Question

ca(oh)2 is a strong base which dissociates fully in water
ca(oh)2 to ca2+ and 2oH-
BUT ca(oh)2 is also a sparingly soluble salt which dissociate only partially ,and has ksp of about order of -6.
why are so?when discuss about acid-base ,it dissociate fully but when talk about salt ,it dissociate partially .why ?can any expert helping me .

Answer 1

$\mathrm{Ca(OH)_2}$ is a strong base which dissociates fully in water: $\mathrm{Ca(OH)_2}$ to $\mathrm{Ca^{2+}}$ and $2\mathrm{OH^{-}}$. BUT $\mathrm{Ca(OH)_2}$ is also a sparingly soluble salt which dissociates only partially, and has $\mathrm{K_{sp}}$ of about order of -6. Why are so? When discuss about acid-base, it dissociates fully but when talk about salt, it dissociates partially. Why?

Everything is right EXCEPT:

...Ca(OH)₂ is also a sparingly soluble(!!!) salt which dissociates dissolves only partially...

The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as acid or alkali.

For example, sucrose (common sugar) dissolves fully in water but it doesn't dissociate at all!

Dissociation and dissolution are not the same process. So $\mathrm{Ca(OH)_2}$ dissolves sparingly, but dissociates fully. In other words: far not all the molecules of $\mathrm{Ca(OH)_2}$ get to the solution from solid, but all that did it - dissociate.

Although calcium hydroxide is only slightly soluble, all of the compound which dissolves is completely ionized.