Why Do Some Hydroxides Form Precipitates in Water?

[rogerfreak]

why are compounds like copper(II)hydroxide, calcium hydroxide and so on are precipitate solids? I thought they were dissolved in water to form alkali already and they suppose to be in liquid form?

 

[chem_tr]

They have low ionization products and hydroxide cannot get outside as copper(II) ion needs it very much to provide neutralization; and it would be better to say that even in copper(II) hydroxide, it is already hydrated a bit with water to yield tetraaquacopper(II) hydroxide, so additional water ions cannot reach the core, this may be the cause for precipitation. But if you add concentrated ammonia to this precipitate, you'll encounter that a rapid dissolution occurs to yield a dark blue solution, namely tetraamminecopper(II) hydroxide.

I cannot say the same for calcium hydroxide, it doesn't prefer additional coordination very much. It fits better here to suggest that it is a very stable ionic solid, not very soluble in water. Although you can prepare a saturated solution of it, this is not likely to be very concentrated, as its ionization product may be somewhat lower than ordinary very soluble ionic solids.

It is a matter of hydration and in general form, solvation, we decide a solid is to be dissolved in water providing that a lot of molecules surround it; if not, the solid is not very soluble in aqueous solution.