[lena]

 ???I wonder why do some ions precipitate as  hydroxides while other do not?

[HongKongALevelboy]

   It s about the solubilities of metal hydroxide . According ot the equation : enthalpy change of solubility = enthalpy change of hydration - enthalpy change of lattice . The more negative the solubility enthalpy is , the more soluble the metal hydroxide is .
   For group 1 , the size of metals are larger than that of other metal ions .Going across the period from group 1 , the change in lattice enthalpy is larger than that of hydration enthalpy . Therefore ,the enthalpy change of solubility become positive . Group 2 and transition metals are mostly insoluble in water and form precipatate .
   that s all i know , can anybody explain more about it ?

[Donaldson Tan]

the gist is: if the final hydroxide product is insoluble in water, precipitation occurs.

[pizza1512]

It all depends on where it is in the relativity table...