How can an insoluble compound be a strong electrolyte

Question

Here's a quote from Petrruci General Chemistry (pg. 160):

Silver chloride, $\ce{AgCl}$ is an insoluble ionic compound. When $\ce{AgCl}$ dissolved in water, it is 100% dissociated into $\ce{Ag}^+\text{ and } \ce{Cl}^-$ ions; there are no $\ce{AgCl}$ pairs.

I'm confused:

1) If $\ce{AgCl}$ is insoluble, how is it dissolved in water?

2) If $\ce{AgCl}$ is a strong electrolyte but insoluble, does it mean a $\ce{AgCl}$ molecule is ionized, but isn't separated - i.e. the silver and chloride remains close (they're not solvated by water molecules) but each becomes and ion? I'm confused, can someone explain what's going on?

Answer 1

It sounds contradictory, but soluble and insoluble are relative terms. Silver chloride's $K_\mathrm{sp}$ is $1.77\times 10^{-10}$, so one can generally think of it as insoluble, but actually about a milligram will dissolve in a litre of water.

Much like the terms strong acid and weak acid, strong electrolyte and weak electrolyte refer to the dissociation of a substance in a solvent, though they include all electrolytes, not just acids. In the case of silver chloride, though little of it dissolves, what does is present only as $\ce{Ag+}$ and $\ce{Cl-}$, not solvated $\ce{AgCl}$. In water, all salts are strong electrolytes, but in other solvents, things can be different. For example, while perchloric acid is a strong electrolyte in water, it doesn't dissociate completely in acetic so it's a weak electrolyte.

Source: (1) Electrochemical Dictionary; Bard, A. J.; Inzelt, G.; Scholz, F., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2008.

Answer 2

I sort of believe that AgCl is strong electrolyte; when we decide whether a given solid compound is strong electrolyte or not , it comes that basically there are 3 stages to look into

(1) how much of solid is taken to dissolve
(2) how much of it indeed, dissolved ( that is to say, what fraction of solid goes into aquous state) (3) how much of dissolved state (fraction of dissolved solid) really dissociate in to ions.

Some books consider amount of solid taken in order to dissolve in water and compare the fraction of which turned into ions eventually in solution to decide if it is a strong one, on this basis, AgCl is weak electrolyte .because ,amount that turned into Ag+ and Cl- is negligible, when compared to solid AgCl originally taken

Another version is that, some other books consider Amount of solid dissolved ( not the solid amount taken) and compare fraction of dissolved salt that turned into ions: on this basis, almost 100%of AgCl dissolved fraction turns into ions, hence it is strong electrolyte

Now coming to your question of AgCl pairs, when solid AgCl is taken, it is, infact, ionic network solid. a little fraction of which is dissolved in water that exists as "discrete AgCl molecules "in water; in other words as (,Ag+&Cl- )combined ion pairs (AgCl ionic pairs) ;now 100% of this AgCl ionic pair completely dissociate into constituent Ag+and Cl- ions that is to say 100 percent of dissolved salt dissociates completely ; thus,it is deemed strong electrolyte.