# How can an insoluble compound be a strong electrolyte

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?

• I think I've found a solution - The silver chloride dissolved very slightly, but whatever is dissolved - is 100% ionized. source: Yahoo Answers
– blz
Dec 24, 2014 at 8:58

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.