# Which of the following has highest solubility in water?

Which of the following has highest solubility in water?

A. $Ca(OH)_2$

B. $Fe(OH)_3$

C. $Cr(OH)_3$

D. $Zn(OH)_2$

And also please explain how can we predict their solubilities when we are not provided with their solubility products.

I know that calcium hydroxide is sparingly soluble. So it cannot be the right option. I think Iron hydroxide is also soluble to a good extent. I haven't got any idea about option C. I think D may be the right answer.

• Hi SkyDreamer welcome to ChemistryS.E.! Can you show us your efforts and your attempts, we can't do your homework see our homework policy! :-) – G M May 14 '14 at 17:30
• I know that calcium hydroxide is sparingly soluble. So it cannot be the right option. I think Iron hydroxide is also soluble to a good extent. This is evident as Iron rusts in presence of water vapours, but it is a slow process. I haven't got any idea about option C. I think D may be the right answer. – azaidi May 14 '14 at 17:44
• Please help! I have got my final exam tomorrow. – azaidi May 14 '14 at 17:47
• What are your thoughts on what the answer is? We can't solve problems for you; we only guide you through them. In addition, iron hydroxide is a gel-like substance and is hydrated. I'm not aware of how this gel-like precipitate may rust. – Dissenter May 14 '14 at 17:57
• Ohh. I think I was wrong about Iron hydroxide. I just have got a rough idea that the answer is D. – azaidi May 14 '14 at 18:04

1. Most hydroxide salts are only slightly soluble. Hydroxide salts of Group I elements are soluble. Hydroxide salts of Group II elements (Ca, Sr, and Ba) are slightly soluble. Hydroxide salts of transition metals and $\ce{Al^3+}$ are insoluble. Thus, $\ce{Fe(OH)3, Al(OH)3, Co(OH)2}$ are not soluble.
So in this case $\ce{Ca(OH)2}$ even if slightly soluble theoretically should be the right answer.