soluble, because even if they are all soluble but in tiny amounts, in practice you can't base much on that, the Ksp may be negligible.

The word "salt" comes from a rather old chemical theory, established in the $$19$$th century, before the "discovery" of the ions. At this time, acids were substances that turn tea yellow and cabbage red. Bases were substances that turn tea brown and cabbage blue. Other indicators were also used.

Acids formula were written $$\ce{H_nX}$$ and bases $$\ce{M(OH)_n}$$, with M being a metal, X a non-metal or a group af atoms, and $$n$$ an integer. At this time, chemists discovered that each base can react with any acid according to the general equation :

Acid + Base --> Salt + Water

This definition of "salt" (as a result of the reaction acid + base) was considered as valid even if the acid or the base was not soluble in water. For example, if an acidic solution was not acidic any more after reaction with an insoluble substance, this last substance was accepted in the category "base", even though it does not react with indicators. If metallic oxides were also reacting with and destroying acids, they were admitted in the category "bases". So any acid can react with any base to produce a salt. A salt is by definition the result of any reaction acid + base, after loss of water.

Experiments show that $$\ce{AgCl}$$ can be obtained from $$\ce{Ag2O}$$ as a base after reaction with $$\ce{HCl}$$ as an acid, even though $$\ce{Ag2O}$$ and $$\ce{AgCl}$$ are sparingly soluble. So $$\ce{AgCl}$$ is a salt. Its poor solubility was not considered as important, in the 19th century.