[vmelkon]

If you have a non-saturated solution of let's say MgCl₂ and you add some solid Cu(OH)₂, what would happen?
Would some of the Cu(OH)₂ dissolve and the OH^- binds with the Mg²⁺?
I imagine some kind of equilibrium would be reached where you have perhaps a little of one hydroxide and a lot of the other hydroxide based on the solubility of each compound.

[Borek]

Its all in K_sp values. pK_sp for Cu(OH)₂ is 18.6, pK_sp for Mg(OH)₂ is 10.7. You start with mostly neutral solution - so concentration of OH^- is around 10^-7. Ignoring shift of water autodissociation solubility of copper hydroxide is 3.6*10^-7 - so pH doesn't change by much, it slightly goes up by just a few tenths of the unit. Even if it goes up to 8, that means [OH^-] = 10^-6, [OH^-]²=10^-12, so you would need 20 M Mg²⁺ to see any precipitation. It ain't gonna happen.

[vmelkon]

Would the pK_sp mean that the shift would be towards Cu(OH)₂?
So if you add a solution of CuCl₂ to some solid Mg(OH)₂, the Mg(OH)₂ would slowly dissolve while some Cu(OH)₂ forms?

[Borek]

So if you add a solution of CuCl₂ to some solid Mg(OH)₂, the Mg(OH)₂ would slowly dissolve while some Cu(OH)₂ forms?

That's what I would expect.