When an "insoluble" solid is placed in water we know that some of it actually does dissolve and enter the solution as separated ions (or molecules). The extent of this dissolving is quantitatively described by the Ksp value for the substance.

When silver chloride is added to water the following equilibrium is eventually established:
AgCl(s) ⇄ Ag+(aq) + Cl-(aq)
The Ksp expression and value are:
Ksp = [Ag+][Cl-] = 1.8 x 10-10

We have already seen that for a simple binary salt like silver chloride, the molar solubility will be the square root of the Ksp, in this case, 1.3 x 10-5 M. The silver or chloride ion concentration would be the same.